Project Electronics For Everyone Projects for all · 2020. 4. 13. · 740173 0 72 740174 1.05...

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Project Electronics For Everyone Projects for all Digital ThermometerA Audio Signal Generator Echo-Reverb Unit4 Scaling the Hi-fi Heights -the final installment NEW NEW NIFW NEW Popular Computing Series Into Radio First steps MAY "82 ISSN 0142-6192 70p HE CABLE TRACKER Cable Tracker

Transcript of Project Electronics For Everyone Projects for all · 2020. 4. 13. · 740173 0 72 740174 1.05...

Page 1: Project Electronics For Everyone Projects for all · 2020. 4. 13. · 740173 0 72 740174 1.05 740175 1.05 740192 1.08 740193 1.08 740195 1.08 740200 4 52 740221 1 06 74C901 0 38 740902

Project Electronics For Everyone

Projectsfor all

Digital ThermometerA Audio Signal Generator

Echo-Reverb Unit4

Scaling the Hi-fi Heights-the final installment

NEW NEW NIFW NEWPopular Computing SeriesInto Radio First steps

MAY "82ISSN 0142-6192



Cable Tracker

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....! Avallable at/ieWSagent orcur


Ambit's new style catalogue continues to lead the market withAmbit'slow prices, new items, info, 3 £1 discount vouchers. In a recentsupplier survey, we were one of only two suppliers listed in allcategories!There's a few examples of some super low prices78XX 1A 37p All the "usual" stuff at rockBC237/8/9 8p bottom prices + Toko coils,3SK51 54p crystal and ceramic filters,10MHz XTA LS micrometals toroids, Fairite

£2 ferrites, Alps switches, OKI8 Pole 10.7MHz XTAL LSI, Piezo sounders, RF, IF

filters £14.50 Modules + Kits etc.

2GHz coax relay150W £10.95 Price the



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Prices shown EXCLUDE VAT.Access/Barclaycard may be usedwith written or telephone orders,official MA details on application.E & E0.

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and PACKING50p per order

74153N 0 5574154N 0 5574155N 0 5574156N 0 557415714 0 5574159N 1 9074160N 0 5574161N 0 5574162N 0557416314 0 557416414 0 5574165N 0.707416614 0 7074167N 1 25

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AMBIT internationalTELEPHONE

200(STD 0211) 230909

north ServiceTELEX 995194 AMBIT 6 POSTCODE CM14 4SG

Rood, Brentwood,Essex

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PROJECTS* DIGITAL THERMOMETER 10Reads Fahrenheit or Celsius.* HE ECHO-REVERB 33The beat goes on, on, on, on, on* AF SIGNAL GENERATOR 49A simple, useful test generator.* QUICK PROJECT 55Light Seeker.* CABLE TRACKER 65An essential accessory for the DIY handyman.

FEATURES* POPULAR COMPUTINGTHE ZX81 REVIEWED 20Sinclair's micro -masterpiece reconsidered.* MICRO -HISTORY 26A short, short history of the computer.* SCALING THE HIFI HEIGHTS 44The final installment - adding a cassette deck.FAMOUS NAMES 53Charles Babbage.INTO RADIO* RADIO RULES 57Sines of the Times.INTO ELECTRONIC COMPONENTS 61Part 1 1: Digital Dealing.


REGULARSMonitor 6* BUYLINES 68* POINTS OF VIEW 16Clever Dick 30HE Subscriptions 38Backnumbers 56Bookshelf 70What's On Next 18PCB Service 60PCB Print Out 72Index to Advertisers 73Classified Advertisements 73

Editor: Ron KeeleyEditorial Assistants: Paul Coster BSc

Tina BoylanSenior Art Editor: Kieran WadeAssistant Art Editor: Enzo GrandoAdvertisement Manager: Gary PriceManaging Editor: Ron Harris BScManaging Director: T.J. Connell

Hobby Electronics is normally published on the second Friday of the month prior to the cover date.Hobby Electronics, 145 Charing Cross Road, London WC2H OEE, 01-437 1002. Telex No 8811896. Published by Argus Specialist Publications Ltd.

Distributed by Argus Press Sales Et Distribution Ltd, 12-18 Paul St, London EC2A 4JS. Printed by QB Ltd, Colchester. Covers printed by Alabaster Passmore.Notice: The contents of this publication including all articles, designs, plans, drawings and programs and all copyright and other intellectual property rightstherein belong to Argus Specialist Publications Limited. All rights conferred by the Law of Copyright and other intellectual property rights and by virtue of

international copyright conventions are specifically reserved to Argus Specialist Publications Limted and any reproduction requires the prior written consent ofthe Company. Copyright 1982 Argus Specialist Publications Ltd. Member of the Audit Bureau of Circulation.

Hobby Electronics, May 19823

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FOR THE ZX81and other computers

by Stephen Adams





Make the most of your micro with this great bookof construction projects.Projects include: a numeric keypad; giant seven -segment display; score board; light pen;thermometer; shift lock; computerised voltmeter;and an 'unbeatable' burglar alarm.

INTERFACEDepartment HE,44-46 Earls Court Road,LONDON W8 6EJPlease send me a copy of Stephen Adam's book20 SIMPLE ELECTRONIC PROJECTS FOR THEZX81. I enclose £6.45, plus 30p p&p



HE PROJECT KITSMake us your No. 1 SUPPLIER OF KITS and COMPONENTS for H.E. Projects. Wesupply carefully selected sets of parts to enable you to contruct H.E. projects. Kitsinclude ALL the electronics and hardware needed. Printed circuit boards (fully etched,drilled and roller tinned) or Veroboard are, of course, included as specified in the originalarticle, we even include nuts, screws and I.C. sockets. PRICES INCLUDE CASES unlessotherwise stated. BATTERIES ARE NOT INCLUDED. COMPONENT SHEETINCLUDED. If you do not have the issue of H.E. which includes the project - you willneed to order the instruction reprint at an extra 45p each.

Reprints available separately 45p each + p. & p. 40p.

CABLE TRACKER May 82 E9.37DIGITAL CAPACITANCE METER Apr 82£21.69SIGNAL TRACER Apr 82 £3.61BIKE ALARM Apr 82 £10.98DUAL ENGINE DRIVER Apr 82 £48.78DIGITAL DICE Mar 82 £6.82BICYCLE SIREN Mar 82 £10.18NOISELESS FUZZBOX Feb 82 £9.77SOUND SWITCH Feb 82 £8.31MASTHEAD AAMPLIFIER Feb 82 £13.74DRUM SYNTHESIZER Dec 81. Full kit£19.98GUITAR HEADPHONE AMPLIFIERDec 81 £3.48IN CAR CASSETTE POWER SUPPLYDec 81 £4.46SCRATCH FILTER Nov 81 Mono £5.44Stereo £8.40LED VU METER Nov 81 less case E4.56SIMPLE STYLUS ORGAN Nov 81 lesscase f4.74METRONOME Nov 81 £11.88TELEPHONE BELL REPEATER Oct 81£12.78Med Linking wire extra 14p metreCOMBINATION LOCK Oct 81 lesssolenoid E17.43BABY ALARM Oct 81 £8.14, Fig 8 linkingwire 7p metre'DIANA' METAL LOCATOR Sept 81£33.85POWERPACK Sept 81 £9.58REACTION TESTER GAME Sept 81£11.98VARIABLE BENCH POWER SUPPLYAug 81 £25.35ULTRASOUND BURGLAR ALARMJuly 81 £18.67ELECTRONIC DOOR BUZZER July 81£5.65ELECTRONIC METRONOME July 81£4.67CONTINUITY CHECKER June 81 £5.34

ENVELOPE GENERATOR June xi£16.85AUDIO MIXER June 81 £4.99PUBLIC ADRESS AMPLIFIER March81 £1821, Extras - horn speakers £6.83each, PA MIC £4.40FUZZBOX March 81 £10.35WINDSCREEN WIPER CONTROLLERMarch 81 £7.67STEAM LOCO WHISTLE March 81


E3.28HEARTBEAT MONITOR Feb 81 £23.40AUDIO SIGNAL GENERATOR Feb 81£18.93TWO-TONE TRAIN HORN Feb 81 £5.24less caseMEDIUM WAVE RADIO Feb 81 £7.67BENCH AMP Jan 81 £10.10NICARD CHARGER Jan 81 £7.67CHUFFER Jan 81, less case £7.04BATTERY CHARGE MONITOR Dec 80£5.40MEMORY BANK - MINI SYNTH-ESISER Nov & Dec 80 E28.40TRANSISTOR TESTER Nov 81 £6.12 inctest leadsGUITAR PRE -AMP Nov 80 £5.65 caseIdiecastl extra £2.29INTRUDER ALARM Oct 80 £19.61TOUCH SWITCH Sept BO £2.57 less case& contactsGUITAR PHASER Sept 80 £15.22SOUND OPERATED FLASH TRIGGERJuly 80 no skt £4.99FOG HORN June 80 £6.21SPEED CONTROLLER FOR MC April 80£16.41 (less case)DIGITAL FREQUENCY METER april 80£39.35DIGI-DICE Jan 80 £10.97GUITAR TUNER Nov 79 £11.99CAR ALARM Feb 79 £12.07





MEMORY BANK SYNTHESISERMiniature synthesiser featuring vibrato, envelope, tempo, volume + pitch controls.Uses 24 push button switches in a keyboard style layout. Based on a customdesigned L.c. The accessible memory stores a 32 beat length sequence of notes +spaces. Can be played 'live'. Fitted with an internal speaker. Jack socket allows theuse of an external amplifier if wished.

Memory Bank Synthesizer £28.40Complete kit inc. case, pcb's etc. Reprint extra 45p. Available separately 45p+ 45p p&p.



4 Hobby Electronics, May 198

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SOLDERING/TOOLSANTEX XS SOLDERING IRON 25W£5.48SOLDERING IRON STAND £2.40SPARE BITS Small, standard, large. ForXS and X25 irons 65p eachSOLDER Handy size 99pSOLDER CARTON £1.84DESOLDER BRAID 69pDESOLDER PUMP £6.48HEAT SINK TWEEZERS 29pHOW TO SOLDER LEAFLET 12pLOW COST CUTTERS £1.69LOW COST LONG NOSE PLIERS f1.68WIRE STRIPPERS + CUTTERS f2.69HELPING HANDS JIG £6.30Heavy base. Six ball and socket joints allowinfinite variation of clips through 360°.Has 2 'h" diameter (2.5x) magnifierattached. Used and recommended by ourstaff.VERO SPOT FACE CUTTER £1.49PIN INSERTION TOOL £1.98VEROPINS 1pk of 10010.1" 52pMULTIMETER TYPE 1 11,000 opv) £6.66MULTIMETER TYPE 2. 20,000 opv withtransistor tester. Very good £14.75ILLUMINATED MAGNIFIERS Small 2"


*Yet* 41- -it






,0 leads with 20 clips 99p

* CALCULATOR4(. eCoOpsNespECTING WIE PACK 15 x 5 vd .011


4r M2 pack .

M3WC 1A. /H. Eiv11114S£401 Lea c h *SOL TAGSDER M 333p/50. M4541150 *





BASIC ELECTRONICS. A super book covering theory and practice. Educational. Suits age 14upwards. Lots of useful projects - circuits built on an S -Dec breadboard. Approx 525 pages .. £7.98

CONSTRUCTOR'S PROJECT BOOKSSpecially written for the electronics enthusiast. Each book contains a collection of constructionalprojects, giving details of how the circuit works, how it may be assembled and how setting up andtrouble shooting problems may be solved. The colour in the text helps to clarify details. Delightfulbooks.ELECTRONIC GAME PROJECTS Rayer £3.35ELECTRONIC PROJECTS FOR HOME SECURITY Bishop £3.35ELECTRONIC PROJECTS IN AUDIO Penfold £3.35ELECTRONIC PROJECTS IN MUSIC Flied £3.35ELECTRONIC PROJECTS IN PHOTOGRAPHY Penfold £3.35ELECTRONIC PROJECTS IN THE CAR George E3.35PROJECTS IN AMATEUR RADIO fr SHORT WAVE LISTENING Rayer £3.35PROJECTS IN RADIO AND ELECTRONICS Sinclair ... £3.35ELECTRONIC PROJECTS IN HOBBIES Rayer £3.35ELECTRONIC PROJECTS IN THE HOME Bishop £3.35ELECTRONIC PROJECTS IN THE WORKSHOP Penfold £3.35ELECTRONIC TEST EQUIPMENT PROJECTS Ainslie £3.35MORE ELECTRONIC PROJECTS IN THE HOME Flind £3.35110 ELECTRONIC ALARM PROJECTS FOR THE HOME CONSTRUCTOR Marston £5.35

PAPERMAC - ELECTRONIC PROJECTSA set of 4 books full of interesting circuits and projects.



POPULAR LOW COST BOOKSMost titles new this month



FREE PRICE LISTPrice list included with

orders or send sae (9 v 4)CONTAINS LOTS MORE



Illustrations, product descriptions, circuits all in- -cluded. Up-to-date pricy list enclosed. All productsare stock lines for fast delivery.Send 70p in stamps or add 70p to Order.


C106D 58p 2N5457 Sep BEY61 24p INTEGRATED CIRCUITSTIC46 48p 2N54:14 63p BFY 52 23p .4001 20p LM386N 92p0A47 lip 40675 98p 13FX88 32p .4011 20p LM387 £1.100A90 9p AC158 29p BRY39 48p J4013 32p LM389N £1.100 A202 16pW005 33p

AC141 38pAC142 39p

MPSA65 39pRPY58A £1 16

4017 62p LM1830 £1.984020 6515 L M3909 8215

WO6 47p AC176 37p TIP31 A 52p 4024 60p LM3914 f2.9925J £2 92 BC182 lip TIP32A 83p laws LM3915 f3.95WW1 5i -fa BC182L lip TIP33A 94p 4081 20p MC3340 £215IN4005 6p BC183 lip TIP34A 99p 4093 38p NE570N £4.98IN4148 5p BC1E4 11p TIP121 £1 12 140174 f1.20 NE571N £4.84I N5404 18p BCIS4L 11p TIP2955 69p 4522 £1.25 0M335 £8.98IN5408 19p BC212 11p TIP3055 69p 555 26p SN 76477 02.6913E244 Sip BC212L lip TIS43 381, 556 79p TL064 E1.64MPF102 69p BC 213 lip TPSA13 35p 741 22p TL071 47pTIS88A 57p BC214 11p SN3053 25p CA3130E 98p TL082 74pVN67AF £1 21 BC214L 11p 2N3055 59p CA3140E 52p TL083 E1.522'53819 28p 30131 48p 2N3702 lip HA13138 £320 TL084 £1.542N3820 78p BEY5U 25p 2N3704 lip ICL7611 88p TL430 96p

ICM7555 99P U2378 £1.69.1W CARBON FILM RESISTORS LF351N 56P U267B £2.20E12 SERIES. 1R -10M 1 ip each LF353N 9915 U L N2283 E1.57MIN. HORIZ. PRESETS. 1005- M7 12p each i LF355 £1.14 2N414 98pMIDGET POTS. LINEAR. 470R -4M7 37p each LF356 £1.15 2N419CE £221LOG. 4K7 -2M3 38p each LM301A 36p 7400 22pSWITCHED POTS. 4K7 -1M. LIN. 75p. LOG 76p LM317K £320 7447 £1.14

LM335 £1.16 7490 56PLM3130N 89p 7495 80pPOLYESTER (C280) CAPACITORS, 250V

iOnF; tSnF; 22nF; 33nF; 47nF 7p each. 68nF;100nF Sp. 150nF; 220nF 12p. 330nF 15n. 470nF 20p.680nF 28p. 1,4F 33p. 1 505 49p. 2 20F 65p.

OPTOBPX25 £2 242N5777 60pORP12 99pTIL32 81pTIL78 74pLEDS WITH CLIPS3mm. Red 15p. Green 18p. Yellow

20p.5mm. Red 16p. Green 28p. Yellow

SUB MINIATURE PLATE CERAMICS. 63VValues in OF; 2.2; 3 3; 4 7; 5 6; 6 8; 8 2; 10; 15;22: 33; 47 & 56pF 7p each. 68pF; 100pF 7p each.150pF; 220pF; 330pF 11p each. 390p F, 470pF,1000pF Sp each. 2200pF 8p each. 3300pF, 4700pF7p each. lOnF 13p. 100nF 22p. 47nF 14p.

ELECTROLYTIC CAPACITORS 29p.FLASHING LED 78pAXIAL Leads: 1,,F/16V 11p; 1,1F/63V, loF /100V RECTANGULAR. Red 24p12p; 2 2/,F1133V, 3 3uF/63V, 4 7uF/63k/ 12p; ICIpF/ MAINS PANEL. Neon 32p

16V 11p; 10oF/25V, 10uF/63V 12p; 22pF/I0V 22, F/25V 12p; 220F/63V 15p; 33uF/40V, 472/F/25V 12p; ZENER DIODES. 400mW.47jeF/ 40V 15p; 470F/63V 11115; 1000F/16V 12p; BZY88. Range 2V7 to 33V. 12P103rtF/25V 15p; 1007,Ft40V 18p; 1000F/63V 29p; each.220uF/10V 15p; 22021F/25V 19p; 470uF/16V 29p:470,,F/25V 38r: 470trF/40V 55p; 6800F/16V 32p; I.C. SOCKETS1033gF/10V 30y; 1000uF,16V 33p; 1000,1F/25V 48p: 8 pin.. 16p 18 pin. 22p100011F/40V 58p; 10090F/6311r 79p; 22000F/I0V 39p; 14 pin.. 17p 24 pin.... 48p2200rtF/25V 64p; 2200nFi63V £1.10. 16 pin....ilp 28 pin.... 45p

SWITCHES JACKSONMIN. TOGGLE spst 59p; spdt 69p; dpdt 79p. 300pF di I econ £236MIN. PUSH ON. 19p. PUSH OFF. 22p. 500pF dilecon £2 92FOOTS WITCH & ALT. ACTION spco £1 39, C804 Ver. Capac.; 10pF C2 28.

dpco £1 88. 25pF £2 46. 50pF £2 48. 100pFROTARY SWITCHES. 1 p 12 way, 2p 6w, 3p 4w, £2 83. 150pF £3 48.

4p 3w 69p each '01' 365pF £3 48.'02' 365pF. £4 4912V 185R DPCO RELAY £2 98 '02' 208 + 176pF £3 98.


VEROBOARD 0.1" COPPER STRIPS PP3 CLIPS 10p10 strips 24 holes .... .£120 per 5 PP9 CLIPS lip24 strips 37 holes 78p


24 strips W holes 99P EUROBREADBOARD f6.20 1A, 25V. 100uA-0-36 strips 37 holes 89P 5 DEC BREADBOARD £3.98 100uA, 5A. AU36 strips W holes 09P BIMBOARD 1 BREADBOARD £6.98 £4.98 each. StateTerminal pins 0.1" 52p/100 VEROBLOC BREADBOARD £4.20 value


by TO111Duncan

An easy to follow book suitable for all ages. Ideal for beginners.No soldering. Uses a Bimboard 1 breadboard, gives clear instruc-tions with lots of pictures. 11 projects based on integrated circuits-includes dice, two-tone doorbell, electronic organ, MW/LWradio, reaction timer, etc. Component pack includes a Bimboard 1breadooard and all the components for the projects.Adventures with Microelectronics £2 55. Component pack £2964less battery.


An easy to follow book suitable for all ages. Ideal for beginners.No soldering, uses an S -Dec breadboard. Gives clear instructionswith lots of pictures. 16 projects -including three radios, siren,metronome, organ, intercom, timer, etc. Helps you learn aboutelectronic components and how circuits work. Component packincludes an S -Dec breadboard and all the components for theprojects.Adventures with Electronics £2.40. Component pack £1798 lessbattery.

MAGENTA gives you FAST DELIVERY OF QUALITY COMPONENTS & KITS.All products are shtick lines and are new & full specification. We give personal service &quality products to all our customers -HAVE YOU TRIED US?



OFFICIAL ORDERS WELCOME.IRISH REPUBLIC & B.F.P.O. EUROPE:Deduct 10% from prices shown Payment.must be in Sterling.ACCESS and BARCLAYCARD (VISA)ORDERS ACCEPTED BY PHONE ORPOST.SAE ALL ENQUIRES.Normal despatch by return of post.

Hobby Electronics, May 19' 5

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111114 SSSS

IT NEVER rains but it pours, quite often.This month's new releases include, forthe first time, a selection of videomonitors suitable for use as computerVDUs.

Most home computers are equippedwith a 'video' output intended for usewith ordinary domestic television sets.This output is, in fact, a frequencymodulated RF signal which is tuned in onChannel Six. This system provides acheap and convenient display, but thedefinition is not as good as it could be, dueto lack of stability and bandwidth of mostvideo modulators - and there is often aconflict over the use of the set! The simplesolution to the second problem has beento buy or hire a monochrome TV whichcan be used solely as a VDU. Until now,however, there has been no easy, safemethod of achieving improved displayresolution (most domestic TVs cannot besafely modified to accept video input), sothe appearence of a range of monitorsdesigned for home computer users isespecially welcome.

The most attractive of the new monitorsis the 12" Monochrome Data DisplayMonitor (above) from Chable ElectronicsLimited. It operates from either 12 V DCor 240 V AC, measures a compact 370 x290 x 30 mm and can be lifted with onehand; the display area is 24 x 80characters. However, the DDM's mostappealing feature, to the home computeruser, is it's price - £85.68 includingVAT and carriage. For further informationcontact Chable Electronics Limited, 3ACommercial Street, Batley, WestYorkshire WF1 7 5HJ or 'phone (0924)441128.The range of monitors from Thandar (mid-dle) are intended for professional users,although they are also suitable for thehome user. The range starts with theTV2S, a 2" model, and extends through5" , 9" to 12" types. They are supplied inchassis format, with a choice of B/W orgreen phosphor tubes and the option ofstandard or non -glare screens. Pricesrange from £119, including VAT, up to£155. Colour monitors, from 14" to26" , are also available on application.Thandar Electronics Limited are located atLondon Road, St. Ives, Huntingdon,Cambs PE17 4HJ; Tel. (0480) 64646.

Two new monitors (below right) fromStotron Ltd. are intermediate in price. The12" model costs £99.50 and the 9"one, £92.75; both prices include VATand carriage. The picture tubes are greenphosphor, showing 1920 characters in80 x 24 format and comply with DHEWregulations on X-radiation. Video input isvia either RCA phono jack or SO -239 UHFconnectors.

Another new product line from Stotronis a range of low -profile IC sockets(left), featurir,g beryllium copper four -jaw gold plated contacts and insulationresistance of 1014 ohms. Full details ofthese and other new lines from Stotronappear in their 1982 catalogue, which isavailable from Stotron Ltd, HaywoodHouse, Ivyhouse Lane, Hastings, EastSussex TN35 4PL; Tel. (0424) 442160.

6Hobby Electronics, May 1982

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The best of the test equipment releases,this month, is a new hand-held digitalmultimeter (above) made by SabtronicsInternational Inc., of Tampa Florida (theydon't just grow oranges there, you know)and marketed in the UK by Black Star Ltd.The Model 2033 features 0.5% basic DCaccuracy, a large 3 Y2 digit LCD, push-button range and function switches and ispowered by either a single PP3 battery oran optional AC adaptor. The 2033 willmeasure AC or DC voltages from 100 uVto 1000 V in five ranges, resistance from1 RO to 20M in five ranges and AC or DCcurrent from 10 uA to 2 Amps in threeranges. It is supplied complete with testleads for £42.26 including VAT; a highvoltage probe is available as an optionalaccessory. The Sabtronics 2033 isavailable from various distributors aroundthe country or directly from BlackStarLtd., 9A Crown Street, St, Ives, Hun-tingdon, Cambs PE17 4EB or 'phone(0480) 62440 for the location of ournearest dealer.

Readers of our new POPULAR COM-PUTING section who are also owners ofSharp, Casio or similar pocket computerswill be interested in a Manchester basedmail order firm who specialise in pocketand portable computer systems. They areElkan Electronics of 28, Bury New Road,Prestwich, Manchester M25 8LD. Theirnew catalogue, shortly to be released, willinclude the Sharp PC -1211 and PC -1 500and Casio's FX-702P, together with newsoftware and books.

Obtaining the mechanical parts for a pro-ject is often more difficult than actuallyconstructing the circuit, so new sourcesfor hardware are always welcome.Fieldtech Heathrow Limited aremarketing a new range of midget toggleswitches, both single and double -pole,with switching from basic ON/OFF tovariations of ON/OFF/ON through,various retailers, nationwide. The newrange, called Type 550, is also availabledirect from Fieldtech Heathrow at pricesranging from 32 pence to 51 pence each.Data sheets will be supplied to interestedreaders who write to Fieldtech HeathrowLimited, Components Division, HuntaviaHouse, 420 Bath Road, Longford, MiddxUB7 OLL; Tel. 101) 897 6446.

Still on the subject of hardware, OKMachine & Tool (UK) Ltd have announceda new addition to their PacTec caserange. The Series CLH (above) has a han-dle which doubles as a tilt -leg and ismoulded from heavy weight ABS plastic.The basic size is 318 x 296 mm, withheights ranging from 115 to 146 mm.The cases accept a variety of mountingsystems - PC card guides, mounting railsand other hardware - so allowing con-siderable flexibility for creating a numberof internal configurations. The Seriescomes in four standard colours (blue,black, tan and grey) and can be supplied inkit form. The CLH Series is available directfrom OK Machine &Tool (UK) Ltd, DuttonLane, Eastleigh, Hants S05 4Aa (Tel.0703 610944) or from Watford Elec-tronics.

Still on the subject of cases, Adsum Ltdhave a range of standard instrumentcases designed to suit a wide variety ofapplications. They are made from ABSplastic in standard colours (black or grey)and supplied blank, with internal fixingpoints. Two sizes are made, with externaldimensions of 170 x 250 mm or 310 x250 mm, and a range of heights. Themaximum PCB size is, in each case, 10mm less than each of the external dimen-sions. Details are available from AdsumLtd., Kiln Acre, Wickham Road, Fareham,Hants P016 7ZH, Tel. (0329) 285858.

Latest news from the leading edge oftechnology concerns the introduction ofstereo sound video machines. Leadingthe way will be Grundig, the West Ger-man manufacturers of almost everythingelectronic, with Philips and JVC not farbehind. This long awaited (and long over-due) innovation will be welcomed by all- but particularly those who have beendisappointed by the quality of sound onvideo cassettes of pop music or spec-tacular movies such as Flash Gordon,Superman or Star Trek. Already, pro-ducers of video -pop are preparing soundversions of popular tapes, as they willalmost certainly benefit most from the im-proved quality of stereo.

Grundig are also reported to be the firstcompany ready to introduce stereo TVreceivers to the UK. However, althoughstereo sound is broadcast in West Ger-many, it is unlikely to broadcast in the UKfor some time, yet. True stereo TV willalmost certainly come into general usewhen satellite television commences.Several European countries, notablyWest Germany and France, have theirsatellite TV systems well under waywhile, in the UK, a joint venture by threemajor companies, British Aerospace,British Telecom and Marconi Avionics,will put a UK satellite into orbit around1986.

There are two possible methods bywhich satellite TV can be received -directly, by a small dish antenna on theroof, or via cable from a central receivingstation. The Government appears tofavour the idea of cable distribution, so itis likely that cable and satellite TV will beclosely linked in this country. HE

Hobby Electronics, May 1982 7

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RESISTOR BARGAINS"Pa Net Qtr Doscsipties Nice5110 400 Mixed "All Type Resistors £15111 400 Preformed 14 15 watt Carbon

Resistors ElS112 200 ft watt Carbon Resistors ElS113 200 tv watt Carbon Resistors El5114 150 Yr watt Resistors 22 ohm

2m2 Mixed El5115 100 I and 2 watt Resistors 22

ohm -2m2 Mixed ElPaks St 12.15 contain a range of Carbon Film Resistorsof assorted values from 22 ohms to 2 2 meg Savepounds on these resistor paks and have a full range tocover your protects'Quantities approximate. count by weight

rTRIACS - PLASTIC4 AMP - 400. - 10202 - TAG I36GTOFF 10 OfF 50001400 £3.75 £17508 AMP 400. - TO220 - TAG 42560, £5.75 L27.50

100 OFF



213055 The best known Power Transistors in theWorld - 213055 NPN 115wOur BI-PAK Special Offer Price10 WI 50 off 191 oft£3.50 E16.00


Equivalent M12955 - 80312 - TO3SPECIM. POKE 13.70 sock /

111 WI 00.50 , v -a

MORE BARGAINS!5X51 60 metres PVC covered Hook-up

wire single and stranded Miredcolours LI

8t50 25 Assorted TEL Gates 7400Series 74017460 El

5159 10 Assorted flip flops and MSITIE El

5140 20 Assorted SliderPotentiometers El

5161 25 Assorted Potentiometers.Rotary. Dual. etc LI

5162 40 Assorted Pre -Sets Hor/Vertetc El

$179 10 Reed Switches - glass type3 Micro Switches - with lever El A


Or Description Price250 Capacitors Mixed Types LI200 Ceramic Capacitors Miniature

Mixed El100 Mixed Ceramics 1pf- 5601 El100 Mixed Ceramics 6Rpf- 0.5m I El100 Assorted Polyester/Polystyrene

Capacitors El5121 60 Mixed C280 type capacitors

metal toil El5122 100 Electrolytics, all sorts El5123 50 Quality Electrolyfics

50.1000ml El5124 20 Tantalum Beads. mixed El

L.:Quantitiesapproximate. count by weight

Pak No.




2 pieces of Audio Plugs Sockets and Connectors5 to include DIN 180°140" Inline 3-6 Pin.

Speakers, Phono lack. Stereo and Mono etc etc Valuedat well over E3 normal Order No 5025 Our Price El 50per pak Guaranteed to save you money

5326 3 Prs of 6 pin 240° DIN Plugs and Chassis


5I27 1 x Right Angle Stereo lack Plug 6 3mm plusmatching metal chassis mounting socket

50211 4 Phono plugs and 2 dual phono connectors

5129 1 x 2 5mm Plug to 3 5mm Socket adaptor

LSSX30 I x 3 5mm Plug to 2 5mm Socket adaptor

X31 1 x 3 5mm Plug to Phono Socket adaptor





TECASBOTYThe Electronic Components and Semiconductor Bargain of the Year A host of Electronic

components including potentiometers - rotary and slider. presets - horizontal and vertical

Resistors of mixed values 22ohms to 2M2 - 1.810 2 Watt A comprehensive range of

capacitors including electrolytic and polyester types plus disc ceramics etcetera

AodiO plugs and sockets of various types plus switches. fuses. healsinks wire. nuts bolts.gromets. cable clips and ryes. knobs and P C Board Then add to that 100 Semiconductors

to include transistors. diodes. SCR's opto's. all ol which are current everyday usable devices

In all a Fantastic Parcel No rubbish all identifiable and valued in current catalogues at well

over £2500 Our Fight Against Inflation Price --- Beat the Budget

- Down with Depression JUST £6.50.Olno 5%85



5163 5 x 470 ohms Lin 5X67 5 1 47k LinSX64 5 e Iklin SIBS 5 1 471i Log

5145 5 a 22k lin 5169 5 x 100k Lin- S166 5 x 22k Log 5170 5 f I meg Lin

AO et 500 POf NILS171 50 BC108 "Fallouts. Manufacturers out ol

spec on voltsor gain You test LIS172 A mixed bundle of Copper clad Board, Fibre

glass and paper Single and double sided Afantastic bargain El

5170 Genuine MULLARD 0E71 Germanium PNPhib Transistors. Brand New 10 for El


Valued at over EIO-Normal Retail -we offer you a pack of 25 Opto

devices to include LEDs Large and Small in Red. Green. Yellow andClear 7 Segment Displays both Common Cathode and Common Anode

PLUS bubble type displays -like DL 33 Photo Transistors -similar to

OCP7 I Photo Detectors -like MEL11 12 This whole pack of 25

devices will cost you rust

BOOAND we guarantee your money back if you are not

completely satisfied FULL data etc included

Order No. Sacs 7.

5 watt (ANSI Audio AmpHigh Quality audio amplifier Module Ideal for use inrecord players tape recorders, stereo amps andcassette players etc Full data and back up diagramswith each module

Specilical ion Power Output 5 watts RMS Load Impedance 8.16ohms frequency response 50Hz to 25 KHz-3db 0Sensitivity 70 my for lull output Input Impedance50k ohms Sire 85 r 64 r 30mm Total Harmonic

distortion less lhan.5%131 PAK'S give away price

You could not Build onefor this mice.

1 Amp SILICON RECTIFIERSGlass Type similar 1144000 SERIES N4001444001

50 - 500v !Pealed - you select for VLTS

ALL perfect devices - NO duds Mm 50v50 tnr £1.00 -- worth double ORDER NO SX76

Silicon General Purpose NPN Transitors TO -18 Case

Lock lit leads - coded CV7644 Similar to BC147- BCI07- I189 ALL NEW, VCE 70v IC500mA '-',

Hte 75-250 50 off 100 off 500 off MOO on

PRICE £2.00 £3.10 £17.50 £30.00g,Silicon General Purpose PNP Transistors 1O-5 Case

Lock fit Rods coded CV9507 similar 2N2905A to

BFX30 VC 60 IC 600mA Min Ale 50 ALL NEW,

50 on 100 off 500 oft 1000 off

PRICE £2.50 £4.00 £19.00 £35.00Order as CV9507 A


BARGAINS20 wall 125 Red LEDs10 Rectangular Green LED s 2

30 Assorted Zeikel Diodes250mw-2 wall mired voltagesall coded New

5112 4 Black InstrumentKnobs -winged with pointer 1/4"Standard screw Fit sire 29


Ste, 20 Assorted Slider KnobsBlack/Chrome etc





UM 12 Neon: and Filament Lamps Lowvoltage and mares - various typesand colours - some panel mounting El

r_Gel a LI FREE PACK Orders over 110 excluding VAT

Choose LI Pack free for 2 X 500 add It to row order and

save even more money

This alter only applies to Ms advertisement

Satislaction or your money back has

always been BI-PAK s GUARANTEE and it still isAll these Sate items are in stock, in quantity andwe will despatch the same day as your

order is received

rSilicon NPN'L' TypeTrantikcTilsTO -92 Plastic centre collector

Like BC182L 183L - 184LVCBO 45 VCEO 30 IC200mA Hte 100-400

ALL perfect devices --- uncoded ORDER AS SX183L

50 off 100 off 500 off 1000 off

£1.50 £2.50 £10.00 £17.00

PNP SILICON TRANSISTORS:Similar ZTX500 - Z10214 - F -LineVCEO 40 VCBO 35 Ic 300mA Hie 50-400

Brand New - Uncoded Petect Devices

50 off 100 off 500 off 1000 off

f2.00 £3.50 £15.00 £25.00Order as ZTXPNP 41r-4 TheThird and

i Fourth Hand...


.... you always need.7 ..," but have never got until nowi.,414 This helpful unit with Rod mounted

horizontally on Heavy Base Crocodile clipsAattached to rod ends Six ball 8 socket Jointsgive infinite variation and positions through360° also available attached to Rod a 2'4 diammagnifier giving 2 Sr magniticatiOn Helpinghand unit available with or without magnifierOur Price with magnifier as illustrated ORDERNO 1402 £5.50Without magnifier ORDER NO 7400 £4.75

mw398 NI -CAD CHARGERUniversal NI -Cad battery charger All plasticcase with lift up lid Charge/Test switch LEDindicators at each of the five charging points

Charges - Power -

PP3 (9VI 220-240v AC012 II 5V peptide Dims -U1111 5V --UT 210 .100 x 50mrnU21 I 5V OH £0.95

POWER SUPPLY OUR PRICE £3.25Power supply his directly into 13 amp socketFused for safety Polarity reversing socketVoltage switch Lead with muiti plugInput - 240V AC 50HZ Output - 3 4 57 5 9 8 12V DC Rating -300 ma MW8L3 A

BI-PAK 'a COMPLETELY NEW CATALOGUECompletely re -designed full of the type of components you require plus some

very interesting ones you will soon be using and ol course the largest range ol

semiconductors for the Amateur and Professional you could hope to find

There are no wasted pages of useless information so often included in

Catalogues published nowadays lust solid facts i e price description and

individual features of what we have available But remember. Bi Pak s policy

has always been to sell quality components at competitive prices and THAT


01 PAK S COMPLETELY NEW CATALOGUE is now available to you You will be

amazed how much you can save when you shop for Electronic Components with

a Eli Pak Catalogue Have one by you all the time -it pays to buy 01-PAK

To receive your copy send 75p plus 25p p&p

BI -PAKSend your orders to Dept HE5 BI PAK PO 00X 6 WARE HERTS.





8 BIt MICROPROCESSORNational INS8080AN 40 Pin OIL N Channel SiliconGATE MOS TECHNOLOGY As used in NationalsN8080 Micro Computer FamilyInstruction Cycle Time 2 uSSupplied with functionalBlock Diagram

BRAND NEW --NOT seconds or reclaims

100% perfect ORDER NO SX8080 onlyNormal Sell price (450 each nn


40 Pin IC Socket lo lit SX80130 Offer price

ORDER NO 1609 30p

Use your credit card. Ring us on Ware 3182 NOW and

get your order even faster. Goods normally sent 2nd

Class Mail.

Remember you must add VAT at 15% to your order

Total Postage add 75p per Total order

8 Hobby Electronics, May 198

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TRaP Tel: 0206 36412Hill Farm Industrial EstateBoxted, Colchester'Electronics Essex C04 5RD


Polyester Radial Leads 250V 280 type0.01, 0.015, (1022, (1031,6p; 0 047, 0 %8, 0 1, 7p;0 15, (1 22, 9p; 0 13,0 47, 13p; 0 68, 20p; 1u 23p.

Electrolytic Radial or Axial leads(147/61V, 1/63V, 2 2/63V. 4.7/63V, 10/25V, 7p;22/25V, 47/25V, Bp; 100/25V, 9p; 220/25V, 14p;470/25V, 22p; 1000/25V, 308.

Polyester. Siemens PCBIn, 2n2, 3n 3, 4n7, 6n8, 10n, 15n, 7p; 22n, 33n, 47n, 68n, 8p;1(I0n, 9p; 150n, 11p; 220n, 13p; 330n, 20p; 470n, 26p;680n, 29p; in, 33p; 2u2, 50p.

Tantalum head(1 1, 0 22, () )3,1)47, 100 1SV 12p; 22, 4.7, 10®25V, 20p; 15/16V 30p; 22/16V, 27p; 33/16V, 45p;47/6V, 27p; 47/16V, 70p; 68/6V, 40p; 100/10V, 90p.

Ceramic 22p -0-01u SOV 3p each.*Mullard Miniature ceramic plate*1 8pr to 1000F 6p each.Polystyrene 5% tolerance100-10(13p 6p. 1500-4700p 8p. 6800-0 01210p.Trimmers. Mullard 808 Series2-10pr 22p. 2-22pr 30p. 5 5.65pr 35p.


20 metre pack single coreconnecting cable, tendifferent colours 65pSpeaker cable 10p, mStandard screened 16p, mTwin screened 24p/m2 SA 3 core mains 23p/m111 way ribbon 65p, m211 way ribbon 120p/m

OPTO SPECIAL!Special pack containing thirty 5mm LEDs.Ten red, ten green, ten yellow.Our normal price is 320pSpecial Offer price just 250p!Offer expires 30th May 82Please note these are top quality grade 1devices

COMPONENT KITSAn ideal opportunity for the beginner or the experiencedconstructor to obtain a wide range of components atreduced prices

%W 5% Resistor Kit Contains 10 of each value from4.752 to 1M (650 resistors) 440P.

Ceramic Capacitor Kit Contains 5 of each value from22p to° 01 (135 caps 1370p.

Polyester Capacitor Kit. Contains 5 of each value from0.01 to luF (65 caps 1575p each.

Preset Kit Contains 5 of each hor Value from1008 to 1M (total 65 presets) 425p each.Nut and Bolt Kit. Total 300 items 140p.25 6BA % bolts i 25 4BA 1/4- bolts25 6BA 1/2" bolts 25 4BA 1/2" bolts50 6BA nuts 50 6BA nuts50 6BA washers 50 6BA washers


Rotary. Carbon track Logor Lin 1K -2M2Single 32p. Stereo 85pSingle switched 80pSlide 60rnin travel singleLog or Lin 5K -500K 63p.Preset. Submin boric.1110/0-1M 7p each.


lA 50V1A 400V2A 200V2A 400V6A 100V6A 400VVM18 OIL0.9A 200V 50

22 18 OM35 18 412M40 19 968M45 18 6667M801 48 om95 libm



HARDWAREPP3 Battery clips 6pRed or black crocodile clips 6pBlack pointer control knob 159Pair Ultrasonics 350p*6V Electronic buzzer 60p* 12V Electronic buzzer 65p* P02720 Piezo transducer 75prO18 transistor socket 15p64mm M ohm speaker 70p64mm 8 ohm speaker 70p

Alfac PCB transfer sheets - pleasestate type (e g . DIL pads etc ) 45pDalo etch resist pen 100pFibre glass board 3.75' x 8' 75pFerry Chloride 250(01 bottle 100p


BC 158 10 BCY 70AC125 35 I *BC159 8 BCY71AC126 25 BC160 45 BCY72AC127 25 BC168C 10 00115*AC128 201 BC169C 10 BD131AC176 251 BC170 8 BD132AC187 22 BC171 10 BD133AC188 22 8C172 8 BD135AD142 120 )3C177 18 BD136AD149 80 BC178 18 BD137AD161 40 BC179 18 BD138AD162 40 BC182 10 BD139AF124 60 *BC182L 8 BD140Ar126 50 0C183 10 80204AF119 40 13C1831 10 50206AF186 70 BC184 10 130222

' AF239 75 *BC1841 7 BF 180BC107 10 BC212 10 BF182BC10713 10

80:1088 12! BC213 10 Brit%

BC2121 10 B1184

,,........_..6. BC2131 10 BF194 12 MIE521 95.°--1.3,- 12 BC214 10 BF195 12 MIE 3055 70*BC109 91*5C214L 8 511% 12 MPF102 40BC109C 12 ,BC237 8 F3F197 12 MPF1O4 40BC114 22 BC238 14 BF198 10 MPSA05 22BC115 22 BC308 15 BF 199 18 MPSA06 25BC117 22 80327 14 1%200 30 MPSA12 30BC119 35 BC328 14 * BE 24413 T1 MPSA55 30BC137 40 8C337 14 BF245 30 MPSA56 30BC139 40 8C338 14 BF2568 45 MPSUOS 55BC140 30 BC477 30 BF257 32 MPSUO6 55BC141 30 BC478 30 BF258 25 MPS1/55 60BC142 25 50479 30 BF259 35 MPSU% 60BC143 25 BC517 40 BF337 40 TIP29 35BC147 8 *80547 7 ,BFR40 23 TIP29A 40)3C148 8 13(548 10 BFF280 25 TIP298 55130149 9 BC549 1010FX29 25 , TIP29C 60

10 BF X84 25 TIP30 1518 BFX85 25 TIP30A 1018 BF X86 28 TIP30B 5018 Br X87 25 TIP30C 6080 Br )(88 25 TIP31A 4535 BPI% 23 TIP31B 4535 BP(51 231 TIP31C 5550 BFY52 23 TIP32A OS

50 BF Y53 32 TIP32B 5530 8FY55 32 TIP32C 6030 IVY% 32 TIP33A 5030 BRY39 40 TIP33C 7535 55020 20 TIP34A 6035 BSX29 35 TIP34C 85

110 B5Y95A 25 TIP35A 160110 550205 160 TIP35C 18085 f3U206 200 TIP36A 17035 13U208 170 TIP36C 19535 MI2955 99 TIP41A 6025 MIL 340 50 TIP42A 6025 MIL520 65 TIP120 90

TIP121 90TIP122 90TIP141 120TIP142 120TIP147 120TIP2955 60T IP3055 55TIS43 40TIS44 4511545 45TIS90 3071591 30*VN1OKM

45VN46AF 75VN66AF 85VN88AF



*ZTX107 812143054 5519,*ZTX108 812143055 50ZTX109 12 2N3442 1201

ZTX300 14 *2N3702 6'ZTX301 16 2143703 9

ZTX302 15 *2143704 6'ZTX304 17 2143705 9ZTX341 30 2143706 9ZTX500 15 2N3707 10ZT X501 15 2N3708 1021)(502 15 2143709 10ZT X503 18 2143772 19027X504 25 2143773 2102N1,97 20 *2143819182N598 40 2143820 402N706A 20 2143823 652N708 20 2N3866 902N918 35 2143903 102N1112 22 2N3904 102511,14 30 *2143905 6252218A 45 2193906 102N2219A 25 2144037 452N2221A 25 2194058 102N2222A 20 2144060 102142368 25 2144061 10

2142369 16 2N4062 10

2N2484 25 2145457 36

2N2646 45 2145458 36

2N2904 20 2145459 302N2904A 20 2145485 36

2N2905 22 285777 45

2N2905A 22 2146027 30

2142906 25 40360 402N2906A 25 40361 502142907 25 40362 502N2907A 25 404082142926 9 40594 1002143053 23


ORDERING INFOAll prices exclude VAT. Please add to total order.Please add 50p carriage on all orders under E10 invalue. Send cheque/PO or Access/Visa numberwith your order. Please note our new address.Callers most welcome - we are just 10 minutesfrom the centre of Colchester. Telephone orderswelcome with Access and Visa. Official orderswelcome from colleges and schools etc. Exportorders no VAT but please add carriage.

EIDEIIES2111SCRsT1C45C1(161)41X1V 8A400V I2A


TRIACs400V 4A400V 8A413111V 16A




likaasjiMI* 3mm red* 3mm green 12

3mm yellow 12

(-lips to suit 1p each

Rectangular*red 12green 17

yellow 17

11138 40

Seven Segment Displays

Com. cathode01704 0.3- 95*FND500 0.5" 80TIL313 03" 105TIL322 0.5" 115

All seven segment displays aresupplied with connection details.2N5777 45 Dual Colour LED 60

10(1KHz 290200KHz 3701MHz 3001.(108M 37018432M 3002 Otvl 2702 4576M 2201.276M 2401.579M 1204 OM 1504.194M 1504 43)4 I 125

5 0M 1 24060M6 144M7 wan80M10 OM12 OM16.0M


18.0M 24018.432M 22019.968M 30038.6667M 32048 OM 220116M 300


*5Frn red*Sinin green* 5mm yellow





Com anodeDI.707 0 3- 95I ND507 0 5" 90111112 0.3" 105111.121 0 5- 115

Just phone uson 0206 36412

with your order!


*8 pin 7p*14 pin 9p*16 pin 10p18 pin 15p20 pin 189

22 pin 200.24 pin 22p28 pin 26p40 pin 32p

Soldercon pins



UNEAR CA3189E*709 25 ICL7106* 741 14 ICL8038748 35 ICM7555AY -3-1270 MO *LF351AY -3-8910 700 LF353 85

*AY -3-8912 11356 90625 LM10 395 *LM709

CA3046 60 , *LM301A 25 110710CA3080 651 LM311 70 LM725CA3089 215 LM318CA3090AQ 375 * LM324CA3130E 90 LM339CA3140E 45 LM348CA3160E 100 LM3513CA3161E 140 LM377

* Verobloc 350p.Size 0.1 matrix2 5 x 1"2 5 x 3 75'2 5 x 5"3 75 x 5'

VQ Board

Veropins per RIOSingle sidedDouble sidedSpot face cutter





CMOS4000 14*4001 124002 144006 6S4007 174008 584009 304010 35*4011 134012 17*am s 264014 604015 60*4016 26

DIL PLUGS14 pin 45p.16 pin 50p24 pm 90p


Positive*781_05 257811278115* 7805* 78127815





ODESNegative BY 127 12 154001 3

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1)4723 40 *1N4148 2 400mW pen 6PLM338K 475 -

78H05 *EIZX61 Series zeners5A 51/ 550 *1 IW 4V7 -39V 15p each.

Dimensions in inches Aluminium

x 2 x 1 70px 1 x 11/2 850

40.102 100p

6 x 4 x 1

86 x 4 x 1

x 6 x 2Plastic Project BoxesComplete with lid and screws3 x 2 x in S5941/2 x 3 x 1'/in. 88p7 x 4 x tin. 160p



Submin ToggleSPST 55p SPDT 60p *()PDT 50pMiniature toggleSPOT 80p SPOT centre off 90pDPDT 90p DPDT centre off 1009Standard toggleSPST 35p DPDT 48p* Miniature DPDT slide 12p* Push to make 12p Push to break 22pRotary type adjustable stop1P12W 2P6W, 3P4W, 4P3W all 55p eachDIL switches4 SPOT 800 6 SPST 60p 8 SPST 100P





Antex CX 17W Soldering Iron2 3mrn and 4 7mm bits to suitCX 17W elementAntex X25 25W Soldering Iron3 3mm and 4 7mm bits to sunX25 25W elementSolder pump desoldering toolSpare nozzle for above10 metres 22 swg solder


DIN Plug Ski2 pin 99 9P3 pin 12p 10p5 pin 13p tipPhono 10p 12p





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120. *LM3915 220 SN76018 150

100 LM13600 120 *SN76477 250MC1310 150

25 MC3302 15050 MC3340 135 TBA820

350 NE515 270 TBA95075 NE529 225 TCA94014 NE531 150 TDA100475 NE544 185 TDA100835 *NE555 16 TDA101040 *NE556 45 TDA1022 560

200 NE565 120 TAD1024 125

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80 ZN424290 ZN42SE170 ZN426E380 ZN427E320 ZN428E225 ZN1034E



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4502 70 4529 1504501 50 4532 954507 38 4534 4954508 200 4538 1104510 65 4541 110*4511 50 4549 3804512 70 4553 2954514 180 4555 454515 180 4556 484516 75 4559 390*4518 45 4541) 1804520 70 4584 454521 200 4585 994526 80 4724 1404527 90*4528 75

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LS161 42 LS221 60 L5365 38L5162 42 L5240 90 L5366 3815163 42 LS241 80 LS367 38L5164 50 LS242 80 LS368 50L5165 120 LS243 85 LS373 80LS166 85 15244 80 LS374 80LS170 170 L5245 120 L5375 50LS173 70 15247 75 15377 901.5174 60 15251 40 L5378 7015175 60 L5257 48 LS390 75

190 55 L5258 45 15393 75LLSS191

55 15259 95 LS399 220L5192 55 L5266 25 15541 135L5193 60 LS273 90 LS670 175LS195 50 L5279 50

1_51% 60 15283 4515197 15353 100

lack2 5mm3 5mmStandardStereo

1mm 139 13p

UHF (CB) ConnectorsPL259 Plug 40p Reducer 14p50239 Square chassis socket 38p502395 Round chassis socket use

IEC 3 pin 250V/6APlug chassis mounting 38pSocket free hanging flop

Socket with 1m lead 120p






The Rapid Guarantee Top:

TRANSFORMERSPlease add carriagecharges to ourormal post charges

Miniature mains606V, 909V, 12012V all a 100mA 100p each

High quality, Split bobbin constructionOVA 0-6,0-6 a 0 5A, 0-9, 0-9V a 0.4A

0-12. 0.12V a 0 3A 220o each.12VA 0-6, 0F6 IS 1A, 0-9. 0-9V a 1.2A,

0-12, 0-12 0 0.5A, 0-15, 0-15V 0 0.4A275p each (Plus 40p carriage)

24VA 0-6 0-6V a 1.5A; 0-9, 0-9V 1.2A;0-12, 0-12V 1A, 0-15, 0-15V a 0.8A,330p each (plus 60p carriage)

50VA 0-12, 0-12V 2A, 0-15, 0-15V a 1.5A;400p each (plus 70p carriage)

100VA 0-30, 0-30V a 1 6A920p each (plus 80p carriage)


1/4W 5% Carbon film E'2series 4.752 .10M. 1p each1/2W 5% Carbon film El2series 4.711-4M7 2p eachV.W 1% Metal film. E24series 108 -1M. 6p each.

1:1211=1211rSire 60 x 46 x 35mrn0 -SODA 0-500mA13-100" 0-1A0-5008A0-1rnA 0-50V AC


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Hobby Electronics, May 19E 9

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DigitalThermometerA neat, compactproject for indoor oroutdoor use.

THIS PROJECT was designed as abattery -powered, digital thermometergiving a readout on a seven -segmentLED display. Its neat appearance makesit equally at home in the study or theliving -room, or it can be used for remotetemperature sensing.

The sensor is an IC which generatesa current proportional to thetemperature of its surroundings. Oneuseful feature of this IC is that thecurrent is completely unaffected by thelength of the wire connecting the sensorto the rest of the circuit. Changes inresistance of the connecting wire haveno effect either. The project,therefore,is suitable for remote sensingof temperature. The sensor could easilybe situated in the greenhouse, forexample, allowing you to monitor thetemperature from indoors and warningof the risk of overheating in summer, orof frost in winter.

Although weather forecasts,nowadays, are given in degrees Celsius(some people wrongly persist in calling itCentigrade - why not pay our respectsto Celsius, the man who invented thisscale?), many people still think in termsof degrees Fahrenheit, so this circuit canbe built to measure temperature in eitherscale.

The CircuitIC1 is a constant -current generator, thecurrent being determined by the value ofR1 and by the ambient temperature.The current flowing to R2 is given by:

1 = 227 x T/R,

where I is in microamperes, R is thevalue of R1 in ohms, and T is theabsolute temperature, in Kelvin.Absolute Zero, on the Kelvin scale isapproximately equal to -273°C or-459°F. A room temperature of 20°Cis, therefore, 273 + 20 = 293 K. Thusat room temperature, I = 227 x293/220 = 302 uA. As this currentflows through R2 it generates apotential of 3V02 across it. Note that ifIC1 is to be used as a remote sensor, R1must be mounted as close to it aspossible; the leads to V+ and V- may beas long as necessary. Current flowing

from V- is constant and all of it flows to -R2. Hence the length of the lead and itsresistance do not affect the voltageacross R2.

IC2 is a CMOS phase -locked -loop IC,which is used because of the VCO itcontains (among other functions). Witha fixed capacitance across pins 6 and 7,the rate of oscillation at any givenvoltage is determined by the resistanceat pin 11. The resistor at pin 12determines the minimum (or offset)frequency. With no resistor at pin 12,the rate is 0 Hz at 0 V. We chooseminimum (offset) and maximumfrequencies which correspond to thecorrect temperatures when the'hundreds' digit is omitted. The scalemaximum is, in practice, only 799(99°F) for Fahrenheit.

Timing for the system is under thecontrol of IC3, a CMOS timer wired asan astable, operating at 0.5Hz. The dualmonostable (1C4) is wired to generate ashort pulse at the beginning and end ofeach pulse from IC3. The count is'enabled' as the output of 1C3 goes highand the counter is reset to zero, by thepulse from IC4a. When the high pulsefrom IC3 ends, the counter is disabled;simultaneously, the low -going pulsefrom IC4b to the drivers (ICs 6 and 7)stores the resulting count, which is thendecoded and displayed.

IC5 contains two decade counters;the units counter (IC5b) is enabledwhenever input 'EN' (pin 2) is taken

low. Its '8' output (pin 6) goes lowwhenever the count changes from 9back to 0; this low -going edge triggersthe other counter (IC5a) which isincremented and counts 'tens'. Enablingor disabling IC5 automatically has thesame effect on IC5a. Note that the inputto IC5b is the clock input (pin 1), whichtriggers a count on a positive -goingedge; the input to IC5a is at its 'enable'input (pin 10) which is negative -triggered; its clock input (pin 9) is wiredto 0 V to hold it permanently low.

To ensure that all the digits areequally bright, there should be a resistorin each lead between ICs 6 and 7 andthe displays. We have economised byusing only a single resistor (R9) in thecommon cathode line. When thedisplayed value alters, there is a smallchange in brightness because thenumber of illuminated segments haschanged. In practice this is not worthbothering about, since the display doesnot change often.

The regulator circuit is fairlyconventional. A constant voltage of2V7 is maintained at the junction of Q2and D1; with the forward voltage dropacross the base -emitter junction of Q2,the voltage at R12/R13 must be 2.7 +0.6 = 3V3. Since R12/R13 is apotential divider across the output, thecircuit is only stable when the outputvoltage is 4V5, giving 3V3 at the baseof Q2. As the supply voltage or loadvaries, the negative feedback through

10Hobby Electronics, May 1982

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NOTE01,02 ARE ZTX300ZD1 S BZY88C2V7C1 S 3342C2 S 4046C3 S 7555C4 S 4098C5 S 4518C6,7 ARE 4511





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Figure 1. The complete DigitalThermometer circuit. Although it usesquite a few chips, all except thedisplays are mounted on one board.The sensor IC may be taken off -boardfor remote sensing of temperature.

Figure 2. Cut -away view of internal 0.layout, showing the PCB mountedupside down.

Q2 (via R12, 13) will either turn Q1 onharder or turn it off slightly, to maintainan output of 4V5. This circuit givesgood regulation over the voltage rangeto be expected from batteries, as theygradually discharge.

ConstructionThe thermometer circuit is built on asingle PCB mounted upside down in thetop section of the case. The case weused has a built-in battery compartment,holding the four HP7 cells which providethe power supply of 6 V. The circuit willwork directly from the batteries, but it isaffected by falling voltage, as the cellsage, as mentioned For this reason thesimple voltage regulator is included,though it is not essential if you arewilling to renew the cells fairlyfrequently. The regulator is mounted atthe rear of the PCB, away from thesensor. It supplies only the sensor andoscillator, the remainder of the circuitbeing supplied directly from the battery.

The switch is a push-on/push-offtype, mounted on the bottom of thecase, near the front; It also acts as atilting leg, giving a better view of thedisplay. To switch the thermometer onor off, simply place your hand on top ofthe case and press down!Hobby Electronics, May 1982





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Digital Thermometer

Parts ListRESISTORS(All 1/4 W, 5%)R1 220RR2 10k(R* 3k9R4* 120kR5 1kR6 2M7R7 1kR8 1kR9 100RR10 10kR11 330RR12 1kR13 3k3

See Text

POTENTIOMETERS(All sub -miniature horizontal presets)PR1 10kPR2 47kPR3 1M

CAPACITORS(All polyester, except whereindicated)C1 * 47n polycarbonateC2 470n polycarbonateC3 100n polycarbonateC4 330p polystyreneC5 330p polystyreneC6 100n polycarbonate


constant current generatorIC2 . . . . 4046B phase -locked loopIC3 7555 CMOS timerIC4 4098Bor 4528B

dual monostableIC5 4518B dual BCD counterIC6,7 4511B

BCD -to -7 -segment latch/decoder/driver

01, 2 . . . . ZXT300 NPN transistorZD1 Zener diode, 2V7

MISCELLANEOUSDISP1, DISP2 . . . . 7 -segment LED

display (2 off)SW1 . Push-to-make/push-to-break

switch, SPST

Battery instrument case (Vero type2) 2.5 mm matrix stripboard,127 mm x 63 mm main board,63 mm x 25 mm for regulator,55 mm x 28 mm 120 strips x 11holes) for displays; 1.0 mm terminalpins, (7 off); Small angle bracket,4BA bolt and nut set (2 off); Self -tapping screws for 2 mm holes (4off).

BUYLINE page 68

Figure 3. The component layout. Becareful to make the correctconnections between the displayboard (right) and the main board.

1 IV)


It is more convenient if you leavemounting the board and wiring theswitch until the thermometer board iscomplete and working. We stronglyadvise that sockets are used for the ICs,as this allows them to be removed, ifnecessary, when attempting to locatefaults. Soldercon pins were used tomount ICI and proved perfectlysatisfactory. Since the circuit usesCMOS ICs, take the usual precautionsto avoid static charges and wire up allthe connections to each IC beforeinserting it.

Begin by building the regulator board;this is very simple so there should be noproblems. When the regulator isconnected to the 6 V supply, its outputvoltage should be about 4V5. Continuewith ICI and its resistors (RI , R2); usea heat sink when soldering this IC to theboard. Test this section by switching onthe power and measuring the voltageacross R2. This should be about 3 V,whether you are using the 4V5 supplyor the direct 6 V supply. Hold the ICbetween your finger-tips for a minute ortwo (or hold a warm soldering iron nearit, but don't actually touch it); thevoltage should rise. When the heatsource is removed, the voltage shouldfall back to its original level.

Next, wire up IC3 (the CMOS timer),together with R5, R6, RV3, C2 and C3.To test this section you will need to takeout IC1. The output of IC3 (pin 3)should rise and fall at approximately0.5 Hz (2 seconds between successivepulses). Run it for one minute, countingthe pulses. Use a 'scope', if you haveone; if not, connect a high impedanceearphone or audio probe between pin 3and 0 V; you should hear a 'click' eachtime the edge of a pulse rises or falls.Adjust RV5 for 30 pulses per minute(60 clicks per minute); the pulses shouldhave a 50% mark -space ratio ie, 1 Sechigh, 1 Sec low.

Now you are ready to fit IC2, thevoltage -controlled oscillptor (VCO). Atthis stage, you need to decide whether

the thermometer is to read degreesFahrenheit or Celsius, and to choose theappropriate values for Cl, R3 and R4.Those given in the Parts List are fordegrees Fahrenheit; for Celsius, changethese to 100n, 10k and 56k,respectively.

Next, mount PR1 and PR2 and, withIC1 still out of the circuit and IC3removed, look at pin 4 of IC2. A 'scopeis preferable but, if you do not have one,the frequencies can be adjusted bymatching them against audio tones froman accurately calibrated audio oscillatoror, if you have a musical ear, from apiano or organ, using a high impedanceearphone or probe, as before.

The 'offset' frequency of IC2, pin 4should be 241 Hz (about B belowMiddle C) for the Fahrenheit scale or127 Hz (C below Middle C) for readingCelsius; adjust PR2 for the correctfrequency (PR1 is not adjusted at thisstage). If you find it impossible to obtainthe frequency, replace R4 with a higheror lower value, to reduce or increase thefrequency accordingly.

Before going on to wire up thecounting and display circuits we mustput together the pulse generators ofIC4. The output of the leading -edgepulse generator (pin 6) is normally low(0 V), with a very brief (250 nS) high-going pulse as the output from IC3goes low. If you have doubts that IC4 isworking, connect a 100n capacitor inparallel with C4 or C5, to stretch thepulses.

Now, replace all the ICs in theirsockets and mount ICs 5, 6 and 7. Thetwo LED digits are mounted on a small,separate PCB, plugged in to Solderconpins. Run wires from the display boardto the pins beside ICs 6 and 7, beingcareful to follow the wiring diagram (agoes to a, b' goes to b' and so on).Remember that the main board will bemounted upside down in the top of thecase; the displays must be mountedwith their upper edges nearest the mainboard. Securing the display board is

12 Hobby Electronics, May 1982

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Digital Thermometer

The sensor produces a voltage which isproportional to its temperature and this isfed to a voltage controlled oscillator; thefrequency of the VCO is therefore, pro-portional to temperature. When switchedon, the counter is reset to zero, thenallowed to count for a fixed length of time(approximately 1 second). At 0°C itcounts to 400, and at 40°C it counts to440 - or, if the device has been built tooperate on the Fahrenheit scale, it countsto 732 at 32°F and to 804 at 104°F.The counter has only two stages, for unitsand tens; hundreds are ignored, so thecounter actually shows '00' at 0°C and'40' at 40 °C (or '32' at 32°F and '04' at104°F). The astable multivibrator is aslow oscillator which controls the timingof events in the system. It turns thecounter on (enables it) for one second inevery two. It is also connected to twopulse generators. One gives a rising pulse(high) to reset the counter, the other givesa falling pulse (low) to store the result, ascounting finishes.






How It Works











0 VOLTAGE 2.82 3.23--273 TEMPERATUREI°C) 0 40

0 TEMPERATURE(K) 273 313
















easy if you use the bolts on the LEDbezel; if your budget can't afford these,you will have to mount the display boardto the main board using small L-shapedangle brackets.

Assuming that all connections havebeen correctly made, and the ICsplugged in the display should show '00'when first switched on. About onesecond later it should change to anumber between '00' and '99'. It willthen remain at that number, or changeby one or two as the temperaturechanges. To begin with, you willprobably obtain a nonsensical reading,but don't panic - here are some thingswhich may be wrong:1) Both displays blank - check the

power supply and the displayconnections.

2) Some segments never light -faulty wiring between the display andIC6 or IC7.

3) Peculiar symbols - wires betweenIC6 and 7 and the display are crossedor shorted; you may have wired in the

decimal point instead of one of thesegments.

4) Certain figures never displayed buta blank occasionally appears on one(possibly both) displays - wirescrossed between IC5 and ICs 6-7.

When the display is workingproperly, all that remains is to set thedevice to indicate the correcttemperature. At average roomtemperature, the frequency from IC2should be about 768 Hz (G in theoctave above Middle C) for theFahrenheit scale or 420 Hz (Ab aboveMiddle C) for Celsius. Adjust PR1 toobtain this, but on no account re -adjustPR2. It is possible, though unlikely, thatyou may need to replace R3 at thisstage, if the correct frequency cannotbe obtained. The display should now bewithin range of the correct temperature,but could be as much as 30 or 40degrees out, in either direction. Nowadjust RV1 to bring the display readingto its correct value, using an ordinaryroom thermometer as reference. Both

thermometers will take several minutesto acquire the temperature of theirsurroundings - do not attempt thiscalibration until both have been atsteady temperature for at least 10minutes.

Before mounting the boards,drill thecase to take the switch. Also drill plentyof ventilation holes to allow air tocirculate to the sensor or, for greateraccuracy and rapid response, drill a holelarge enough to accept the package ofthe sensor IC and attach someleads so that it protrudes slightly out ofthe hole, into the air outside.

Secure the main board in the top halfof the case, using self -tapping screws or'Sticky Fixers'. Cut a rectangularopening in the front panel, level with thedisplays and slip the panel into its slots.If you're using the display bezel, drilltwo holes to take the mounting bolts.Before closing the case, read the displayto check that PR1 and PR2 have notaccidentally been altered. The HE DigitalThermometer is then complete! HE

Hobby Electronics, May 1982 13

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MATTEL OWNERS CLUB - Why not join our Mattel Owners Club and recive ourregular newsletters containing details of all the latest cartridge releases. Telephone uswith your name and addresses and we will add your name to our computer mailing listFREE 16 PAGE CARTRIDGE CATALOGUE - If you are interested in owning aMattel, we now have available a 16 page catalogue describing the latest six cartridgesto be released, as well as a new Mattel colour leaflet with brief descriptions of all 25cartridges. Telephone us for further details.


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SPACE INVADERSBased on one of the most popular arcade games. Space Invader'. packs in tots ofaction with multrshaped. brilliant LED vessels, including attack ships, bombs.defenders. missile rockets and a beam force cannon, plus exciting electronic soundeffects The attack ships attempt to bomb the ground defenders as they are movingYou manoeuvre the missile rockets - avoiding alien bombs -- and destroy theinvader force as quickly as you can Progressive degree of difficulty Includesautomatic digital scoring For ages 6 to adult

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Clubbing TogetherDear Sir,I became interested in electronics a fewyears ago, just before your first issuecame out, so I was pleased to see itappear on the shelves. Since then / havecome a long way, with your help. I haveevery copy from Issue one on mybookshelf, although I must admit thatsome of the earlier ones are becoming abit tatty, now. I also attended nightschool for two years and, between nightschool and your magazine, I have beenable to build a lot of my own testequipment.

I sometimes pick out a couple of oldissues and read through them. InSeptember 1980 you printed a letterfrom a Mr. George Edwards, who askedabout clubs. You replied that you wouldmention any clubs that wrote to youbut, as you have not printed any (apartfrom the one mentioned in the reply), Itake it that either no other clubs exist orthat they are very secretive about theirexistence.

There must be thousands of peoplewho're interested in electronics andwho are pursuing the subject on theirown, like myself. I would be veryinterested in contacting anyone in myarea so that we can share ideas and helpone another. Perhaps people interestedin forming a club could write to you.Then others in the area, whether theylive in London or Edinburgh, would beable to contact them. Electronics is afast-growing industry, but the hobbyistseems to be left out, to fend for himself.

I think the potential for this is endless- who knows what might be developedthrough the sharing of equipment andideas? Perhaps you could spare a pageof HE to be used for Club News andsimilar ideas?

Just one other thing - whathappened to 'Short Circuits'? I used toenjoy experimenting with them.R. Mitchell,Portslade,Sussex.

Mr. Mitchell is apparently correct in hisassumption that no other electronicsclubs exist - certainly, none havecontacted us. The club mentioned in theSeptember 1980 issue is still goingstrong, however. They are the BritishAmateur Electronics Club and interestedreaders should contact the Chairman,Mr. Cyril Bogod, at the BAEC,'Dickens', 26 Forrest Road, Penarth,South Glamorgan. Tel: 0222 707813.The BAEC is a national organisation andthey should be able to put you in touchwith other electronics hobbyists in yourarea.

We will shortly be re -introducing anew series of circuits for experimentersunder the title "Breadboards". Look outfor them in the June issue!

Reactions TestedDear Sir,I have just completed the ReactionTester (September '81 issue) and itdoesn't work. The bottom LED lights upwhen switched on but pressing the 'go'button has no effect whatsoever. I havechecked everything I can think of,including the PCB and the two ICs.

Maplin recommended a C106thyristor instead of the C103 - couldthey have different gate voltages andtherefore be the cause? The 4017 Ibought has CD4017 printed on it -could this be the fault?

The only other thing I can think of isthat there is a mistake in theinstructions. If there is, could you pleaseadvise me?A. Bricknell,Leigh -on -Sea,Essex.PS Keep up the good mag.

The 'CD' is simply the maker's codename for the IC - the type or 'generic'as it is sometimes called, is 401 7;that's what counts. Since it appearsthat either the 555 IC is not oscillating,or the 4017 is not counting. indicatedby the fact that is 'stuck' in resetcondition (zero count), it's unlikely thatthe thyristor is at fault (the gatevoltages are the same, anyway).

The circuit has no errors, other thanthat the IC types are interchanged(which should be immediately obvious),and the PCB layout seems correct.Having eliminated all those possibilities,we can only suggest that you check,very carefully, all the circuitryassociated with the 555 astable and theinput to pin 14 of the 4017.

Mast Head AmplificationDear Sir,I purchased your magazine this monthfor the first time and I find it excellent,but for one thing; no information onwhere to buy the kit for the TV MastHead Amplifier (February '82 issue).Who and where are RS Components,please?Yours,(signature indecipherable)South Wigston

We're not even sure about the address,but hopefully the writer will recognisehis own letter! Our misplaced commentsabout RS Components continue tohaunt us - RS Components will acceptorders only from "Full time industrial,educational and trade users..", howeverthe mail order firm Crewe Allen, of 51Scrutton Street, London EC2A 4PJ (Tel:01 739 4846) can supply componentsfrom the RS catalogue.

The TV Mast Head Amplifier is notavailable as a kit, as such, but Magenta


Electronics can supply all of the parts,including the double -sided PCB. WouldOliver Davis of Co. Donegal, Ireland andG. Inchbald in Manchester please copy!

Delay Time

Dear Sir,I have been trying, without success, toobtain a circuit diagram for a solid stateecho unit using Bucket Brigade Delaytechnology.

Can you suggest where I mightobtain such a circuit?D. VanderwolfTorpoint,Cornwall.

This is one letter we are happy toanswer without delay. Look no further,Mr. Vanderwolf! The circuit you seek ison page 41 - the HE Echo-Reverbproject.

Overseas Mail

Dear Editor,I have read with great interest yourmagazine, Hobby Electronics.

I am particularly interested in booksabout trouble -shooting in electronics -these are particularly lacking on the HEBookshelf. I will be very glad if youcould suggest any suitable books. lamvery interested in regulators, audio,radio and test equipment.R. Kimura,Swaziland,Southern Africa.

Trouble -shooting (fault-finding) is a veryinteresting topic - one that we will begiving detailed attention to, in the nearfuture. However, Mr. Kimura and otherinterested readers might also like to readIan Sinclair's book, "Electronics FaultDiagnosis", published by Keith Dickson(Publishing) Ltd., 17, Hendon Lane,London N3 1 RT.

Satisfaction Guaranteed.Dear Sir,I own a stereo radio/cassette playerwhich is still under guarantee and it hasa very annoying defect; when recordingfrom FM, especially from stereo FM, themotor causes definite distortion to therecording.

I have isolated the motor as thesource of the interference by playing atape while in the FM radio mode. Whilethe motor was running the interferencecould be heard distinctly and as soon asit was stopped, the interferencestopped. Obviously this should nothappen, as it means I cannot getsatisfactory recordings from the radioand this defeats my purpose in buyingthe machine. I wondered if there isanything I could do to remedy thesituation or whether I should return it

16 Hobby Electronics, May 1982

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under guarantee. I would be verygrateful if you could give me youropinion.E. Weeks,Godalming,Surrey.

We do not, normally, advise on anythingto do with commercial equipment butthis case is so clear that we decided tomake an exception - also, it is anexcellent example of fault-findingtechnique! Our opinion is: do not eventhink about fiddling with it - return it atonce.

Specifically Speaking

Dear Sir,Recently I have been thinking of buildingyour power amplifier from the March1980 issue of Hobby Electronics, theSystem 5080A. The amplifier seemsexcellent but you did not supply thespecifications - power output,distortion, frequency response, etc. Ishould be grateful to receive a copy ofthe specifications and, in future, Isuggest that you supply them with theprojects, as lam sure many peoplewould be interested.Yours,P. Upton,Salisbury,Zimbabwe.

To the best of our knowledge, a set ofspecification figures for the 5080A wasnever produced - or if they were, theyare now lost! However, the followinginformation can be extracted from thetext: the power output is 25 W perchannel; distortion is "so low as to beinaudible" and the frequency responseis "well above and below the humanhearing range". As you say, it seemspretty good. Nevertheless, yoursuggestion is a good one, and in futurewe'll certainly publish the fullspecification figures for any qualityaudio project.

Famous Names

Dear Sir,The article Famous Names (HE,December 1981) paying tribute to thevisionary Campbell -Swinton, containeda serious error. We read: "Theunanimous rejection of Baird's 30 -linesystem in favour of Schoenberg's405 -line system, in 1936, proved howright Campbell -Swinton was. In fact,no such choice was ever made.

The Baird 30 -line system was startedas early as 1929 and, by 1930, hadbecome fairly refined. In 1931 the BBCtook over all responsibility for the dailybroadcasts as a light entertainmentprogramme on the medium waveband,receivable nationwide. This left Bairdwith time to develop his 120 and 180line systems, which he broadcast on theseven meter band. Later, he developed a240 -line system.

So the contest of the two rivalsystems in 1936 (transmitted onalternate weeks) was between theSchoenberg 405 -line system and the

Baird 240 -line system. The BBC(30 -libel transmissions had actuallyceased the previous year. The number240 has been chosen, not by Bairddirectly, but by the PostmasterGeneral's Television Committee, underthe chairmanship of Lord Selsdon,which commenced sitting in May 1934and gave its decision on recommendedstandards in February 1935. The choicemay well have been influenced by thechoice of 240 lines by RCA, in theUnited States of America, which wastrying to develop Zworykin'slconoscope camera, without muchsuccess.

J.L. Baird was not only the firstperson to demonstrate true television(ie, moving half -tone portraits of livesubjects) anywhere in the world, butone of the quite small number ofpioneers in the field of high -definitiontelevision.D.B. Pitt,Chairman,Narrow Bandwidth TeleVisionAssociation,Nottingham.

Ian Sinclair, the author of the FamousNames series, replies:I regret that my efforts to compress a lotof information into a short phrase wasthe cause of a serious error of fact. Asyou rightly point out, the oldexperimental 30 -line system had ceasedby the time the dual broadcasts started.The Baird system had moved to higherdefinition and also to a more electronicsystem. Whether this would have beendone but for the determined work ofSchoenberg, Blumlien and McGee, onthe completely electronic system, is amoot point.

At one time, in fact, the only way inwhich live TV could be transmitted onthe Baird high -resolution system was byscanning the light source and, even bythe time of the experimental broadcast,the Baird system could not cope wellwith live working. For that reason, hedeveloped a fast film processor and itwas usual to film each programme, thentransmit from a flying -spot scanner afew minutes after the end of the show.It was extremely dangerous to stand inthe corridor after a concert because thewhole orchestra, brandishinginstruments, would come running fromthe studio to the processing rooms tosee themselves on film as theprogramme was being transmitted.Some of this film was preserved, but theshort processing cycle made most of ituseless after a few months.

Thank you for your comments. Thisseries has aroused a lot of interest andsome most interesting personalrecollections.

We certainly agree and would be mostinterested to hear from any otherreaders with personal knowledge of thepioneers or recollections of the earlydays of electronics.

Kit FeverDear Sir,Please could you tell me if you can do akit of the Bicycle Speedometer (March

'81 issue)? I intend to build this as partof my course at Technical College.A.E. Davis,Weston-Super-Mare,Avon.

M.A. Elatta, of Portsmouth and P.Longman, from Hull also wrote in withsimilar requests.

It seems to be a commonmisunderstanding, among some of ourreaders, that we supply kits for ourprojects; we don't. Kits are available forsome projects, true, but not from us. Ifyou have a copy of the issue in whichthe project appears, the Buylines make itclear whether or not there is a kit and, ifso, who is supplying it and the cost.

Dear Sir,I have been buying Hobby Electronicssince the first issue and I am generallyvery impressed.

I was, therefore, annoyed to findthat two projects in the December1981 issue (Drum Synthesiser andGuitar Graphic Equaliser) had no PCBfoil patterns printed in the magazine.They do seem to be available, but onlyas part of a full kit.

I stopped buying another electronicsmagazine a few years ago because itwas becoming a 'kit catalogue' and Ihope Hobby Electronics is not on thesame downhill path.Yours sincerely,D. Squibb,Weymouth, Dorset.

The issue of the PCBs for theseprojects has been dealt with in previousissues, but to clear up this matter onceand for all:

We try to present, in HobbyElectronics, projects that are useful,interesting or instructive. To achievethis, we accept material from a numberof sources; many of our contributorsare private individuals, but some aresmall companies, designing and makingup circuits for industry and private sale,as well as for magazines such as ours.In the past, several of thesecontributors have wished to retain thecopyright acknowledgement to therelevant company. We also attempt topersuade the kit supplier to make PCBs- and any other special components- available separately.

While we would prefer norestrictions whatsoever attached to ourprojects, the continual and difficultsearch for new, innovative ideasthe kit supplier to make PCBs - andany other special components -available separately.

While we would prefer norestrictions whatsoever attached to ourprojects, the continual and difficultsearch for new, innovative projectsdemands that we accept such projectswhenever they become available, fromwhatever source. This newarrangement, while less than ideal, atleast permits the individual at home tomake a project from the PCB upwards,as many of our readers prefer.

We hope that this long-windedexplanation closes the subject! HE

Hobby Electronics, May 1982 17

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Dols a3cr4.60 I esiFF-C311:10Flara910DI I I 13.,..C3ES....aiann-ra3rr6EOir-R4.4.1101131:1C.,E200EgriF EDC.,.-0000

Er I T 32"'X4 0000 reP I 4.601 FCIR DI $ 2 13

Next month we begin a brand new computerhardware project, the HE MicroTrainer. Basedon the 1802 microprocessor, it is simple toconstruct and easy to operate, the perfectmachine to introduce the 'nuts and bolts' ofmicrocomputer technology. The MicroTrainerhas been designed specifically for HobbyElectronics' readers, and offers a number ofdistinct advantages over other developmentsystems:* Low cost.

Modulated video output for direct input to adomestic television receiver, permitting moreinformation to be displayed than is possiblewith LED readouts. The 12 line x 12character display shows the complete set of1802 registers or 32 bytes of programmemory.

* Displays the current instruction, in mnemonicform.

* Unique single-step operation from RAM orROM based software.

* Cassette tape storage of user programs.* Twenty command functions, eg RUN, STEP,


* Twenty key keypad for direct input ofHexadecimal data orcommand functions.

* Includes 1.5 KBytes of RAM for userprograms.

* Optional 24 line I/O port* High quality, double -sided through -hole -plated

printed circuit board simplifies construction.

The HEM icroTrainer.


* Light -Operated Power Switch* Auto-Wah Effect

* Greenhouse Monitor* Telephone Timer

* Power Supply Design Project

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The Electronic RevolutionThe development of electricity and electronics,

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The June issue of Hobby Electronics will be inyour newsagents on May 14th


Simply fill out the coupon and hand it to yourlocal newsagent so that your copy will be

reserved for you.

Please reserve copies of the May issue of

Hobby ElectronicsforName


18 Hobby Electronics, May 1982

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IL IF'It's the chance

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It's something you have always wanted.... something to buildyour equipment into that's smart, modern, strong, adaptableto requirement and not expensive. The 'UniCase' is yet anothertriumph of I.L.P. design policy. It presents totally professionalappearance and finish, ensuring easier and better assembly tomake it equal to the most expensive cased equipment.The all -metal `UniCase' is enhanced by precision aluminiumextruded panels engineered for speedy and perfect alignedassembly within a mere five minutes. Designed in the first caseto accommodate I.L.P. power amps with P.S.U's, the rangewill shortly be extended to house any other modular projects.

WHAT WE DO FOR CONSTRUCTORSOur product range is now so vast we cannot possibly hope to show itall in our advertisments without overcrowding or abridginginformation to the point of uselessness. So we have devised a solutionwhich we invite you to take advantage of without delay. ALL YOU NEEDDO IS FILL IN AND FORWARD THE COUPON BELOW TO RECEIVEOUR NEWEST COMPREHENSIVE I.L.P. CATALOGUE POST FREE BYRETURN. It gives full details of all current I.L.P. products for theconstructor together with prices, full technical and assembly details,wiring and circuit diagrams etc. and it's yours, FREE. You don't evenhave to stamp the envelope if you address it the way we tell you.

ELECTRONICS LTD.FREEPOST 6GRAHAM BELL HOUSE, ROPER CLOSE, CANTERBURY CT2 7EPTelephone Sales (0227) 54778 Technical Only (0227) 64723 Telex 965780

FREEPOSTMark your envelope clearly FREEPOST 6 and post it WITHOUT a stamp toI.L.P. at address above. We pay postage when your letter reaches us.








I.L.P. are the world'slargest designers andmanufacturers of hi-fi audiomodules?

I.L.P. pioneered encapsulatedpower amps and pre -amps forenhanced thermal stability,mechanical protection anddurability?

There are TWENTY poweramplifiers from 15 to 240 wattsRMS including the verylatest super -quality Mosfets tochoose from?

TWENTY pre -amp modulesallow you to incorporateexciting professionalapplications to your equipmentnever before available toconstructors andexperimenters?

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I.L.P. products are available also from Henry's, Marshall's, Technomatic & Watford.

Hobby Electronics, May 198119

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Popular Computing

THEZX81 REVISITEDThe ZX8 1 is a remarkable machine formany reasons - not the least of which isits price.Phil Cohen

I REMEMBER from my adolescence (I'mnot as old as I feel) the first of the Sinclairdevices to hit the British market. Theseincluded a matchbox sized radio, a

calculator (which hit before theJapanese ones), a digital watch kit at afraction of the cost of competitive ones,and so on.

Although these may raise a yawnnowadays, at the time they were at theforefront of the market. Imagine thereaction of the public to the first digitalwatch, the first calculator - real 'Boys'Own Paper' stuff.

Sinclair entered the computer marketwith a little development kit based on theSC/MP processor, then moved veryquickly up-market with the ZX80 (theforerunner of the ZX81).

I remember my reaction on first see-ing the ZX80 advertised - I looked atthe date of the magazine to see whether

it was an April edition. I really believedthat it was a joke! I didn't think anyonecould put so much into so small apackage at so low a price.

The ZX80 had a couple of dozen chipsand primitive BASIC capabilities. TheZX81 has as powerful a BASIC as manyother machines on the market, and hasfive ICs (ignoring the three -terminalregulator).

Amazing!To look at, the ZX81 seems to be amock-up of itself - it weighs about asmuch as a paperback book, it has nomoving parts, it has no trailing wires, itscase is plastic.

The keyboard looks as if it has simplybeen printed onto the front of the case. Infact, it's 'elastometric', made up of aconductive rubber sheet over a set ofprinted circuit contacts. When you press

the sheet down, it makes contact withthe circuit board. It's just difficult tobelieve that it is what it's claimed to be.

HardwareInput and output for the ZX81 could notbe simpler - literally. Output is direct toa TV set (not a monitor), and there are acouple of sockets for connection of acassette recorder for program and datastorage.

The power for the ZX81 comes from9 V DC and that's it! Unless you want toadd peripherals.

There is an exposed section of edgeconnector pads at the rear of themachine designed for the addition of thetwo peripherals so far released. One is aRAM pack which brings the totalmemory from its existing, ratherlimited,1 K up to a more sensible 16K.The other peripheral available is a printer.It features full alphanumerics plusgraphics and prints 32 characters to theline, nine lines every 25 mm.

The processor used in the ZX81 is aversion of the ubiquitous Z-80. Thekeyboard is laid out in the normaltypewriter 'QWERTY' manner -although much smaller than a typewriter

20 Hobby Electronics, May 1982

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Popular Computing MI

computers is the fact that the key func-tion don't stop at the letters of thealphabet.

Statement EntryThis 'doubling up' of function is due tothe extremely clever way in whichSinclair have arranged their program en-try.

Say you want to enter the line "10PRINT A". In most computers, youwould press the '1' key, then the '0'key . . . through to the 'A' key at the endof the line. On the ZX81 you start withthe line number, then simply press the 'P'key. Up on the screen comes "10PRINT". There's no need to type the restof the word in. Then you press the 'A'key (no need for a space - the computersupplies that).

The ZX81 BASIC is arranged so thatthe first word on each line is a keyword(like PRINT, FOR etc). So when you pressthe 'P' key, the machine knows that youmean PRINT, because the next entrymust be a keyword. On the keyboard,the word PRINT appears over the 'P' key.


1 oAND2





3 o




























,E E.







The ZX81's multi -function keyboardsupports a very large number ofoperations. Graphics characters arecalled up by the GRAPHICS key(SHIFT/9); alternate characters(+, etc) by holding down the SHIFTkey; functions (AND, etc) or any ofthe words printed beneath the keysare called by FUNCTION(SHIFT/NEWLINE) while programkeywords are entered automaticallyafter entering a program line number.If all this sounds confusing - don'tworry. The keys are all colour -coded(eg, SHIFT and SHI FTed keys are red)and the input mode is indicated bythe cursor character.

keyboard. One user I heard from (he'sten) said it was just the right size.

I, unfortunately, have normal sizedadult hands, so it's a bit small for me. Infact, that's one of the few criticisms I

have of the ZX81 - you can't type on it,you have to use it like a calculator.

In fact, there's no 'feedback' to tellyou that you've pressed a key - so youhave to keep moving your eyes from thescreen to the keyboard and back again.After a while (particularly during pro-gram entry) this gets very tedious in-deed, but I suppose that most of thebuyers (myself included, if I'd boughtone instead of being loaned one forreview) would rather have the cash thana better keyboard. It's very tempting forreviewers to catalogue the facilities thatare missing from a device while at thesame time forgetting that if they hadbeen included, the buyer would in-evitably have to pay. The incredibly lowprice of the ZX81 is one of its main at-tractions.

The other difference between ZX81keyboard and those normally found on

A ZX81 disassembled.

Hobby Electronics, May 1982

In fact, nearly all of the keys have akeyword associated with them. Oneconsequence of using this system is thatthe old 'LET' statement (introduced inthe very first version of BASIC, nearly 20years ago) is resurrected. Most systemsthese days allow you to miss out theword LET in an assignment statement.

Another consequence of the systemis that the software doesn't have tocheck the spelling to see if it's akeyword. Each of the keywords isentered and stored as a single character(although it appears spelt out on thescreen). This is not only faster, it alsosaves memory.

There is also a SHIFT key, and a com-bination of other keys (FUNCTION andGRAPHICS) to select other options fromthe same letter key. In fact, some of theletter keys have five different functionscrammed onto their ultra -small face!

The ZX81 is not for those who havetrouble with small print!

DisplayThe display on the screen is ratherunusual for two reasons: the first is thatall characters are shown normally blackon white. I found this a rather pleasant,and less of a strain on the eye than thenormal white -on -black.

The second rather unusual feature isthat there is no automatic scrolling of thedisplay. In most systems, when thePRINT statements in the program haveput enough lines out to fill the availablespace, the screen 'scrolls' up one leavinga blank line at the bottom for the next lineof output. The ZX81 does not have thisfeature - and in fact, if the PRINTstatements try to put too much onto thescreen, an error will result and the pro-gram will halt!

There are two ways to get round this.One is a SCROLL statement, whichmoves the screen up one line. The se-cond is the CLS statement, which clearsthe screen.

It is rather surprising that Sinclairhave chosen not to implement theautomatic scroll - perhaps they havesome good reason. I can't think of one.

The character set consists of upper-case letters, numbers, and the veryminimum of other symbols. In factSinclair have kept the character set sosmall that I think some users may run intoproblems. For example, the symbol formulitplication is an asterisk '*', and thesymbol for exponentiation (ie raising to a


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II The ZX8 1 Revisited

higher power) is two asterisks °* *'.Now, it is quite possible to put two of themultiplication symbols into line side byside. However, this is not interpreted bythe computer as exponentiation. Thathas to be the special '**' symbol. Unfor-tunately, there is no easy way that theuser can tell the difference between thetwo on the screen. So it is quite possibleto do as I did - to type two multiplicationsymbols to mean exponentiation, andthen wonder why it didn't work.

Syntax CheckingEach line of the program is checked forsyntax as it is entered. Not only does thismean that problems will be shown up asthey occur, but also that the machinedoesn't have to check the syntax againas it runs the program.

The graphics symbols are fairly com-plete - allowing each character positionto be split into four segments each ofwhich can be black or white.

There are also symbols which allowshading of each character position, splithorizontally into two segments (seephoto of the keyboard). Each of the sym-bols in the alphanumeric set can beshown 'reversed', also.

As each line of program is entered, itappears on the screen in its correct posi-tion. So the normal method of looking atthe program - a LIST command whichscrolls the listing onto the screen - isnot used.

Instead, the bottom couple of lines ofthe screen are an 'entry area', where thecursor appears. The top part of thescreen then shows whatever part of the

program the last line was entered into.In fact, this method of entry is very

much easier to sue than the normal'scroll' method. It means that you canactually see the program change as youenter lines - this is very useful for begin-ners, who sometimes have troublevisualising what is happening inside themachine.

There is also an EDIT facility - one ofthe already -entered lines can be called in-to the bottom part of the screen andmodified, before being replaced in themain part of the program.

The operating system has a couple offeatures which are unique to the ZX81- one of these is the ability to run in twomodes - SLOW and FAST.

In the SLOW mode, the machine givesa 'flicker -free' display - the screendisplay is constant while calculations arein progress.

In the FAST mode (about four timesas fast), the screen blanks while themachine is calculating, only coming onwhen it is paused for input (or during ex-ecution of the PAUSE command). This isbecause Sinclair are using the CPU tooutput the display!

The cassette saving routines have theability to label the programs with analphanumeric string, and to search forthat string when the program is read offtape, only starting to load when they findthe right program.

Another unusual feature is that whena program is saved, all the currentvariable values are saved, too. This isnice for fitting very 'tight' programs intothe machine - the data initialisations do

The computer lets you know that it expects a keyword by the 13 that you had to start oftwith There is almost always some ,,nyefse.yideo, letter either 11"il or ti ierwe shall set, later, 9 or tit, called the cursor The mean. 'whatever key you press, I ;hallinterpret a as a keyword As you saw. alter you had pressed P le' PRINT the 13 changed toan II

This system of pressing lust one key to ger moroinan one symbol is used a lot on theZX81 In the rest of the manual words wth the, own keys are panted .n BOLD TYPE

You must remember that it is useless trying to spell these words our 11 full. because thecomputer lust went understand

2 Now' tybo 2 This should cause no pf,b,r, Again, you should see 2 appear or, thescreen and the L move along one place

Note also how a space's autOnkatically put between PRINT & 2 to make 1 if need

This is done as much as possible so that yea hardly ever rave to type a space II you do rypta space orll appear on the screen but .1 a,.II not;rifer, thr meamno of the metesrye ar

3 Now type + This is a shitted character ittrey are Aimed in red me colour u1 SHIFTitself on as key - in the top right hand corner of each key) and lode! You must c',' J. -

the key 131-41Ft and while vOri are Stai riti nu that, Ihri

4 Now type 2 again The screen w II

5 Now -and you must always remember this - press NEWLINE. the key, . This

means 'message complete', or 'all right, cemputer, let's see some action' The computerwill now read the message, work out what has to be done, and do it In this case. the screen

will change to


not need to take up any memory. The on-ly space they need would already be us-.ed by the variables themselves.

ManualThe documentation that comes with theZX81 is really excellent - the author,has taken a very down-to-earth ap-proach, and the whole thing (over 200pages of it) hangs together very n cely in-deed. It is well peppered w th ex-planatory examples, and is written in aneasy style that will not confise orfrighten anyone. It's also spiral bound,so that it will lie flat while you copy pro-grams from it!

The manual for a machine I ke theZX81 is almost as important as the hard-ware itself - it is, after all, primarily ateaching machine.

The only thing that's missing from themanual is any sort of comprehensivehardware details. I suppose, though,that given the probable audience thiswould not be worthwhile.

The manual not only describes theZX81 BASIC in loving detail, it also goeson to describe the internal software insome depth, including a full listing of thesystem variables and their interpreta-tion, and a section on how to usemachine code programs with BASIC.

Using itNow we come to the most importantpart - how the machine performs.

I didn't try any 'benchmark' programson the ZX81 - there's not much point,because all they would show is that the

4 is the answer - but of course you do not need to buy a computer to work that outPb' INote how zero is written with a slash to distinguish it from capital 0 This is fairly

common in computing circles 1 is the report in which the computer tells you how it got onThe first 0 means 'OK, no problems' (In appendix B there is a list of the other report codesthat can arise, for instance if something goes wrong I The second 0 means 'the last thing Idid was line 0' You will see later - when you come to write programs - that a statement canbe given a number & stored away for execution tater it is then a program line. Commandsdo not actually have numbers. but for the sake of reports the computer pretends that theyare line 0.

You should imagine a report as hiding a [2I cursor - if you press P for PRINT now, thereport will disappear and the screen will change to

A sample page from the excellent manual which is supplied with the machine

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The ZX81 Revisited

machine is significantly slower thanalmost any other on the market.

I say again - it's a teaching machine.So the speed doesn't really matter.

I wouldn't recommend the ZX81 tosomeone who wants to do a lot ofnumber -crunching, though - you'd bebetter off with a programmablecalculator.

The display is sharp enough to be readwithout too much strain - even on mylittle portable. The characters are a little'blocky', but not outrageously so.

Apart from the problem I mentionedearlier about the keyboard having nofeedback, the only other major trouble inusing the ZX81 is that 1K of RAM isreally rather small - even with onecharacter per keyword.

Then again - if it's the only computerthat you can afford, and it's the first onethat you've used, then it's not likely totrouble you.

ZX81 BasicFinally, I've included a list of the com-mands and features of the ZX81 BASICso that you can see that the ZX81'slanguage is every bit as comprehensiveas that of other machines on the market.Variables may have an alphanumericname of any length, starting with a lettercontinuing with letters and numbers -and spaces! This is due to the uniquekeyword entry method.Values are stored to 9 digits, with arange between about 10- 38 and 1038.Array names are a single letter, and ar-rays may have any number of dimen-sions of any size.String arrays are allowed - but all of thestrings in the array are the same length.String variables are any length, but thestring name is only a single letter.Functions supported include: absolutevalue, arccos, AND, arcsin, a rctan ,CHR $ , CODE (the same as ASC in otherBASICs - but it's not ASCII), cos, Xe,INKEY$ (gets a key press from thekeyboard), integer part of a numC inother BAISCS - but it's not ASCII), cos,Xe, INKEY$ (gets a key press from the

The inside. That's all - true!

keyboard), integer part of a numberlength of a string, NOT, OR, PEEK, pi,random number, sign of a number,sin,square root, STR$ tan, user machinecode routine call, and VAL.

Statement types are:CLEAR deletes all variablesCLS clears the screenCONT after 'break', continues executionCOPY sends a copy of the screen con-tents to the printerDIM dimensions arraysFAST sets machine into fast mode (seetext)FOR . . TO . . . STEP forms a loop (thevariable used must have only one letter inits name)GOSUB sends program to a line number(line number may be expressed as an ex-pression)IF . . . THEN allows changes in programflow - but multiple statements per lineare not supportedINPUT allows the user to input a string ornumeric characters

LET is required for assignmentstatementsLIST allows the user to call up any part ofthe program on the screen's display areaLLIST sends it to the printerLOAD searches for the program name onthe tape, then loads itLPRINT sends output to the printerNEW initialises the whole systemNEXT ends a FOR loopPAUSE stops the program for a setperiod from 1/50 of a second to about10 minutesPLOT makes one quarter of a characterprint in the position specifiedPOKE allows the program to altermemory directlyPRINT puts information onto the screen.Features supported are: comma (givingfixed tab), semi -colon (at the end of thestatement, preventing line feed and car-riage return) and TABRAND allows randomisationRND variable sequenceREM for remarksRETURN ends subroutineRUN runs a program. RUN (line number)starts the program from the line numberSAVE puts the program onto tape, with aname of any lengthSCROLL moves all the lines in the displayarea up oneSLO puts the machine into slow mode(see text)STOP halts executionUNPLOT turns off one quarter of acharacter position, in the positionspecified

SummaryThe ZX81 is a very high value -for -moneymachine. It's designed as a teachingmachine, and at a price of £49.95 forthe kit or £69.95, ready assembled it isvery well targeted.

It is not a machine for those who havenumber -crunching applications in mind.For that is rather slow and a bitawkward.

It does, however, have almost all theadvanced features found on otherBASIC systems. Having mastered theZX81, you will be able to drive almostany other machine after a couple of days. HE

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Sinclair ZX81 Personal Compthe heart of a systemthat grows with you.1980 saw a genuine breakthrough -the Sinclair ZX80, world's first com-plete personal computer for under£100. Not surprisingly, over 50,000were sold.

In March 1981, the Sinclair leadincreased dramatically. For just£69.95 the Sinclair ZX81 offers evenmore advanced facilities at an evenlower price. Initially, even we weresurprised by the demand - over50,000 in the first 3 months!

Today, the Sinclair ZX81 is theheart of a computer system. You canadd 16 -times more memory with theZX RAM pack. The ZX Printer offersan unbeatable combination ofperformance and price. And the ZXSoftware library is growing every day.

Lower price: higher capabilityWith the ZX81, it's still very simple toteach yourself computing, but theZX81 packs even greater workingcapability than the ZX80.

It uses the same micro -processor,but incorporates a new, more power-ful 8K BASIC ROM - the 'trainedintelligence' of the computer. Thischip works in decimals, handles logsand trig, allows you to plot graphs,and builds up animated displays.

And the ZX81 incorporates otheroperation refinements - the facilityto load and save named programson cassette, for example, and todrive the new ZX Printer.

BASIC manualEvery ZX81 comes with a comprehensive, specially- writtenmanual -a complete course in BASIC programming, fromfirst principles to complex programs.

Kit:149.95Higher specification, lower price -how's it done?Quite simply, by design. The ZX80reduced the chips in a workingcomputer from 40 or so, to 21. TheZX81 reduces the 21 to 4!

The secret lies in a totally newmaster chip. Designed by Sinclairand custom-built in Britain, thisunique chip replaces 18 chips fromthe ZX80!

New, improved specification Z80A micro -processor - newfaster version of the famous Z80chip, widely recognised as the bestever made. Unique 'one -touch' key wordentry: the ZX81 eliminates a greatdeal of tiresome typing. Key words(RUN, LIST, PRINT, etc.) have theirown single -key entry. Unique syntax -check and reportcodes identify programming errorsimmediately. Full range of mathematical andscientific functions accurate to eightdecimal places. Graph -drawing and animated -display facilities. Multi -dimensional string andnumerical arrays. Up to 26 FOR/NEXT loops. Randomise function - useful forgames as well as serious applications. Cassette LOAD and SAVE withnamed programs. 1K -byte RAM expandable to 16Kbytes with Sinclair RAM pack. Able to drive the new Sinclairprinter. Advanced 4 -chip design: micro-processor, ROM, RAM, plus masterchip - unique, custom-built chipreplacing 18 ZX80 chips.

Built:169.95Kit or built - it's up to you!You'll be surprised how easy theZX81 kit is to build: just four chips toassemble (plus, of course the otherdiscrete components) -a few hours'work with a fine -tipped soldering iron.And you may already have a suitablemains adaptor - 600 mA at 9 V DCnominal unregulated (supplied withbuilt version).

Kit and built versions come complete with all leads to connect toyour TV (colour or black and white)and cassette recorder.

24 Hobby Electronics, May 1981

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16K- byte RAMpack for massiveadd-on memory.Designed as a complete module tofit your Sinclair ZX80 or ZX81, theRAM pack simply plugs into theexisting expansion port at the rearof the computer to multiply yourdata/program storage by 16!

Use it for long and complexprograms or as a personal database.Yet it costs as little as half the priceof competitive additional memory.

With the RAM pack, you canalso run some of the more sophisti-cated ZX Software - the Business &Household management systemsfor example.

ZX816 Kings Parade, Cambridge, Cambs., CB21SN.Tel: (0276) 66104 & 21282.

30 ,40


" -0C)R iC---

Available nowthe ZX Printerfor only 149.`ADesigned exclusively for use withthe ZX81 (and ZX80 with 8K BASICROM), the printer offers full alpha -numerics and highly sophisticatedgraphics.

A special feature is COPY, whichprints out exactly what is on thewhole TV screen without the needfor further intructions.

At last you can have a hard copyof your program listings -particularly


useful when writing or editingprograms.

And of course you can print outyour results for permanent recordsor sending to a friend.

Printing speed is 50 charactersper second, with 32 characters perline and 9 lines per vertical inch.

The ZX Printer connects to the rearof your computer - using a stackableconnector so you can plug in a RAMpack as well. A roll of paper (65 ftlong x 4 in wide) is supplied, alongwith full instructions.

How to order your ZX81BY PHONE - Access, Barclaycard orTrustcard holders can call01-200 0200 for personal attention24 hours a day, every day.BY FREEPOST - use the no -stamp -needed coupon below. You can pay

by cheque, postal order, Access,Barclaycard or Trustcard.EITHER WAY - please allow up to28 days for delivery. And there's a14 -day money -back option. We wantyou to be satisfied beyond doubt -and we have no doubt that you will be.

rTo: Sinclair Research Ltd, FREEPOST , Camberley, Surrey, GU15 3BR. OrderlQty Item Code Item price


£Sinclair ZX81 Personal Computer kit(s). Price includesZX81 BASIC manual, excludes Mains adaptor. 12 49.95Ready -assembled Sinclair ZX81 Personal Computer(s).Price includes ZX81 BASIC manual and mains adaptor. 11 89.95

Mains Adaptor(s) (600 mA at 9 V DC nominal unregulated). 10 8.95

16K -BYTE RAM pack. 18 49.95Sinclair.ZX Printer. 27 49.95

8K BASIC ROM to fit ZX80. 17 19.95

Post and Packing. 2.95

Please tick if you require a VAT receipt TOTAL £

*I enclose a cheque/postal order payable to Sinclair Research Ltd, for £*Please charge to my Access/Barclaycard/Trustcard account no.

*Please delete/complete as applicable. I I I I I I I I I I I I IIIPlease print.

Name: Mr/Mrs/Miss IiAddress. I IIIIIIIIIIIIIIIIIIIIIIILFREEPOST -no stamp needed. Offer applies to UK only. HEL051

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Popular Computing

Pictured above is ACE, the National Physical Laboratory's first computer. The vertical sliding panels are to allow access formaintainance - and also to provide better cooling for the racks of valves. (Courtesy NPL, Crown Copyright).

MICRO -HISTORYA slice through the years, as we look at some of the men and

machines that preceeded today's powerful computers.

ALMOST every day, we read in the na-tional press, or in other publications, thatthe age of computers is upon us. This isundoubtably true, but it is neverthelessworthwhile looking back at some of thedevelopments which have led to the cur-rent 'state of the art', before thetechnology overwhelms us! In writingabout computers, there is often a tempta-tion to baffle readers with science - thejargon looks good, and soundsauthoratative. In fact, we should be tryingto do the opposite; explain exactly whatcomputers are and how they effect ourlives, in plain, simple language. So, let's

start at the beginning, and get the wholething in perspective.

Looking at the names associated withearly computers, three stand out asbeacons of foresight and understanding.

Taking them in chronological order,which is logical, we first meet BlaisePascal.

Born in France, in the SeventeenthCentury, he was best known as amathematician. His father was a tax col-lector, an unpopular occupation, eventhen, and the sight of his father spendinglong hours over columns of figures wasyoung Blaise's first inspiration; he pro-

duced a mechanical engine that removedthe drudgery of the work. The basicdesign was a success, which was a con-siderable feat of purely mechanicalengineering, and it enjoyed a limited com-mercial success, too. The main disadvan-tage was that it was only capable of eitheraddition or subtraction - one operation ata time.

This machine was called the'Pascalline' and it is still, surprisingly, inuse today, recording the mileage thatyour car has covered. Its improved suc-cessor, which was capable of multiplica-tion, survived for nearly as long. Only the

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Popular Computing

first electronic calculators sent it to theScience Museum. If you should see itthere, it's worth remembering that it wasinvented when the Inducalculators sent itto the Science Museum. If you should seeit there, it's worth remembering that itwas invented when the Industrial Revolu-tion was still to come and that even simplemechanical parts could not be manufac-tured accurately. It was this difficulty thatcaused the failure of the next innovator incomputer science.

The Difference EngineCharles Babbage (who is the subject ofthis month's Famous Names), also beganlife as a child prodigy and it was his earlyinterest in tables of logarithms that in-spired his life's work. His first achieve-ment was to produce a machine forcalculating log tables which were moreaccurate than those previously published.His next task, the construction of the Dif-ference Engine, was more challenging,hut after eleven years' work and aGcnernment grant of £17,000 (an enor-mous amount of money, in those days),the mechanical problems finally defeatedhim.

Despite this 'allure, he conceived aneven more spectacuia; device, the greatAnalytical Engine, which incu. porated allof the concepts of the Difference Enginebut was, in addition, programmable; itwas intended to carry out mathematicaloperations as required, on whatever datawas available. This was the first visualisa-tion of the computer as we understand ittoday - it actually included most of thefeatures incorporated in modernmicrocomputers. Once more, though, theinability to make presicion parts spelled

A first generationcomputer - thousandsof circuit cards andtransistors beyondcounting!

(Courtesy NPL,Crown Copyright).

the doom of the Analytical Engine. Bab-bage died, aged 80, with not much left tohis name except a pile of cogs and gearwheels. His son finally managed to puttogether a working model, which can beseen in the Science Museum.

Herman the WiseLess than twenty years later, the thirdgreat inventor in the history of com-puters, Herman Hollerith, made the finallink in the chain of innovations that led tomodern computers. His 'Tabulator' wasdesigned as an entry in a competition todevelop a system to analyse the mass ofinformation collected for the 1890American Census. It was an electro-mechanical device and completed theCensus results in record time, makingHollerith a very rich man. Indeed, thecompany he founded (InternationalBusiness Machines - IBM) is still one ofthe largest mainframe computermanufacturers and it is only recently thatits dominance of the business has beenchallenged.

Hollerith's Tabulator was by no meansan ideal computer; it was designed to dojust one specific job and its 'programm-ing' could not easily be changed. This wasnot the flexible, all-purpose machine con-ceived by Babbage. However, it is in-teresting to note that, followingBabbage's adaptation of the punchedcards used to control weaving machinery,Hollerith used similar cards to enter theCensus data. These cards are still in use inmany computer systems, today - somethings never seem to change!

The breakthrough was made shortlyafter World War II. The development ofballistic computers and code -breaking

machines during the War produced someextremely sophisticated electro-mechanical devices but, like all suchdevices, they were 'dedicated' machineslimited to a specific application. The firsttrue electronic computer was, in fact,developed in Britain, in 1948 by a Man-chester University team.

The development of the true electroniccomputer rested largely on the work oftwo mathematicians; Alan Turing, a Bri-tain whose work on code -breaking laidthe foundations and the American, JohnVon Neumann. Another vital step was theintroduction of the binary numberingsystem. If Pascal or Babbage had design-ed their systems to use binary numbersthey would have simplifed their problemsby a factor of nine and it is possible thatthe world would have had steam -powered computers before it had elec-tricity.

Having made the key evolutionaryleap, computers went from strength tostrength. The first ones were monsters,its true (Ferranti's first commmercialcomputer stood 32 feet long, eight feethigh and four feet deep, used 3,500valves, 2,500 capacitors, 15,000resistors and six miles of wire) and rejoic-ed in names such as UNIVAC, ENIAC andACE, but they were true general-purpose,programmable computers, consisting ofthe same basic elements as are found intoday's computers.

The growth of computers, since thenhas been by a series of steadyrefinements, rather than by revolutionarydevelopments. The thermionic valve wasa notoriously unreliable device, slow andcostly in terms of power consumptionand space and they were soon replaced

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Micro History

Pilot model of the NPL's ACE. (Courtesy NPL, Crown Copyright).

by the newly invented transistor.However, the replacement of valves bytransistors did not involve any change inthe basic design of computers.

The introduction of the transistor pro-duced the 'second generation' of com-puters. A 'generation' in computer termsis loosely defined as a tenfold decrease insize with a tenfold increase in processingpower at a tenth of the original cost. Asthe transistor became the descendent ofthe valve, so the chip or integrated circuitbecame the descendent of the transistor.In those days (some ten to twelve yearsagol, the first integrated devices con-sisted of perhaps a half dozen transistorson a single chip of silicon. Rapid advanceswere made and soon a new kind of com-puter was born.

The MinisJust as the Mini car revolutionised theway the world looked at motoring, so theminicomputer changed the face of com-puters. Up till the advent of the integratedcircuitry there had been only 'computers';now there were 'mainframes' and 'minis'.These were rigidly divided into sectors ofoperation - the mainframes were usedfor serious purposes, the minis were'toys' used in research. Among thenames of companies who were to maketheir fortunes producing minis was DECwho are, probably, still the world leader.Soon, the mini was to be foundeverywhere from research labs toclassrooms and their spread was duesimply to the fact that they were small,cheap and relatively easy to use. Theywere even built into pieces of equipment,such as machine tools. Indeed, it is fair tosay that the mini paved the way for themicro, although the actual distinctionsbetween them have been rapidly eroded.

Firms involved in the business of in-tegrated circuit production tend to followa natural progression in the devices thatthey make. First off the production linecome the standard logic elements, theAND/OR type gates and, once the pro-duction of these is running at a profitablelevel, they attempt to squeeze a littlemore onto the slab of silicon. As soon asthis stage is proved they take another leapforward and so on. In the terminology ofthe business, this is a progression from

SSI (Small Scale Intergration) with about10-20 actual devices on the 'chip'through MSI (Medium Scale Integration),which has a dozen or so gates (rather thandiscrete elements) up to LSI (Large ScaleIntegration), which is taken as beinggreater than 100 gates on the chip. Atthis stage of the game we are still talkingabout, complex TTL type packages, thenext jump is to VLSI which, believe it ornot, stands for Very Large Scale Integra-tion. We are now in the realm of memorydevices and microprocessors.

If we take a look at Figure 1 we can seea generalised block diagram of a com-puter; what kind of computer is not impor-tant because they all have the same func-tional blocks within them, be they microor mainframe. The common misconcep-tion is that the 'mighty chip' is a computer- far from it. Your averagemicroprocessor still needs all the memorycircuits, control circuits, mass storagedevices and other components that eventhe old valve machines needed; they aremerely smaller. The very firstmicroprocessor came about in 1971simply because it was realised that itwould be possible to make a device ofthat complexity' on a single chip. Thedevice was called the 4004 and the com-pany that made it was Intel . The nextdevice they produced was the 8008 andthat was followed by the microprocessorchip which, ten years after its initial in-troduction, still dominates the microcom-puters of the world - ubiquitous 8080.

The rest of the story is not history;since the 8080, new chips have been in-troduced almost every year and so thefuture is still wide open. Indeed, the futureprobably holds as many new and excitingdevelopments as the past and anotherrevolution in computer technology is stillpossible, considering the work currentlybeing done on the 'biological computer'.We may yet see computing 'brains' whichmore closely live up to the description!



1 I 1





li lthitglARITHMETIC







Block diagram of a computersystem. A modern micro-computer is essentially iden-tical - mass storage (usuallyhard disc) is quite possiblewith larger, more powerfulmicrocomputers. Cardreaders, however, are a hold-over from the very earliestdays of computing. They areonly used, now, with verylarge mainframe systems.


The life and career of CharlesBabbage is the subject of thismonth's Famous Names feature.

HE28 Hobby Electronics, May 1982

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Lack of ZX81 memorygiving you headaches .?

The Memotech 64K MemopakThe growth of interest in computer use caused by the introduction of the Sinclair ZX81 has made new andexciting demands on the ingenuity of electronic engineers. At Memotech we have focused our attentionon the design of an inexpensive, reliable memory extension.

The Memopak is a 64K RAM pack which extends the memory of the ZX81 by a further 56K. Followingthe success of our 48K memory board the new memory extension is designed to be within the price rangeexpected by Sinclair users. It plugs directly into the back of the ZX81 and does not inhibit the use of theprinter or other add-on boards. There is no need for an additional power supply or for leads.

The Memopak together with the ZX81 gives a full 64K, which is neither switched nor paged, and isdirectly addressable. The unit is user transparent and accepts such basic commands as 10 DIM A(9000)0-8K ...Sinclair ROM8-16K...This section of memory switches in or out in 4K blocks to leave space for memory mapping, holdsits contents during cassette loads, allows communication between programmes, and can be used to runassembly language routines.16-32K...This area can be used for basic programmes

32-64K...32K of RAM memory for basic variables and large arrays. DEM litand assembly language routines.

a powerful computer, suitable for business. leisure and educationalWith the Memopak extension the ZX81 is transformed into EN013111IES

use, at a fraction of the cost of comparable systems. AVIFICMillIEl&

Please rush me:UPEINNCIUMU"nrinIn rN or yin isimmorousumm Pleasedebit I 16 V VIVVOMMOVVIVIMISOM I BARCLAYCARD/ACCESS7Of 1111 111MMIIMMIELNIMINNIMME .

IbliniabilialThillManq I

LTIEVIOTECIli I 'Please deleteIwhichever does not apply


account number:

Memotech Ltd3 Collins StreetOxford. OX4 1XLTel . 722102/3/4/5



I Date

Quantity Price Totalb4K HAM, Assemolea £68.69HINTS & TIPS FOR ZX81(BOOK BY ANDREW NEWSOM £4.25

Stock Control Programme £ 25.00Payroll Programme £ 25.00



Postage £2.00

HE1 TOTALTo: Memotech Ltd., 3 Collins Street, Oxford, OX4 1 XL Telephone (0865) 722102

m. m imm INN me MIS MIN NM MIN







Hobby Electronics, May 1981 29

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Who is this mysterious person? Did Charles Dickens discoverHovis? Did Robert the Bruce discover Australia?

OK. Lets get it out of the way.

Dear CD,I am writing to you about your reply toMr Belfield's letter in the March '82issue of HE. You said that you wouldaward next month's binder to anyonewho could politely and in one sentenceexplain what chocolate digestives haveto do with you.

I believe /have the answer - heregoes. Chocolate digestives have theinitials CD and C.D. = Clever Dick. Ihope it is the right answer.Paul Procter,Chester.

PS Your mag is the best one out.

Dear CD,I write to you claiming a binder, with nogrovelling whatsoever. You offered abinder to anyone who could explain D.Belfield's PPS; well, here is theexplanation.

Clever Dick (CD) and chocolatedigestives (cd) both have the sameinitials, so by saying that his sister likeschocolate digestives, he is implyingthat she also likes you -is thisgrovelling?

Was that polite enough?In expectation of a binder,R. Hart,Mansfield,Notts.

PS Charles Dickens also has the intialsCD - any relation?

Dear CD,In the March issue (we know that bynow, thanks all the same) you askedfor the connection between yourselfand Mr Belfield's sister's craving forchocolate digestives.

Well, firstly you should be honouredto have the same initials as a ChocolateDigestive and, secondly, it is a clue toyour real identity.

If you remove all the letters fromClever Dick that also occur inChocolate Digestive, you are left withthe letters R, K. This would therefore,suggest that you are in fact RonKeeley, the Esteemed Editor.

I hope this answers your questionand you wil keep your promise of anaward?I.M.A. Bruce.

PS I have been reading HE for almostthree years and in that time have notonly developed a warped sense ofhumour but also a very warped stock ofHEs.

Dear Chocolate Digestives,Isn't Mr Belfield's PPS obvious? Hethinks that CD stands for ChocolateDigestives and his sister has fallen inlove with you (but then, which girlwouldn't).

Anyhow, down to business. Whyare all these people complaining aboutthe few mistakes made in HE? Don'tthey know that it is all done on purposeto keep us hobbyists on our toes? Afterall, how can two whole diagrams(Figures 9 and 10, page 43) in theocilloscope article in the March issue bemisplaced and then merely attributedto 'gremlins'.

I'm sure that the HE team, in theirultimate wisdom, know exactly whatthey are doing.Yours, Can/ha veabinderpleasingly,J. Sylvester,Ilkeston,Derbyshire.

PS Why is it that, in some projectcomponent lists, it says, for example,"50k -5 off" instead of "5 off"?PPS I wouldn't mind a binder.PPPS If you send a binder, I might justget back in from this three -stories -highwindow ledge.PPPPS Remember, if I do accidentallyfall off (of?) this ledge, I won't be ableto buy HE anymore!

Oh what clever, witty readers you are. I

should have known that the questionwas far too easy for such erudite,intelligent and perspicacious people.Never mind. You can't win them all -nor can all win. So.

Paul Procter is straight to the point,even blunt, one might say - but is that

polite? G. Adamson, I'm afraid, losesimmediately. What a dreadful pun!

Mr R. Hart has a strong claim, butloses points for subdued grovelling -if your going to do it, lad, do it properly,like. Besides, Charles Dickens is olderthan I am. I.M.A. Bruce came up with avery clever answer (although his'name' is another dreadful pun) which,unfortunately for HIM, has nothing todo with ME. My real identity, sir, is themost closely guarded secret sinceSuperman fell out of the 'phone box.Not even the Editor knows who I reallyam - and sometimes I'm not too suremyself.

That leaves J.Sylvester (any relationto Victor?), who has probably jumpedby now. Just in case he's still hangingabout, a binder is on its way to him,awarded for the most concise answerto the question; his salutation was plainenough. A belated binder is alsoawarded to D. Belfield, together with apacket of Chocolate Digestives for hissister (but he'll have to write again,with his full address this time).

This late reply seems to have missedthe point, though he's quick enough toclaim the reward.

Dear CD,In reply to D. Belfield's letter, theSAB0600 is, in fact, the IC used in theDoorchime project (December 81) andcan be obtained from TK Electronics.

Now a suggestion for a project; howabout a Flanger? I am interested inmaking electronic music - December'sHE was excellent and I have built theGraphic Equaliser and the Drum Synth(with modifications) and they workedextremely well.

30 Hobby Electronics, May 1982

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'&fls aomvir,ISPifoRS 14..D

SFS. -fo S'fArl/Sts4..

Don't forget the binder, thank you.S.C. BonneII,Wolverhampton.PS Keep up the good work.

Right! The Doorchime! I knew I'd seenis somewhere. This is a good as time asany to remind you all that I can onlygive detailed replies to exact questions- the IC referred to could have been inany project published anytime in thepast four years and I certainly don'thave time to check them all!

The Flanger project is a good idea,which follows naturally from the HEEcho-Reverb in this issue. I'll see what Ican do to push it, OK? As for the binder- sorry, you're too late.

It's always nice to receive 'loveletters', but this one is just slightlydubious.

Dear CD,I often buy decent magazines onelectronics from my newsagent andoften have a laugh at HE. Most of theprojects are, to say the least, pathetictea-break rejects.

But what was this I saw in the April'82 issue? A useful project! Amazing! Imust congratulate you for the DigitalCapacitance Meter (usually, anysensible projects are to be found inETI). I actually bought the magazineand the last three pages weren't on CB!M. Garton,Purveyor of Messy Writing,(Address illegible).

He's right about the writing, if nothingelse. All Hobby Electronics projects aredesigned to be useful for somepurpose, even if the aim is only toinstruct - we do have a number ofreaders who are just getting intoelectronics for the first time, you know.

The next letter raises a most interestingquestion - is it 1984, already?

Dear Dick,I remember reading, some time ago,that a Japanese camera company fittedtheir remote control cameras with infrared control so they could get aroundthe Wireless Telegraphy Act of1947/1969. But, after reading the1969 Act, I noticed that any form oftransmission, including infra red, couldonly be considered legal if used by thePost Office or by people with licenses.

4.0a, RP V....7)

It seems that they do not yet havecontrol over shouting or singing, but Iexpect that they're working on it. Is ittrue, Dick? Are these infra red camerasand remote control units against thelaw?M. Scholes,Telford,Salop.

PS Why do all these people waste timebuying or grovelling for binders? Havethey no floors on which to pile theirHEs? I keep mine in suitcases and oldbox files under the bed!

The implication, here, is that anyonewho uses an infra red remote controlsystem for any purpose (changing TVchannels, for example), is breaking thelaw. That is not the case. The Actreferred to defines Wireless Telegraphy(for which a license is required) asfollows:

Wirelss Telegraphy means theemitting or receiving, over paths whichare not provided by any materialsubstance constructed or arranged forthat purpose, of electromagneticenergy of a frequency not exceeding 3x 106 Megahertz.

Fortunately for all of us, the infra -redband starts at 3 x 106 MHz and,therefore, operating an infra -red remotesystem does not require a license,under the Act. Imagine the troubleBuzby would have, if IR was illegal!He'd probably lose all his feathers fromover -work.

Now here's a classic plea for HELP.

Dear CD,I have built the Power Pack (September'81) and - lo and behold - it worked.Yesterday I put in the fuse holder and,instead of 5 to 15 volts, I now get 7V5to 23 V and sparks jump between thecarbon track and the connector. Whathave I done wrong?C. Basnett,Quarry Bank,West Midlands.

See what I mean? On the face of it, thisproject is so simple that nothing shouldgo wrong (so what do you mean, "loand behold"?). The only answer thatoccurs to me is that you've got a solderbridge between -- or nearly between -the PCB tracks and the mains voltage isarcing across. Check the boardcarefully for lumps of solder.

Kio-0 KNOW. I -atiaLuca, It BEITE.k. 1.,AIEN IY

3-v S-1 NAB fkr4 "ON -

SW 11-okk !!


Sometimes, it seems, people just don'tread the magazine - we go to an awfullot of trouble to get all this informationin. Why, of why won't you just READIT!

Dear CD,Please can you answer my simplequestion; where do you get the boxesfor projects, ie Bike Alarm and DigitalCapacitance Meter (April '82). Do youhave to build them yourself?C. Mathews,Egham,Surrey.

No, I certainly don't - and neither doyou. The Buylines page is there to letyou know where to obtain componentsand parts for all our projects - but youmust read it, you know. If you had,you'd know that cases for both theseprojects are available from BICC Vero.OK?

Then, reading does not always lead tounderstanding.

Dear CD,After reading Ian Sinclair's Into DigitalElectronics, I'm hooked on digitalcircuits and displays. The only thing is,I'd like to be able to check IC pins witha logic probe, for fault finding, but I'mnot sure what the logic state of thepins should be in the first place. Iwonder if you could help me?K. Goonan,Preston,Lancs.

You may be beyond all help, buy I'll try.Without repeating the series, thesimple answer is that the logic state isa given IC pin will usually be either a 1(high voltage) or a 0 (low voltage), butexactly which state the pin assumesdepends on the type of IC (AND gate?OR gate? Timer? Counter?) and how itis connected. For example, a 555 maybe wired as an astable multivibrator oras a monostable, to choose twocommon circuit configurations. Beforeyou can know what the logic state of apin should be, therefore, you mustunderstand the circuit. I believe that avery special logic probe is beingdesigned and will be published in thenear future. Perhaps you will be able touse it to gain a better understanding ofdigital logic. In the meantime I can onlysuggest that you go back and read IanSinclair's series from the beginning! HE

Hobby Electronics, May 1982 31

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HE Echo-Reverb UnitA superb project for creative musicians and audiophiles.

AN ECHO-REVERB unit is an accessorythat can be added to virtually anyexisting audio or electronic music systemand used to impart new life to existingsounds. In any audio system, the unit issimply interposed between the output ofthe preamp and the input of the mainamplifier, so that the audio signals haveto pass through the echo-reverb unit ontheir way to the main amplifier.

Some modestly -priced echo-reverbunits use a crude mechanical spring -lineto create the time -delayed echo-reverband provide only a single, fixed delay

time. Commercial (all electronic) echo-reverb units can be rather expensive,but provide fully -variable delay times.Many use clocked CCD (charge -coupleddevice) analogue ICs to implement thedelays (see How It Works). The bestcommercial units use digital techniquesto implement the delays, but these areextremely expensive!

We have used the analogue CCDtechnique to implement the time delaysand the HE Echo-Reverb gives aperformance that is at least as good assome commercial units using CCDs. The

THE STAGES for producing the variousreverb effects from the HE Echo-Reverb,are shown in the block diagram. Themain signal path comes from the outputof the first mixer and through a7 kHz low-pass filter. This filter isnecessary to limit the audio bandwidth toless than half the clock frequency,because the audio input is being sampledat the clock frequency (variable, tochange the length of delay), and it is afundamental principle of sampling(Shannon's Law) that the sampling fre-quency must be at least twice the max-imum input frequency. The filtered signalthen passes through the delay line to asecond low pass filter (at 15 kHz), whichremoves any clock signal residuals. Thissecond filter includes a buffer amplifier togive the unit an overall gain of one. Theoutput then splits into two paths; one issent back to the input, to provide thereverb effect, the other goes to a finalmixer via a switch. In the other position,the final mix can be varied from 'straightthrough' to full reverberation.'

The delay circuitry comprises twocharge coupled devices (CCD's) with1024 delay stages. The diagram rightshows an example of the internal struc-ture of a MOS CCD IC. The principlemay be compared to a line of firemen


How It Works

important thing about our unit, however,is that the design uses some cunningtechniques to reduce the hardware costsof its circuitry, while at the same timeenhancing the overall performance andfacilities.


As the audio signals enter the echo-reverb unit they spilt into two paths,which are re -united again near theoutput of of the unit via a built-in low -distortion audio mixer. One of these




U,1 n2







passing buckets of water from one end tothe other - hence the name 'bucketbrigade'. In electrical terms the 'buckets'are capacitors and the 'water' is an elec-tric charge which is proportional to an in-stantaneous value of the input waveform-a sample. Each sample is stored brief-ly, then passed on to the next stage at thetime of a clock pulse. Although eachsample is stored for a very short time, ateach stage, the time taken to 'clock' asample from input to output can be asmuch as 50 mS.





Hobby Electronics, May 1982 33

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HE Echo-Reverb






PR14k7SET 5V





R82k7 R12



R11100k 16











27k =D1 A D2








16 14




paths is a direct link from the input of theecho-reverb to the input of the mixerstage; the other path is via a variablesignal -delay network. By varying thesignal delay and then the mixture ofdirect and delayed signals, a variety ofinteresting effects can be obtained. Hereare a few ideas to try:(11. With equal levels of direct and

delayed signals, and a fewmilliseconds of delay, a 'double -tracking' or 'mini -chorus' effect isobtained. This makes a single inputsound like a pair of independent buttime -synchronised outputs. Thus, asingle violin can be made to soundlike a duet and a duet is made tosound like a quartet.

(2). With a reduced level of delayedsignal in the mix, and with a delaytime of tens of milliseconds, a simple

echo effect is obtained. The audiosounds as if it were being played in asoftly furnished room where there isa single hard wall or reflectivesurface, facing the sound source.The apparent size of this room isdirectly proportional to themilliseconds delay time of the echounit, and is fully variable up to 50feet (50 mS delay).

A standard feature of most echo units(including ours) is a Reverb facility. Thisallows a fraction of the output signalfrom the delay line to be fed back andadded to the delay line input, so that youend up getting echoes of the echoes ofechoes. By using only small amounts offeedback (often called 'Recirculation' or'Regeneration' on commercial units),you get 'soft' reverb or, by ad sing lots of

feedback you get 'hard' reverb. A varietyof impressive effects can be obtainedfrom the reverb facility, as follows:(3). When equal levels of mixing are

used with maximum ( 50 mS) delayand maximum feedback, the soundsseem as if they are being played in alarge hard -faced cave or chamber.The apparent dimensions of this'chamber' can be varied via thedelay -time control, while theapparent 'hardness' of the chambercan be varied by altering either themixing or reverb level controls.Thus, the apparent sounds can bevaried from those of a hard cave, toa small church, or down to a largebut softly furnished lounge.

(4). When equal levels of mixing areused with short (a few mS) delaysand a large amount of feedback, all

34 Hobby Electronics, May 1982

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HE Echo-Reverb





C14 R19

100n 120k
















0 +15V











NOTESD1,D2 ARE 1N4148IC2,6,7 ARE 7411C3,4 ARE TDA1022IC5 IS 4046B

Figure 1. The complete Echo-Reverb circuit.

audio signals sound as if they arebeing played inside a small -diameterhard -faced pipe or drum. Theapparent dimensions of the 'pipe' arevariable via the time -delay controlsand the apparent hardness of the'pipe' is variable via the mixing orreverb controls. The sounds cantherefore be varied from those of,say, a sewer pipe, to a dustbin orbucket!

The CircuitThe principle of the echo-reverb unit isdescribed in How It Works. Audiosignals enter the unit via RV1 and splitinto two paths which are re -unitedagain, near the output, via a low -distortion audio mixer (IC7). One ofthese paths is virtually a direct link fromthe input of the unit (RV1 wiper) to one

input of the IC7 mixer, via level controlRV4. Thus, by varying the delay timeand the setting of RV4, a range ofdifferent echo times and characteristicscan be added to the original audiosignals.

A fraction of the buffered output ofthe delay line can be tapped of via RV3and fed back to the input of the delayline via the IC2 mixer stage. Thisproduces echoes of echoes of echoes,etc ('regeneration' or 'recirculation'),and is the standard characteristic of areverb sound. The quality of the sounddepends on the setting of RV3 (Reverb)and the delay time.

The delay line is formed by IC3 andIC4, a pair of series -connected TDA 1022CCD (Charge -Coupled Device) "bucket -brigade" analogue ICs. They are clockedby a two-phase variable frequency

oscillator formed by IC5, a 4046B phase -locked -loop chip. The TDA 1022s are512 -stage delay lines, so our circuit usesa total of 1024 CCD stages. The delaytime available from these chips is:

D = P x S2

where P is the clock -cycle period and S isthe total number of delay stages in theline. Our prototype is set up so that theclock periods are fully variable (via RV2)from a minimum of 2.5 uS (400 kHz) to amaximum of 60 uS (16.6 kHz), thusgiving a delay range of 1.28 mS to30.7 mS. In practice, however, the delaytimes can be extended to 50 mS byadjusting PR2 to give a maximum clockperiod of 97.6 uS (10.24 kHz) if someclock -signal breakthrough is acceptable r

Hobby Electronics, May 1982 35

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He Echo-Reverb




Figure 2. The view from the component side.









36 Hobby Electronics, May 1982

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HE Echo-Reverb

Parts ListRESISTORS(all Y. W 5% carbon)R1 1k5R2,3 1k2R4,5,11,13,23,24,25 100kR6,21 82kR7 22kR8 2k7R9,16 6k8R10 1k0R12,14 47kR15 7kR17 10kR18, 20 180kR19 120kR22 4k7R26 15k

POTENTIOMETERSRV1 47 linear carbonRV2 100k linear carbonRV3,4 22k linear carbonPR1 4k7 miniature pre-setPR2 IMO miniature pre-setPR3 220k miniature pre-set

CAPACITORSC1 1000u 40V electrolytic (axial)C2 10u 35v tantalumC3 . . . . . . ..... 680n polycarbonateC4,11,12,18,19 220n

polycarbonateC5 150p ceramicC6,9,14 100nC7 330p polystyreneC8,10 100n polyester C280C13 470p ceramicC15,17 220p ceramicC16 33p ceramicC20 luO 35V tantalum

SEMICONDUCTORSIC 1 78L15 voltage regulatorIC2,6,7 ........ . . . . . 741 op -ampIC3,4 TDA1022 bucket bridge

delay lineIC5 4046B CMOS

phase locked loopBR1 50V 1 A bridge rectifierD1,2 1n4148 signal diode


mounting transformerSW1 DPDT miniature mains

rocker switchSW2 SPDT miniature toggle

switchSk1,2 Phono Sockets

Case, PCB, fixing bolts, knobs etc.

BUYUNES page 68



2N 0

NOTESICI 15 78115BR 1 IS W005

E 07177








- 1000u



R3 C3680n


Figure 3. Circuit diagram of the regulated power supply.

on the output signal (see setting -upinstructions, Max Delay Time).

When using CCD delay lines it isimportant that the clock frequency mustbe at least double the maximum audiosignal frequency that will be used. Thedelay line output signal must be wellfiltered to cancel residual clock signalsand the input to the delay line must below-pass filtered, to avoid intermodula-tion problems by ensuring that themaximum input frequency is no higherthan half the clock frequency. With thesepoints in mind, the mixer IC2 with R7and C7 are configured to give a 12dB/octave slope, rolling off at 7kHz atthe front of the delay line. IC6 acts as a12 dB/octave, 15 kHz low-pass filter atthe output of the line.

Final points to note about the circuitare that D1 -D2 -R15 -R16 areconfigured to give a degree of self-limiting on the reverb signals. Thisprotects the delay line against destructivereverb overloads. The entire circuit ispowered from a regulated mains -derived15 volt supply via ICI (Figure 3 above).

ConstructionMost of the circuitry for this project isbuilt on a single PCB, and constructionshould, therefore present very fewproblems. Before you start, however, aword of warning: the circuit includes ahigh frequency clock generator whichtends to produce a fair amount of RFI(Radio Frequency Interference).Consequently, you should build it into ametal box and take lots of care over RFscreening.

Begin construction by fitting the sevenwire links and the PCB -mounting mainstransformer. Then proceed with theassembly of the remaining components,taking the usual care to observecomponent polarities, etc. Use sockets tomount the two delay -line chips (IC3 andIC4) and IC5; handle the chips with care,when fitting them into place.

When the PCB is complete,temporarily wire the unit to all controlpots, switches and sockets, then set-upthe pre-sets.

Setting Up ProcedureThe HE Echo-Reverb unit contains threepre-set pots which must be correctlyadjusted to make the unit fit for use;once these have been set correctlyinitially, they require no furtheradjustment. The pre-sets (PRI , PR3 and

PR2) control the delay line biasing, thedelay line loop gain and the maximumdelay time, respectively. The setting upprocedure is as follows:Delay Line Biasing: With no input signal

present, set all three pre-sets to zero,set SW2 to Echo Only, RV4 (EchoLevel) to maximum and RV2 (Delay)to mid value. Connect a DC volt-meter between the + 15 V line ( + ye)and the wiper of PR 1 ( - ve). Thenadjust PR1 for a reading of precisely5 volts. Remove the meter. Nowconnect an audio (voice or music)signal to the input and check that itcan be played through the unitwithout excessive audible distortionlie the sound never becomes harsh).

Delay Line Loop Gain: With RV2 (Delay)set to mid -range but with RV3(Reveb) and RV4 (Echo Level) set tozero, connect a voice -range (350 Hz- 3k5 Hz) input signal of about 1 Vpeak -to -peak and monitor the outputsignal. Switch SW2 between theNormal and Echo Only positions,adjusting PR3 so that equal outputlevels are obtained in both positons(this test can be done with test gearor simply 'by ear', using a tape ordisc signal source). When thisadjustment is complete, set SW2 tothe Echo Only position. Pass amusic/voice signal through thesystem and use RV2 to check theReverb sound is satisfactory.

Max Delay Time: Set SW2 to Normal,RV3 (Reverb) to zero, RV4 (EchoLevel) to maximum (wiper at zerovolts). Adjust PR2 while monitoringthe output of the unit and note thathigh-pitched tone (whistle) isproduced when PR2 is turned beyonda certain point. Now pass a voicesignal through the unit; note that theecho effect is obtained, then trimPR2 to find a compromise setting atwhich a good delay (echo) isobtained with minimum acceptableintrusion from the 'whistle' sound.Finally, check that the delay can bevaried over a wide range (roughly 2mS to 50 mS) via RV2 and thereverb can be varied with RV3.The setting -up procedure is now

complete and the unit can be cased andmade ready for use, as alreadydescribed.

HEHobby Electronics, May 1982 37

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Part One of our new bi-monthly series following the history of theElectric Society - from Volta to Video.

IF you had to name the great, centralpillar of modern society, the one tech-nological aid it couldn't do without,it would have to be electricity. Coalhad its day back in the 19th Centuryand, though it seems to be having some-thing of a renaissance now, this is onlyas a means of generating electricity.Oil rules the world's economy, but onlybecause so few hands have the power toturn the taps off. Nuclear power isthe coming force we are told, but ithas been coming for the past quarter -century without showing much signof actually arriving. Electricity is thegreat common thread which binds to-gether the various primary sources ofpower. The world might survive an oilembargo, but just try to imagine thedarkness and confusion that would fallon the face of the land if some geniushit upon a way of turning off theelectricity worldwide: no lighting, verylittle heat, (even North Sea gas is

driven by electric pumps), no radio,no TV, no computers, no telephones,no industry and very little agriculture.You only have to look at the few largepartial supply failures of the past twodecades to see the sort of chaos whichwould descend as the generators sloweddown and stopped: the famous NewYork blackout one night in 1965, orthe power cuts of February 1974 duringthe Miner's strike. Or, if you want apractical demonstration, try reading therest of this article by the light of anunpressurised paraffin lamp, which is

the best you would have had in mostEnglish villages until the late 1940s.

Try as they might, no one has comeup with a cheaper, cleaner and moreeasily handled form of power trans-mission for applications other thanroad and air transport. Now, as we gettowards the end of electricity's secondcentury, the silicon chip and the societyit is creating looks set to make us more,rather than less, dependent on themovements of electrons - invisible toall and still rather mysterious to most.

Few people these days, it is true,would go as far as James Thurber'saunt, who used to roam around the

sticking light -bulbs into emptysockets in case the electricity drippingout of them formed dangerous, invisiblepuddles on the floor. Nor are therequite as many people, as there oncewere, who replace fuses with stoutcopper wire saying that they can'tunderstand why folk use that inferiorsilvery stuff you buy in Woolworths!But even if electricity is not regardedby people at large with the same super-stitious horror as nuclear energy(though the author did once see a postersaying "Ban Electricity NOW") it is

still a rather strange and unfathomableforce - which makes you realise justhow strange it must all have seemedwhen the first serious experimentersbegan their work, back at the beginningof the 19th Century.

Electricity has been with us, now, forjust over a century as a public utility.But, it took a hundred years of trial and

house error

Cooke and Wheatstone'sfive -needle telegraph, firstinstalled alongside theEuston -Camden railwayand later used by theGreat Western Railwayfor communicationsbetween Paddington andWest Drayton.

and laborious experimentationbefore that to turn it from a uselessscientific curiosity into the drivingforce of the second Industrial Revolu-tion. That movement, beginning inthe 1880s, turned the USA and Ger-many into great industrial powersbecause they hitched themselves to itbut began Britain's long, slow declinebecause she took it up only half-heartedly and late. The century before1880 produced some remarkably far-sighted inventions, projects far aheadof their time like Davidson's electriclocomotive or the transatlantic tele-graph of 1858. But, on the whole,progress was slow, a hesitant, haphazardclearing of the runway for the greatelectrical take -off at the end of thecentury.

Myth and MagicEver since men first began to wonder

Hobby Electronics, May 1982 39

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Special about the world around them, they hadnoticed some inexplicable happeningswhich could best be put down towitchcraft or angry gods. Lightningwas the most spectacular, of course,but the first Stone Age settlers in SouthAmerica must soon have learnt to steerclear of a certain clumsy -looking, slug-gish river -fish. As early as 321 BC, theGreek natural philosopher Theophrastusremarked on the strange attractiveproperties of amber - though hecouldn't explain them. The Chineseseem to have used lodestone compassesabout 800 AD and, by the 1560s,scientists were familiar enough withlarge artificially -made permanent mag-nets for the surgeon Ambroise Pare tohave had the brainwave of using oneto extract a steel splinter from a

patient's eye. It was, presumably, aboutthis time, as seagoing ships grew larger,that shipwrights learnt the inadvisabilityof using copper fastenings near ironnails. But it was left to Elizabeth l'sCourt Physician, William Gilbert, to tiesome of these phenomena together inhis treatise "De Magnete" of 1600 inwhich he coins the word "electricity",associates static electricity with mag-netism and divines that the Earth is

a huge magnet. It also seems thatGilbert constructed the first recordedstatic generator, direct ancestor of the50 -foot diameter Van der Graafmachine currently used in nuclearresearch at Harwell.

Speculation about the nature ofelectricity - or rather, "the electric-ities", since no one had yet linkedstatic, magnetism and lightning con-clusively together as aspects of thesame thing - went on in a desultoryway for the next two centuries. Itwas not until the last two decadesof the 18th Century that seriousinvestigation was able to begin and theElectric Machine became somethingmore than a toy for amusing theladies at a philosophical societylecture. Franklin made his famouskite -flying experiment to investigatelightning - and his less famous,inspired but incorrect guess at pol-arities, which has bedevilled elec-tronics ever since. In Italy, the twoProfessors Galvani - the frog's legsman - and Count Volta worked awayat the problem of chemical electricityuntil, in 1800, Volta was able to pro-duce his first pile battery. The world'smen of learning now had their firstcontrollable, storable, measurablesource of electricity and the experi-ments began in earnest.

Sir Humphry Davy gave his famousarc -light demonstration in 1809 and,eleven years later, in 1820, ProfessorOersted of Copenhagen made themomentous discovery that a com-pass needle could be deflected by anelectric current passing through a

Part of the Gausssystem.

wire. It was left to Michael Faraday,in the summer and autumn of 1831,to make the imaginative leap bystanding Oersted's 'current + wire +magnet + movement = current'. Itmay seem obvious, now, but it was astroke of genius at the time andFaraday's demonstration of a crudemagneto -electric generator to theRoyal Institution on 24 November1831 marks the birth of the electronicage.

The child was born, perhaps, but itwas remarkably slow a -growing duringthe half -century after Faraday's dis-covery. An increasingly powerfulrange of magneto generators werebuilt in Europe and the USA duringthe 1830s and 1840s, but the troublewas that nobody really had much ideaof what to do with the current theyproduced. Arc lighting developedslowly during the middle decades ofthe century and a breed of wanderingelectricity generators grew up (SamuelCrompton was one of the first), roam-ing Europe with mobile steam -drivenapparatus to illuminate railway tunnel-ling work, circuses and fairgrounds.But arc lighting was too harsh and tooexpensive to rival colza oil or keroseneas a source of domestic illumination.A few avant-garde scrap -metal dealersinvested in electro-magnets for liftingand Birmingham factory proprietorsfound electro-plating an excellent wayof poshing up cheap and nasty table-ware, while rheumatism sufferers weremade momentarily forgetful of theiraches by the even worse pains resultingfrom patent Therapeutic ElectricalMachines (which could still be seenat country fairs in the 1930s) Ofcourse, the demand for lighthousespushed forward the development oflarge generators, like Clarkes 2 kWmachine which supplied the one at theSouth Foreland, near Dover, in 1858,but it was all a primitive, haphazardbusiness, this early public electricitysupply.

The men of science were still onlybeginning to grope their way towardsan understanding of this mysteriousforce, so it is hardly surprising thattheoretical knowledge among theearly practitioners was sketchy in the

and Weber telegraph

extreme. It was not until the 1860s,for instance, that any idea began toemerge of there being a distinctionbetween force and current - thepower of the early generators wasreckoned by the size of the spark, orthe loudness of the howls, which theyproduced. Power requirements werejudged, rule -of -thumb fashion, by thenumber of acid -cell batteries neededto make the thing work. Ammeters,where they existed, were simply cali-brated by the man who built themwithout any reference to a commonscale. As we have seen, it took a verylong time to dispose of the notionof several different, independent kindsof electricity. Electricity supply wasplagued well into the 1890s by a

persistent folk -belief that directcurrent was not only less dangerousthan alternating but, in some curiousway, a different kind of power al-together, a belief which led to all theearly generators being fitted withcommutators to convert their outputto DC. In fact, some historians goso far as to say that the obsession withDC over AC held back the develop-ment of industry by a good half -century.

All the same, there were someremarkably far-sighted experimentsmade in these early days, projects likeDavidson's battery -powered electricrailway locomotive of 1842 whichmanaged to reach 40 mph on a runbetween Edinburgh and Glasgow, onlyto be smashed up at the end of thejourney by enraged steam-enginedrivers who were frightened that itwould take away their living. Theyneed not have worried, though. OldKing Coal was to remain unchallengedon his throne for another fifty yearsas the main source of power for therailways and factories. It was notuntil the self-taught Serbian inventorNikolai Tesla developed phase -induc-tion, at the end of the 1880s, thatelectric motors were able to startup under their own power and drivethe tramcars which were to do somuch to change the geography ofEuropean and American cities, aroundthe turn of the century.

First LightIf a public electricity supply industrywas to develop, in the way gas haddeveloped early in the 19th Century,it would have to find itself somesimple domestic service which it couldperform more cheaply than any of thealternative forms of energy then onoffer. Domestic lighting was the ob-vious candidate and, throughout thelate 1870s, Crompton and Swan, inBritain, and Edison in the UnitedStates worked feverishly to developa cheap, mass-produceable incan-descent light bulb. They fiddled

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Specialabout with filaments of paper, horse-hair, silk, bamboo and platinum until- just about simultaneously, in theautumn of 1879 - both Swan andEdison hit upon the trick of makinga bulb which would last more thana few hours - exhausting the airin the bulb while the filament washot, in order to get rid of the com-bustion gases. A vicious patents battleensued - Edison was a devil forlitigation - but from 1880 onwards,progress was as rapid as it had beensluggish over the previous fifty years.Edison, being a most unusual com-bination of inventive genius andshrewd businessman, set up his firstpublic supply power station at PearlStreet, New York City, in September1882. It served 85 buildings at firstbut, by the end of 1884, no less than60,000 lamps were connected up tothe supply.

In Britain, things were done morehaphazardly and Godalming, ratherthan London, had the honour ofbeing the first town to go electric,in September 1881. The local gas

company had attempted an earlyOPEC - unilateral price -hike. Thetown council demurred and turned,instead, to a pair of wandering elec-trical entrepreneurs, Messrs. Calderand Barrett, who undertook to lightthe streets from a mobile steam gen-erator and, later on, from an old watermill. The new arrangement was nota technical success. The lights wereconnected in series, like those on aChristmas tree, with the result thatthose nearest the generator werenoticeably brighter than those furtheraway, while a single blown bulbplunged the whole town into dark-ness. Nor was it much better offfinancially and in May 1884, it closeddown, the brothers Siemens (whohad taken over the contract havingbeen unable to find the 500 domesticsubscribers, at £3 per annum, whowere necessary to make the projectbreak even. Later, town supplysystems were more successful. Londongot its Grovesnor Gallery, Holborn andKensington Court Power stations inthe early 1880s, each with an outputof about 60 kW and equipped withstandby batteries to provide off-peakpower in the small hours, so that themachinery could be oiled and cleaned.

Despite gas lighting's new lease oflife, thanks to the invention of theincandescent mantle in 1880, electriclighting caught on rapidly with thebetter -off and from the early 1880sonwards, there appeared a successionof gadgets to make use of this new,clean, convenient form of power:kettles, flatirons, hotplates, andheaters, all ornately decorated in thefashion of the time and all guaranteedto cause today's BEAB safety inspec-

Hobby Electronics, May 1982

tors a collective heart attack. True,with the power supply pitched at 110volts, in most cases, the danger wasn'ttoo great, but a cavalier attitude tosafety lasted well into the 240 voltera, in many countries. Most ofEastern Europe still does withoutearthing, which keeps the fire brigadesbusy, if nothing else, and the Frenchwere long famous for the nonchalancewith which they stapled bare wiresaround the attics of their houses.Only a few years ago, a Britishengineer working on an Anglo-Frenchjoint construction project in the Mid-dle East was watching a French elec-trician wiring up a street lamp. Horri-fied by what he saw, he remarked thatif anyone broke into the controlbox they would be killed for sure."Ah M'sieur" said the electrician,"You do not understand. In France,it is forbid the public to open theseboxes."

AC/DCBy the end of the 1880s in Europeand the USA, demand for electricitywas fast outstripping what the small,city -centre power stations were ableto supply. The answer was to buildout-of-town, but this meant hightransmission voltages, if the methodwas to be economically worthwhile.High voltage meant transformers, tostep it down to domestic supply level,and transformers - unfortunately forEdison and the other champions ofdirect current - meant AC, unlessthey wished to tinker with alternatorsand commutators on either side ofthe iron core. The result was deFerranti's Deptford power station, in1891, which drove 10,000 volts anunprecedented 8 miles along under-ground cables to London's centre.Edison published pamphlets to warnthe public that AC and DC weredifferent things, the former being"fit only to power the electric chair",but the battle was lost, especially asParsons steam turbines came toreplace the inefficient and failure -prone reciprocating engines in thepower stations, making accurate fre-quency regulation possible for thefirst time. However, this all took timeand, in the mid -1920s when theGovernment was trying to unify theelectricity supply system, the UK stillhad over 200 DC town supply systemsplus no less than 17 different ACfrequencies. At the start of theFirst World War, only one Britishhome in ten was on the mains and bythe outbreak of World War Two thefigure had barely reached one house-hold in two.

So it was not in the area of publicservice supply that electricity estab-lished itself in the public conscious-ness during the 19th Century. Al-

though the power used was muchsmaller, it was the telegraph and itsyounger near -cousin, the telephone,that made the Victorian man -in -the -street familiar with electricity andbuilt up the manufacturing andoperating skills which were to bedrawn upon by power engineering atthe end of the century. Where wouldthe electricity supply companies havebeen in the 1880s, for instance, ifthe telegraphs hadn't first inspiredthe factories which specialised incables, insulated with that now -forgotten 19th Century wondermaterial, gutta-percha?

Where electric power supply wasconcerned, the trouble in the early1830s was that, while people knewmore or less how to do it, theycouldn't find any really convincingreason why, a question which wouldbe answered only when Swan andEdison came along with the electriclight bulb. But with the telegraph,things were very different; the needwas there, ready-made, just waitingfor electricity to come its way.

Long Distance InformationTelegraphing information over longdistances had excited a great deal ofinterest since the late 18th Century.In France, a semaphore telegraphsystem, developed by one M. Chappe,had been crucial during 1792-93in enabling the Revolutionary govern-ment in Paris to move its raggle-tagglearmies about the country to beatback the invading Allies. As is so

often the case with inventors, theywere so grateful to M. Chappe after-wards, that the poor man cut histhroat in despair. However, theBritish Admiralty was more appreciativeand soon set up its own chain of tele-graph towers which, on one famousoccasion, got a message to Portsmouthand back in three minutes flat. Butthese telegraph systems were slow,useless in fog or at night and soexpensive to man - one from Warsawto Moscow needed 1,300 men onduty at any one moment - that onlythe most urgent government businesscould justify them. They never reallycaught on though the Admiralty,conservative as ever, kept its sema-phore network going well into the1850s on the ground that fiddlingabout with copper wires was damnedun-seamanlike.

It had been known for some timethat, among its many other properties,electricity travelled at amazing speeds.A French savant had proved as muchin the 1760s, by connecting a circleof 200 Carthusian monks holdinghands to a static generator (what apity that photography had not beeninvented). So it is scarcely surprisingthat, by 1830, some 30-40 different


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Special electric telegraph systems had beeninvented; some were merely ingenious,like Soemmering's electrolytic tele-graph of 1810 (the operator watchedthe lettered electrodes in a glass tankof water to see where the bubblesformed) and some very promising,like the Russian diplomat BaronSchilling's electo-magnetic telegraphof 1825, the first to use a simplecode. What the telegraph needed,in order to become a practicablesystem, was a large-scale non -govern-mental user to furnish the sort of mar-ket which would make it worthwhilefor someone to develop a cheap,fast, simple method. That customermaterialised in the shape of therailways, which were spreading acrossBritain and the Eastern USA in themid -1830s. They had soon found thatthey would need some quick and reli-able method for all-weather, 24 -hourlong-distance signalling if the country-side wasn't to resound with the crunchof colliding locomotives. They alsoneeded (people often forget) commontime for their trains to run to, some-thing which simply hadn't existed inthe pre -telegraph days when everytime for their trains to run to, some-thing which simply hadn't existed inthe pre -telegraph days when everytown set its clocks to its own midday,with the result that noon at Plymouthwas 12.20 GMT. In Britain, theanswer was provided by an invalidedIndian Army subaltern and half-hearted medical student namedWilliam Cooke, who saw Schilling'stelegraph at Munich and rushedhome to team up with the scientistCharles Wheatstone in developingan electric telegraph to sell to therailways.

Though slow by later standards,their alphabetic needle -deflection app-aratus could be used by an untrainedoperator. It was installed betweenEuston and Camden stations, in June1937, and two years later a line wasrigged up alongside the Great WesternRailway between Paddington and WestDrayton. Cooke and Wheatstone soonparted company amid great bitterness,but the idea of the electric telegraphremained, first coming into the head-lines in January 1845, when a notoriousmurderer was arrested at Paddingtonafter a message from the booking officeat Slough station - "hanged by a

copper wire" they said. The realbreakthrough came, also in 1845,when the minor portrait painterSamuel Morse invented his code -basedkey telegraph which, in the end, didas much to open up the AmericanWest as the railways themselves (thoughyou can't help wondering whetherall the operators did, in fact, weareyeshields and sleeve -bands as the TVWesterns portray them).

Baron Schilling's magnetic needledetector.

first telegraph line was laidacross the English Channel at theend of August 1850. So, the age ofinternational telecommunications began- though it was cut off rather abruptlythe next morning, when a fishermanhauled up the fragile unarmoured wirethat was fouling his anchor and hackedoff a length of this queer, gold-filledseaweed for a souvenir. Later attemptswere more successful and in 1855, theline laid beneath the Black Sea from theAllied HQ in the Crimea to link upwith the overland cable at Constantin-ople provided dramatic proof of theadvantages of electric telegraphy. TheBritish could get a message from Seb-astopol to London in a couple of hourswhile the War Ministry in St. Petersburghad to wait weeks for news to come onhorseback, across the steppes. In factit is said that they ordered the London"Times" to find out what was goingon at the front. It was only a matterof time before someone attempteda transatlantic link and, after severalfrustrating failures, the American entre-preneur Cyrus Field succeeded in layinga cable between Ireland and New-foundland in August 1858 - almosta miracle in view of the primitivetechnology and crude theoretical know-ledge available to him. The link failedafter less than a month of garbled,fitful operation, probably because theproject's scientific adviser had insistedon using 2.000 volt spark coils to tryto ram messages along the thin wire,thus burning out the insulation.

Further attempts were made, as

business got back to normal after theAmerican Civil War, and success wasfinally and permanently achieved on27 July 1866, when the end of thecable was brought ashore from thevast, redundant passenger vessel the"Great Eastern" - the only ship in theworld large enough to carry the amount


of cable necessary for the job. Fromthen on, it was merely a question ofextending the telegraph around theworld. Technical improvements werefew, until the advent of FrederickCreed and the telex machine in theearly 1900s. In 1902, Creed foundtelegraph paper -tape messages beingprepared, exactly as they had been in1860, by the operator pouding threemetal plungers with a rubber -tippedstick held in each fist, a process whichled to a crippling occupational illnesscalled Telegraphist's Wrist.

Dirty DeedsThe possibilities of the copper wirewere not exhausted (for the timebeing) until the late 1870s when twoinventors arrived - once again simul-taneously - at a solution to theproblem of transmitting human speechdown the wire. This time it was a realphoto -finish with Alexander GrahamBell and Elisha Gray registering theirreed-and-electro magnet telephonesystems at the US Patents Office withinhours of each other, on 14 February1876. The result was a long and nastylawsuit which was only resolved by theindefagitable Thomas Edison stealingboth partys clothes with his carbondiaphragm mouthpiece, the ancestor ofthe modern telephone. The Strowgerautomatic telephone exchange wasinvented, some twenty years later, bya New York undertaker who had beenbrought to the verge of bankruptcyby an unscrupulous rival, who hadbribed an exchange girl to route callsto Strowger through to his firm instead!With that, the telephone and telegraphsystems had achieved very much theshape they still have, eighty years later,on the verge of the digital telecoms era.

The telegraph brought about drasticchanges in the world. Newspapers, aswe know them today, were built uparound the telegraph office; the "DailyTelegraph" acknowledged as much inits title, when it was founded in 1855.In international relations, crises grewmore difficult to handle, if anything,as the ambassador declined from beinga minor potentate, acting on his owninitiative, into a kind of decorativemonkey at the end of a wire. Theworld grew a smaller place, but it stillcomes as something of a shock to re-member that in September 1914, Ad-miral vonSpees German Pacific squadroncould be warmly greeted and soldsupplies by the French communityin the Marquesas Islands who, beinghundreds of miles from the telegraphlines, were unaware that France andGermany had been at war for nearlytwo months. It was not until wire-less telegraphy came along that instantworldwide communication began tomake the Global Village, and GlobalVillage Idiots, a possibility. HE

42 Hobby Electronics. May 1982

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Fr -SUPER HI -Fl SPEAKERCABINETSMade for an expensive Hi-Fi outfit- will suit any decor. Resonancefree. Cut-outs for 6'h" woofer and2%" tweeter. The front material isDacron. The completed unit is mostpleasing. Supplied in pairs, price£6.90 per pair (this is probably lessthan the original cost of onecabinet) carriage £3.00 the pair.

GOODMANS SPEAKERS6'A" 8 ohm 25 watt £4.50. 2Yx" 8 ohmtweeter. £2.50. No extra for postage ifordered with cabinets. Xover £1.50.

UNIVAC KEYBOARD BARGAIN50 keys, together with 5 miniature toggle switches all mounted ona p.c.b. together with 12 i.e.'s, manytransistors and other parts.£13 50 + £2.00 post.This is far less than the value of theswitches alone. Diagram of this key-board is available separately for £1.

440.0411001.10000011111111.000019111B0000004110110.11Attle01111141100 AN


SOLENOID WITHPLUNGERMains operated £1.9910 - 12 volts DCoperated £1.50.

3 - 30v VARIABLE VOLTAGE POWER SUPPLY UNITWith 1 amp DC output, for use on the bench, students,inventors, service engineers, etc. Automatic short circuitand overload protection. In case with a volt meter on thefront panel. Complete kit £13.80

IONISER KITRefresh your home, office, shop, work room, etc. with anegative ION generator. Makes you feel better andwork harder - complete mains operated kit, case£11.95 post £2.00

MORSE TRAINERComplete kit £2.99.


MAINS POWER SUPPLYGives any voltage from 3v to 16v at up to 300mA.Complete kit less case £1.95. Case 90p.

OUR CAR STARTER AND CHARGER KIT has no doubt savedmany motorists from embarrassment in an emergency you can startcar off mains or bring your battery up to full charge in a couple ofhours. The kit comprises: 250w mains transformer, two 10 ampbridge rectifiers, start/charge switch and full instructions. You canassemble this in the evening, box it up or leave it on the shelf in thegarage whichever suits you best. Price £12.50 + E3.00 post.

BUMPER BARGAIN PARCEL10 kilo of unused parts. Minimum 1000 items. Includes - relays,switches, motors, drills, taps and dies, thermostats, neons, i.f.coils, oscillator coils, variable condensers variable resistors andat least one each of the following: panel meter, timer, thermaltrip, and other expensive items. Individually would cost well overE100. Yours for only £11.50 + £3.00 post.

MILLIONS OF HOMES WILL BE BURGLEDTHIS SUMMER - SAY THE EXPERTSDon't let yours be one of them. Install our burger alarm. Installour burglar alarm. Complete kit includes 6" external alarm bell,mains power unit control box with key switch 10 window/doorswitches 100 yards of wire. With instructions £29.50.

TINIEST MICROPHONENot much bigger than a pea, 600 ohm condenser type. Ideal forbugging and similar applications. 50p each or 10 for £4.50.

LEVEL METERSize approximately Y." square, scaled signaland power but cover easily removable forresealing. Sensitivity 200 uA. 75p.

THERMOSTAT ASSORTMENT10 different thermostats. 7 bi-metal types and 3 liquid types.There are the current stets which will open the switch to protectdevices against overload, short circuits. etc., or when fitted sayin front of the element of a blow heater, the heat would tripthe stat if the blower fuses; appliance stets, one for high temp-eratures, others adjustable over a range of temperatures whichcould include 0 - 100°C. There is also a thermostatic pod whichcan be immersed, an oven stat, a calibrated boiler stat, finally anice stat which, fitted to our waterproof heater element, up in theloft could protect your pipes from freezing. Separately, thesethermostats could cost around £15.00 however, you can havethe parcel for £2.50.

3 CHANNEL SOUND TO LIGHT KITComplete kit ofParts for athree -channelsound to lightunit controll-ing over 2000watts of light-ing. Use thisat home ifyou wish but itis plenty rugged enough far disco work. The unit is housed in anattractive two-tone metal case and has controls for each channel,and a master on/off. The audio input and output are by Y."sockets and three panel mounting fuse holders provide thyristorprotection. A four -pin plug and socket facilitate ease of connect-ing lamps. Special snip.price is £14.95 in kit form or £25.00assembled and tested.

ULTRA SMALL 12v RELAYSSingle pole gold plated contacts. Tubular construction, 17mmlong lOmm dia. Ideal for models. PCB or freemounting. £2.30 ea.

MINAITURE PLUG IN RELAYS12v operated. 3 changeover. £2A5 base 45p.12v operated. 2 changeover. £1.67, base 35p.

THIS MONTH'S SNIPROTARY WAFER SWITCHES5 amp silver plated contacts. Y." shaft. 1" dia. wafer.Single wafer types, 29p each. as follows.1 pole 12 way 2 pole 6 way 3 pole 4 way4 pole 3 way 6 pole 2 way 4 pole 3 wayTwo wafer type, 59p each, as follows2 pole 12 way 4 pole 5 way 4 pole 6 way6 pole 2 way 8 pole 3 way 12 pole 2 way3 wafer types 99p each,3 pole 12 way 6 pole 5 way 6 pole 6 way9 pole 4 way 12p 3 way 18p 2 way

EXTRACTOR FANMains operated - ex -computer5"Woods extractor

£5.75, Post E1.25.5" Plannair extractor

£6.50. Post E1.254" x 4- Muffin 115v

£4.50. Post 75p.4" x 4" Muffin 230v.

£5.75. Post 75p.

8 POWERFULBATTERY MOTORSFor models, rnaccanos,remote control planes, boats,etc. E2.95.

TAPE PUNCH &READER For controlling machinetools, etc, motorised 8 bit punch withmatching tape reader. Ex -computers, be-lieved in good working order, any not sowould be exchanged. £17.50 pair. Post£4.00.

MINI -MULTI TESTER Deluxe pocket size precision mov-ing coil instrument, Jewelled bearings - 2000 o.p.v. mirrored scale.11 instant range measures: DC volts 10, 50, 250, 1000.

AC volts 10, 50, 250, 1000.DC amps 0 - 100 mA.

Continuity and resistance 0 . 1 meg ohms intwo ranges. Complete with test prods and in.struction book showing how to measure cap-acity and inductance as well. Unbelievablevalue at only £6.75 + 60p post and insurance.

FREE Amps range kit to enble you to readDC current from 0 10 amps, directly

on the 0 . 10 scale. It's free if youpurchase quickly, but of you alreadyown a Mini -Tester and would likeone, send £2.50.

12V FLUORESCENT LIGHTINGFor camping - car repairing - emergeny lightingfrom a 12y battery you can't beat fluorescentlighting. It will offer plenty of welldistributed light and iseconomical. Weoffer an inverterfor 21" 13 wattminiature fluores-cent tube. £3.45.(tube not supplied).





TRANSMITTER SURVEILLANCETiny, easily hidden but which will enable conversation to bepicked up with FM radio. Can be made in a matchbox - allelectronic parts and circuit. £2.30. (not licenceable in the U.K.)

RADIO MIKEIdeal for discos and garden parties, allows complete freedom ofmovement. Play through FM radio or tuner amp. £6.90 comp.kit. (not licenceable in the U.K.).

FM RECEIVERMade up and working, complete with scale and pointer needsonly headphones, ideal for use with our surveillance transmitteror radio mike. £5.85.

VENNER TIME SWITCHMains operated with 20 amp switch, oneon and one off per 24 hrs. repeats dailyautomatically correcting for the lengthen-ing or shortening day. An expensive timeswitch but you can have it for only £2.95.These are without case but we can supplya plastic base E1.75 or metal case withwindow £2.95. Also available is adaptorkit to convert this into a normal 24 hr.time switch but with the added advantageof up to 12 on/offs per 24 hrs. This makes

an ideal controllerfor the immersion heater.Price of adaptor kit is £2.30.

STEREO HEADPHONESVery good quality, 8 ohmimpedance, padded, term-inating with standard V."jack -plug £2.99 post 60p.


Wit TIME SWITCH BARGAINLarge clear mains frequency controlledclock, which will always show youthe correct time + start and stop switch-es with dials. Complete with knobs.

J 50

DELAY SWITCHMains operated - delay can be accuratelyset with pointers knob for periods of upto Thhrs. 2 contacts suitable to switch 10amps - second contact opens a few min-utes after 1st contact. £1.95.

MOTORS FOR ROBOTICSIf its a toy robot you are making then one of our eight batterymotors (see centre column) may do. If its a bigger one, however,then see below If still not big enough then enquire -- we havelarger motors but these are usually mains driven.

12v MOTOR BY SMITHSMade for use in cars, these are serieswound and they become more powerful as load increases. Size 3Y," longby 3" dia. These have a good lengthof '4" spindle - price £3.45.Ditto, but double ended £4.25.

EXTRA POWERFUL 12v MOTORMade to work battery lawnmower, this probably develops up to

h.p., so it could be used to power a go-kart or to drive acompressor, etc. etc. £8.90 + £1.50 post.(This is easily reversible with our reversible switch . Price £1.15).

SPIT MOTORSThese are powerful mains operatedinduction motors with gear bonattached. The final shaft is a W. rodwith square hole, so you have altern-ative couplingmethods - final speedis approx. 5 revs/min, price £5.50. -Similar motors with final speeds of80, 100, 160 & 200r .p.m. same price.

REVERSIBLE MOTOR WITH CONTROL GEARMade by the famous Framco Company this is a very robust motor,size approximately 7Y," long, 3'4" dia. 3/8" shaft, Tremendouslypowerful motor, almost impossible to stop. Ideal for operatingstage curtains, sliding doors, ventilators etc., even garage doors ifadequately counter -balanced. We offer the motor complete withcontrol gear as follows:

1 Framco motor with gear box 1 x 100w auto transformer1 manual reversing and on/off switch 2 limit stop switches1 push to start switch 1 circuit diag. of connections.£19.50 plus postage £2.50.

J. BULL (Electrical) Ltd.(Dept. HE), 34 - 36 AMERICA LANE,


MAIL ORDER TERMS: Cash, P.O. or cheque with order. Orders under -I£10, add 60p service charge. Monthly account orders accepted from schoolsand public companies. Access & Barclaycard orders phone Haywards Heath0444) 454563. Bulk Driers: Write for quote. Delivery by return.

Hobby Electronics, May 1981 43

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;.; fr


The final installment looks at adding a cassette deck to a basicsystem.CASSETTE RECORDERS appear in justabout all rack systems, these days, andcontinue to outsell tuners as the mostpopular addition to a record playersystem. Home recording of LPs andsingles (with the appropriate license ofcourse!) is a widespread and popularmethod of enlarging a music collection; inaddition there is the advantage of tapingan LP -length selection of the best tracksof several records or artists. Miss out theboring bits, to order!

Reel-to-reel tape recorders have all butfaded from the home hi-fi scene. Thereare only a handful of models remainingand these tend to be at the 'top -end' ofthe market, with prices in the £500+bracket. They are outside our scope atpresent, as they are an unlikely addition toa first system.

Basic TheoryA music signal is recorded on to tape bythe process of magnetisation. It is mixedwith a high -frequency sinewave, ataround 100-170 kHz, and fed to a coiland magnetic pole -piece arrangement,past which travels a magnetically coatedplastic strip - the tape! The high -frequency signal is referred to as the'bias', while the coil and poles form the'recording head'.

Bias is required to ensure that theamount of magnetisation of the tape par-ticles is directly proportional to the audiosignal strength.

Even so, the music has to be passedthrough an equalisation stage, bothbefore and after recording, to get a 'flat'response. This compensates for the vary-ing sensitivity of the tape at different fre-quencies and - in theory, anyway - theequalisation circuitry produces the sameresult in all tape recorders, regardless ofmanufacture. Tapes recorded on onemachine should be able to be replayedsatisfactorily on another!

Years ago, the equalisation was set toproduce large amounts of bass cut (7 dBat 50 Hz) but, with the ever increasingquality of tape and improved noise reduc-tion systems, this has been substantiallymodified to suit the new, better qualitytapes.

Biased ViewToo little bias, for a given tape, results inlacking of frequency extremes ,ie bassand treble. As the amount of bias is in-creased, treble response improves rapidlyfollowed by the bass, some dBs of biasstrength later. Unfortunately, there is nota single level which optimises both HF andLF. To compound matters still further,distortion and some noise effects are alsohighly dependent upon bias level.

A compromise level is usuallyselected, but this still results in a con-siderably reduced HF level, a problemwhich is overcome by boosting the topend of the signal before recording (pre -equalisation).

Reducing NoisePartially because of this high frequencyboost, signal-to-noise levels lie, howmuch louder the music appears above theunwanted hiss and hum, etc) would beunacceptable by hi-fi standards withoutsome method to control tape hiss.

The most popular system is the Dolby'B' noise reduction system, which is to befound on 99% of all cassette machinessold today. It operates by 'sloping' thefrequency response to a degree depen-dent upon the signal level, so that the tre-ble signal is boosted still further beforerecording and and then cut by exactly thesame amount upon replay. Sinde thenoise added during recording is nearly allabove 5 kHz, the cut in response uponreplay will not only balance the boost inthe recorded signal, but will cut down theadded noise by the same amount -usually around 10 dB. The difference is

r i±i

dramatic, and sufficient to take thecassette medium to an acceptable noiseperformance.

A recent development is the Dolby CSystem which is claimed to not only im-prove noise performance still further, butto also increase the dynamic range (thedifference between the loudest and thesoftest signal replayable) of the recordingmachine.

Of late, the dbx company have beenpushing their 'compressor' method ofreducing noise on tape quite hard andhave succeeded in having it adopted bysome professional machine manufac-turers. The system operates differently tothe Dolby variations, in that it com-presses, or squashes down, the entiresignal before recording, considerablyreducing the variations in signal level.Upon replay the original differences arerestored to the signal (expansion) oncemore pushing tape noise down to irrele-vant levels. Much greater reduction innoise is possible with dbx, but it continuesto have problems producing a playback assmooth as that which went into therecord head!

There have been variations upon thesethemes; Philips and JVC have both pro-duced their own noise reductions sytemsneither of which are compatible withDolby B. Interestingly, the JVC machinecarried the Dolby system alongside theirown 'ANRS' system.

Performance ParametersThere are a fair number of specificationsconnected with tape machines and veryfew are irrelevant. Some are accorded animportance they do not actually possess- much in the same manner that a par-ticular feature of an amplifier, operatingclass for example, can be made into anadvertising lead story for no other reasonthan that of convenience. We will be tak-ing a brief look at the specs concernedwith performance and working through

44 Hobby Electronics, May 1982

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4540353025 -20 -15-10

5 -1 I

10 100 1K 10K


Figure 1. The frequency responsegenerated by a tape head. This is thecurve produced by recording a set offrequencies which have identicallevels. As you can see, they don't staythat way!



100K010 II.

100 1K


Figure 2. This is the equalising responserequired by the tape pre -amp circuitryto compensate for the curve of Figure 1.This will also help reduce the noisegenerated by the machine.

10K 100K


_ A_010100 1K 10K 100K


Figure 3. The result of combining theresponses of Figures 1 and 2. A muchmore linear signal is recorded ontothe tape.

the pros and cons of the different tapetypes on the market.

Ratios of noise: With whatever noise -reducer it has operating, a cassettemachine should be able to better 65 dB(weighted) signal-to-noise ratio. Whathappens with the noise reduction outis really irrelevant, unless you happento enjoy hiss!

Responding to frequencies: As thelimits of human hearing are approx-imately 20 Hz -20 kHz, it is the aim ofall hi-fi equipment to reproduce thisrange as faithfully as possible. Earlycassette machines and tapes couldhardly better 10 kHz as an upper limitof performance. Nakamichi took theart to 20 kHz before anyone else, buthave since been followed beyond 1 5kHz by the majority of manufacturerswith metal and/or C,02 tapes. A goodtape deck, these days, should achieve15 kHz without too much strain.When choosing a tape deck, pay par-ticular attention to the manner inwhich it replays the extremes of thefrequency specturm; all too often, anincorrectly adjusted machine will pro-duce an uneven bass end.

Speedy Variations: Speed control ofthe tape itself is obviously of prime im-portance. Wow and flutter will be asnoticeable on cassette machines as itis on record decks. On all but the most

'budget of budget models there shouldbe no audible speed variations. As longas the peak W + F lies below 0.2%, allwill be well. (Wow is the term appliedto long-term speed charges and flutterrefers to short-term, rapid oscillationsin speed). Some of the better (ie higherpriced) decks produce figures in theorder of 0.05%, some four times bet-ter than the acceptable threshold.Distorting the Truth: There are twotypes of distortion produced by acassette deck; that generated by theelectronics, and the odd harmonicdistortions produced by the act ofrecording onto the tape itself - someintermodulation distortion (one fre-quency interfering with another) isalso added to an signal by the processof recording. The electronics will addsomething of its own but, if thespecified input levels are kept to, thenthe additions will be minimal. If theyare exceeded, then, just like the free-standing amplifier, the deck will 'clip'the tops off the incoming waveform,distorting it badly. Acceptable totaldistortion figures - bearing in mindthat results will vary with differenttypes of tape - are anything less than1 %. Considerably more than foramplifiers, is it not? With the correcttape, and if the machine is set upperfectly, it is possible to better 0.2 %,but it's a struggle!

Non -Facile FacilitiesOf all the many, many facilities you will beoffered on decks these days, which areworth having? Not many, but these areworth consideration.

Variable bias: The most useful add-onyou can have. It must be backed -upwith test oscillators, which allow youto set up the correct level of bias forgood results at both low and high fre-quencies. Setting up the correct biaswill optimise noise, frequency res-ponse and distortion; simple switchingfor different tape types is not reallysufficient, any longer.

Peak metering: Or as near to peak as youcan get, anyway. It is imperative forbest quality results that the metersread as close to signal peaks as theycan. Those parts of the signal whichexceed the recommended maximumrecording level will be grossly distortedand heavily compressed. Not good forthe aurals. Good metering is expensiveso if you are serious about making yourown tapes, look closely at what eachmachine in your price range offers.Don't assume that just because thedisplay is electronic ,ie LCD, or LED,that the meters are brilliant. The cir-cuitry behind the displays might not beset up to peak -read.

Three heads: Are not necessarily better





Figure 4. Without bias, distorion willocurr when signals are recorded (above).Using high -frequency bias shifts thesignal into a more linear region of themagnetisation curve. Result: less distortion.










1 kHz

10 kHz


Figure 5. Bias level has a critical point,beyond which, high frequencies areattenuated. Reduction of distortion hasto be balanced against loss of treble.

Hobby Electronics, May 1982 45

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Scaling The Hi-Fi Heights




r -






H -






0 >I I

L _J L _J







Figure 6. Block diagram of the Dolby C noise reduction system. The top circuit is the encoder, the lower is the decoder.

than two. Without the third headmonitoring what is actually beingrecorded on to the tape is impossible.The importance of this, however,depends upon your personal taste;it's nice to have, but unlessyou're making your own music, notessential. Don't spend on the extrapeice of metal unless the rest of thepackage is right for you.

Two Speeds: Some decks are now offering another speed in addition to theuniversal 1'/8 ips. Not all manufac-turers offer the same alternative,however. The excellent, but nowdeceased, Elcaset ran at 33/4 ips andobtained higher performance as aresult. There are now machines to bebought which offer 1 5/ 1 6 ips - butwith reduced performance, naturally.Their main usage is for backgroundmusic

MPX Filters: FM radios employ a tone at1 9 kHz and another at 38 kHz as partof their method of operation. Theseshould be filtered out at the tuneritself, but might not be completelyremoved from the signal presented tothe tape deck. As the bias signal is alsoa high frequency tone, if the two are inany way connected ,ie one a wholenumber multiple of the other, they willinteract and 'beat' like two tuningforks. An MPX filter is there to removethese multiplex (MPX) tones, ensuringclean recording. With the advancedstate of tuner technology today, theMPX filter is probably a bit redundant,although it is good insurance to haveone somewhere on the front panel,should you wish to make tapes off -air.

Tape TypesThere are four catagories of tapesavailable for cassette machines,nowadays and even manufacturers arebeginning to refer to them as "groups

I -IV". Bias controls can thus be labelled1 -4 to avoid any possible confusion. Tapepackaging should carry either a bias/equalisation setting or a simple "GroupX" indication. The four types are: -

(i) Ferric (or normal)(ii) Chromium dioxide

(iii) Ferri -Chrome (or FeCr)(iv) Metal (or iron)

Taking the four in order, lets have a look atthe pros and cons of the different formula-tions.Group 1: Ferric Oxide. These were the

first tapes to be released and were setup by the Philips specification forequalisation, etc. A fairly severe bassroll -off was employed; the equalisa-tion time constant associated with thiswas 1 20 uS. Setting the equalisationswitch to '1' or 'Normal' achieves thisfigure.Advantages: the tapes are cheap,widely available and compatible withcheap portable recorders.Disadvantages: limited output levels,high noise (comparitively) and high fre-quency compression.Generally not recommended for hi-fiusage.

Group 2: Chromium Dioxide. Introducedto overcome the drawbacks of ferricformulations, C,02 allowed a reductionin replay boost, thus cutting downperceived hiss considerably. The maindrawback to chrome tapes is their low'headroom' at mid -range frequencies- they will not take a high recordinglevel at all; the particles 'saturate' andrefuse to accept further magnetisa-tion. Around 5 dB more bias must beused with C,02 tapes, applied by swit-ching to position '2'. For several otherreasons, their manufacture is beingdiscontinued by some manufacturers;chrome tapes may soon be seen as ashort term answer, since superseded.

Some manufacturers have producedtapes which are basically ferric oxide,in chemical make-up, but are designedfor use at the higher bias and differentequalisation settings of Group 2 tapes.

Group 3: Ferrichrome. Since ferric oxidehad problems and chrome tapes hadfailed to solve them, some bright sparkhad the idea of producing a tape usinglayers of both ferric and chrome - Fer-richrome! The idea was to overcomethe need for excessive bias levels andprovide a better noise performance.Ferric bias and Chrome equalisationsettings were recommended, butquite a few machines have no position'3' - use half of '1 ' and half of '2'! Im-proved noise performance over ferricwas achieved and some newer deckswork well with these formulations.Disadvantages: Increased mid -rangedistortions and higher cost.

Group 4: Metal. Iron is the most easilymagnetised material of all and it follows,therefore, that if you could coat a tapewith iron filings, it would hold a musicsignal beautifully! Group 4 tapes are anattempt to gain the maximum possibleperformance by using this best of allmaterials. Very high bias currents are re-quired to imprint the signal on to the ironcoating of the material - some 9 dB morethan for normal tapes, in fact. This has ledto improvements in recording headtechnology, simply to accept all this extrabias signal. The early metal decks gave lit-tle, if any, improvement in replay qualityover Ferrichrome, but this has changeddramatically recently and the latestmodels offer superlative performancewith Group 4 formulations. Immediateadvantages are an improved high fre-quency response, less noise and cleanermid -range with lower distortion.Disadvantages: Cost, cost and morecost. Very expensive approximation toperfection, this. Try a couple on yourmachine before buying in any numbers.

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Scaling The Hi-Fi Heights

Some Model ModelsOnce more, we round off by offering ashopping list of units with which we havehad some experience. All the decks listedherein have been tested by us at somestage and found to offer good value formoney and high performance. All areworth a look.

Under £100Pioneer CT -320Aiwa AD3100

£100 - £200Technics RSM250Pioneer CT -200Trio KX-800

£200 - £300Sony TCK81Nakamichi N480

ExpensiveAlpage AL300Nakamichi N1000XLNakamichi N681Z


18dB/OCT Ed























Figure 7. Dolby C actually operates in two stages. At no time is the signal processedby more than a single stage. Combined, the two stages produce 20 dB noise reduction.

30Hz 100k H, 300U,
















100k Hz


18dB/ 18dB/OCT OCT












150H, 30H2 10kHz 30Hz 10kHz 1506,



Figure 8. The dbx Type II noise reduction system in block diagram form. Although shown being used for a tape recorder,the system is equally applicable to disc.


Figure 9. By introducing 2:1 compression when recording --and accurately reversing the process on playback, the dbxsystem can reproduce almost the original dynamicrange of the signal. Tape noise is also reduced, as a by-product!






913213 45t/1B




HEHobby Electronics, May 1982 47

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-- = ---


Micro -processoruniversalTimerThis incredibly versatileprogrammable timer cancontrol up to 20 functionsat accurately timedintervals over a periodof a week. Originallydeveloped for industrialand laboratory use itoffers many interestingand excitingpossibilities for theamateur constructor.Based on a pre-programmedTMS 1000 Microprocessor,the unit provides a 24hour clock with fourindependent relaycontrolled outputs witha programmable period of one week. Up to 20 daily or weekly programmablefunctions can be set via a keyboard. Any of the timer functions can beassigned to control any one of the four relay outputs thus providingalmost unlimited programming possibilities.No previous experience of microprocessor programming is necessary sincethe manual explains all the possible operations, clearly and simply,enabling the inexperienced user to be fully conversant within onehour. Completed programme steps are indicated by LED'sThe kit comes complete with printed panel and may be installed eitheras a 'built-in' or a 'free-standing' unit. A stabilised power supply mountedon a separate printed circuit board is supplied with the unit. It requiresthe addition of a 12V, 1A transformer. There is space on the board forup to four output control relays. One is supplied with the kit. Further relaysmaybe ordered separately as required. Price: (excluding wooden housing asillustrated) £48.37 inclusive of VAT and DELIVERED FREE on U.K. mainland.

APPLICATIONSThe programmable timer can provide central control of domesticelectrical cooking, heating and entertainment equipment.The possibilities are limited only by the imagination of the user.Control of house lighting to discourage intruders; control of TV oraudio equipment; sound or video recording control; automatic plantwatering; automatic pet doors or feeding-area few simple examples.For the professional or industrial user many uses in this area of processcontrol will be found.TECHNICAL DATA:Power supplyMounted on separate pcb with space for up to four outputcontrol relays. Requires 12V/1A transformer.

CONTROL SWITCHING:Standard relays lone supplied with kill will switch 2A.Additional relays may be ordered separately.

National relay, order no. HT 12VSiemans relay, order no. R1 INV12


DISPLAYS:12mm 7 segment LED numerical display. LEDprogramme function indicators



THE VELLEMAN KIT RANGE2.2 Watt mini amplifierMono VU using LED's7 Watt amplifierDimmer 1000 WattDimmer 1000 Wattldeparasite)High precision stopwatchMicroprocessor Universal timer20 Watt monolithic amplifierFM oscillatorStereo VU using LED'sUniversal mono preamplifier60 Watt power amplifierPower supply 1 AmpPower supply for stereo 60 Watt amplifierRunning lightDigital panel meterSingle digit counterTransistor ignitionComplex sound generator50 Hz crystal base4 channel infra red remote control (transmitter

or receiver)Infra -red detection system (transmitter or

receiver)Central alarm unitFM stereo decoderHigh quality FM tunerDigital frequency counter for receiversCB power supply 3.5 Amp 12VDigital thermometerFM stereo receiver (19 in. rack -mounting)2 channel infra -red remote control light

dimmer (transmitter or receiver)Infra -red receiver for tuner K2558Infra -red transmitter for tuner K2558Tape/slide synchronizer3 channel coloured light organ20 cm display (common anode)20 cm display (common cathode)Three tone bell5-14V DC 1 Amp Universal power supplyLight computerUniversal stereo pre -amplifierStereo RIAA corrector amplifierUniversal 4 digit up/down counter with

comparatorMicroprocessor doorbell with 25 tunes40 Watt audio amplifierElectric drill speed controlMicroprocessor -controlled EPROM

programmer (kit form)Microprocessor -controlled EPROM

programmer (built and tested)Universal start/stop timer

Repair Service available (for anominal charge) if your solderingtechnique is not quite what itshould be!

Any technical enquiries welcomed-in writing-and will be answeredpromptly by letter.


VELLEMON OKLimitedP.O. Box 30, St. Leonards-on-Sea, East Sussex TN37 7NL Tel: Hastings 10424) 753246

Please send me your free catalogue of Velleman electronic kits:





Hobby Electronics, May 1981

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range EVEL

AudioSignal Generator

R.A. Penfold

A useful and inexpensive addition to your workshop.

IF you are interested in building,servicing, or perhaps even designingaudio frequency (AF) equipment of anykind, you are unlikely to get very farwithout the aid of a few basic items oftest equipment. One of the mostessential is a signal generator. For mostaudio tests and measurements, what isrequired is a good quality sinewavesignal at any frequency within the audiospectrum. It should provide an outputlevel that can be adjusted from zero toat least 1 volt RMS and that does notchange significantly with alterations ofthe output frequency.

Our design fulfills theserequirements, as it covers a frequencyrange of approximately 20 Hertz to 30kHz in three ranges (20 Hz to 300 Hz,200 Hz to 3 kHz, and 2 kHz to 30 kHz)and has a built-in stabilisation circuitthat allows no significant change in theoutput level over this range. It isimportant that the output should bereasonably pure sinewave since anydistortion of the waveshape mayproduce unwanted frequencies at theoutput. These could result in misleadingreadings when making frequencyresponse tests and similarmeasurements. The total noise anddistortion on the output of our design isless than 0.05%, which is good enoughfor any normal audio testing.

The unit has a maximum output levelof about 1.6 volts RMS and this iscontinuously variable down to zero bymeans of a potentiometer and a threeposition switched attenuator. The lowoutput impedance of the unit helps to

maintain the output level under highload conditions.

The CircuitThe full circuit diagram of the unit isshown in Figure 1. It is built around anaudio power amplifier IC and, althoughthis may seem an unusual choice, it isactually ideal for this for this applicationsince it gives low levels of noise anddistortion plus a low output impedance,so that loading of the output is unlikelyto cause a significant reduction in the

The internal view - not much of it, is there?

output level or an increase in distortion.The IC employed in the circuit is the

TDA2006 which is similar to anordinary operational amplifier IC, but hasa high power output stage. This enablesit to be readily used in a Wien Bridgeoscillator, the configuration normallyused in high quality sinewavegenerators. Like many operationalamplifier circuits this one has dualbalanced supplies, provided by two 9volt batteries.

Hobby Electronics, May 1982 49

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Audio Signal Generator

A Wien bridge oscillator uses twocapacitors and two resistors to providezero phase shift at the operatingfrequency of the circuit, butappreciable phase shift at all otherfrequencies. The Wien network,therefore, gives maximum positivefeedback at its operating frequencyand, provided the amplifier hassufficient voltage gain, oscillationoccurs at this frequency. The lossthrough Wien network, at its operatingfrequency, is about 10 dB (threetimes), so the amplifier needs to havesome voltage gain to sustainoscillation.

In this circuit the two capacitors inthe Wien network are whichever twoare switched into circuit by SW1; theuse of three switched sets of capacitorsgives the unit three ranges. Oneresistance of the Wien network isformed by R1 and RV in series, whilethe other is formed by R3 and RV1 b. Bymaking the two resistances adjustable,the unit can be tuned over each of thethree frequency ranges. The operatingfrequency of a Wien network,incidentally, is fo = 1/2 RC.

The voltage gain provided by IC1 iscontrolled by TH1 and R2, which form asimple negative feedback network; thevoltage gain of the circuit is equal to theresistance of TH1 plus R2, divided bythe resistance of R2. TH1 is a selfheating, negative temperaturecoefficient thermistor. This simplymeans that it is enclosed in a glassenvelope (in a vacuum) so that it isisolated from the surroundingtemperature. It is intended to respond tothe heating effect of a current passedthrough it, with a rise in temperaturegiving a reduction in resistance.

At switch -on TH1 will be 'cold' andwill have a high resistance, giving IC1 ahigh voltage gain. This results in thecircuit oscillating very strongly and afairly high current fed through TH1 andR2 from the output. This current has astrong heating effect, so that theresistance of TH1 drops sharply; ICI'svoltage gain is reduced and the circuitoscillates gently, but reliably, giving agood output waveform. If loading on theoutput, or some other factor, shouldcause a reduction in the amplitude ofthe signal at the output, the currentthrough TH1 will be reduced slightly, itsresistance increases and the outputsignal level will be restored to its originallevel. If the output level should increasefor some reason, the opposite effectoccurs; the current through TH1 rises,the voltage gain of IC1 is reduced andthe output signal level falls back downagain. Thus the thermistor AGC circuitensures a pure output signal of constantamplitude. The output of IC1 is coupledby C8 to variable attenuator (RV2) andthen to a conventional three stepswitched attenuator attenuation factorsof 0, 20, and 40 dB. Although thisattenuator might seem to be excessive,with VR2 in the circuit, it is in factuseful for reducing the output levelwhen a very low amplitude signal isrequired - adjustments using RV2alone would otherwise become verycritical and difficult!

100. 3



C2 C3 C46n8 68n 680n


RI R28208 6808





R3 Sib8208


TH1 RA53



R4 R5 R61k 100R 11R




SKI 1:],,OUT


Figure 1. The circuit diagram.



B2 9V


ConstructionStart by cutting the stripboard to size(13 strips by 24 holes) using ahacksaw, and drill the two mountingholes to take 6BA or M3 fixings (about3.2 mm in diameter in either case).Next, solder the components into place(there are no breaks in the copper strips)following the component layout shownin Figure 2. TH1 is mounted horizontally

on the board and should be treatedreasonably carefully as it has a glassencapsulation. Splay the leadout wiresof IC1 slightly to compensate for adifferent pitch to that of the board.Make sure C8 is connected the rightway round.

Now drill the front panel of the caseto take the five controls and outputsocket. We used a sloping front casebut you can of course, use an ordinary


B2-B1 +







1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24



0 0 00 0 00 0 000 0 0 00 0 0 00 00 00







0 7 R2


0 0 0 0 0 0 0 0 0 0 0




R3 0

0 0000

















M 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 00

0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 00 00 00 0 0 00 00 0 0 00 0 0 00 0 0 0 0 0 00 0 00 0 0 0 0 0 00 0 0 0 0 0 0 0 0 00 0

0 0 0 0 0 0 0 00 0 0 0 0 00 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 k 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 O 0 00 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0

0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 00 00 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 00 0 0 0

0 000 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 000 0

0 00 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Figure 2. The component layout (top) and the solder 'dot pattern' (bottom). Note thatthere are no cuts in the Veroboard tracks.

50 Hobby Electronics, May 1982

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Audio Signal Generator

type is you prefer. Mount C2, C3, C4,C5, C6 and C7 and R4, R5 and R6 onSW2, as shown in Figure 3. Thencomplete the rest of the wiring - this isalso illustrated in Figure 3.

A couple of PP3 batteries are used topower the unit, and have a resonablylong operating life. However, the currentdrain from each battery is about 15 mA,and it would be advisable to use largerbatteries (say a couple of PP6s or PP7s)if the unit is likely to receive a great dealof use.

Parts ListRESISTORS(All 1/4 W 5%)R1,3 820RR2 680RR4 1kR5 100RR6 11R

POTENTIOMETERSRV1 10k linear dual gang carbonRV2 1 k linear carbon

CAPACITORSC1 100n polyester (C280)C2,7 6n8 polycarbonateC3,6 68n polycarbonateC4,5 680n polycarbonateC8 100u 10V electrolytic

SEMICONDUCTORIC1 TDA2006 Audio Amplifier

MISCELLANEOUSS1 ,2 . . . 3 way 4 pole rotary switchesS3 Rotary DPST switchB1 ,2 PP3 size batteriesSK1 3 5 mm jack socketTH1 RA 53 ThermistorCase, veroboard, control knobs, twoPP3 type battery connectors, test leads,wire, solder, etc.

BUYLINES page 68

Figure 3 (top). One of the reasons theVeroboard layout is so straightforwardis that most of the components arewired to the panel controls! Follow thediagram carefully, making sure that thewires go to the connection pointsindicated.

Figure 4 (bottom). Templates for thefrequency scale (left) and the fine levelcontrol (right). They are not intended tobe accurate, only to give an indication ofthe frequency or output level. Thefrequency scale is multiplied by one onRange 1, by 10 on Range 2 and by 100on Range 3.

How It WorksThe circuit is built around an audiopower amplifer IC which, as usual, hastwo inputs - an inverting ( - input anda non -inverting (+)input. Positive feed-back is applied from the output to thenon -inverting input and it is made fre-quency selective so that the frequencyof oscillation can be set. However, thecircuit will oscillate only if the amplifierhas sufficient gain. Excessive gain willresult in the output waveform becomingdistorted and producing unwanted fre-quencies, so the gain is maintained atjust the right level by an AGC circuit inthe negative feedback loop from theoutput to the inverting input. A switch-ed attenuator at the output enables theamplitude of the signal to be reduced bya factor of 10 or 100; there is also a'volume control' attenuator whichenables the signal to be continuouslyvaried from zero up to the maximumlevel permitted by the switched at-tenuator.









20 300






H 1


HEHobby Electronics, May 1982 51

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you a MC" X°1Cho latest kit gowneary

lated lone sequence ' CHINEnot a microprocessorco trolled bray or the

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ingrated CirCuitsupplied complete witha punted circuit boardloudspeaker and Ohl,.bob and requires only9V battery and push buttoncommon to most householdsIt may also be switched by logo in suchapplications as car alarms clocks toys.

P A systems etc The unit produces a150.111 MrdPsit and draws less than one tuAfrom a PP3 battery when the lone ceasesSupplied complete with circuit andaSSernbly inStruCtiOn5


REMOTE CONTROLPobt.shed remote coma -systems tend to be guilecomplet requ.rtng&thou. -to -get components and a wellequipped lab loge, them to work If this has putyou oh making your own system we have rustthe tils lot Y. Using infra -red. our KITSrange from simple on r off controllers to codedtransmitted removers with 16 onion outputsor threeanalogue outputs torcontroiling e gTV or Ito -F, systems The kits ate east' to buildand simple to set up-and they are eytternetyversatile controlling anything from garagedopes `PP.ngnhng Net by adding therequored output circuits le relays macs etcII you can design your own system we stock awide range of remote control components atvery compelotive pacesWe have compiled a booklet on remotecont. contaming circuits hints dataSheets and of our remote control totsand components So don I contra yoursed -SENO US 30p and slopped adelteeeed

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DISCO LIGHTING KITSDLICIOOK. This yalUe-for-rhOheykit features a bi-directionalsequence. speed of sequenceand frequency of directionchange. being variable by means of potently.meters and encorporates a master dimmingcontrol13t21000K. A lower cost version of thetab1o4v1r.featuring undirectional channel sequencewith speed variable by means of a pre-set potOutputs switched only at mains zero crossingPoints to reduce eadeo interference to arninirnum Only MOOOptional opto input CAM 60gLAllowing audio I -beat-e- light response.

LEDs3m-& Syron Round Red 09P Green '2P

Clip, 3dini & 5mrn. 03p Yellow 12p

(Shaped Lads)Rectangular I( n ayestune) 16p 16PRectangular [ ]5r 2 5tnni 170 2011&luxe 05.501un 17p 20pTr langular Z...\ 5rnin 1 1p 2011Al rowhecK1 <2.5 x 5Intn 17p 201,

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150°Ct reading to 0.1 °C The basic kit has asensitivity of 200mV for a full scale reading.automatic polarity indtcateon and an ultra lowpower requirement-giving a 2 year typicalbattery life from a standard 9V PP3 Allen used`hours a day. 7 days a week. Price £15.50.A




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52 Hobby Electronics, May 1981

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Feature Famous Names

CHARLES BABBAGECharles Babbage was born in Teign-mouth, South Devon, to wealthy parentswho gave him that most precious gift -a good education, with considerable en-couragement to his mathematicaltalents. These were evident, as is oftenthe case, at a very early age. By the ageof 20 he had founded the AnalyticalSociety, which was intended to en-courage the teaching of mathematics inthis country and, in particular, to makebetter known in Britain the develop-ments in mathematics which were tak-ing place across the Channel. His workearned him a place as a Fellow of theRoyal Society in 1816, making him oneof the youngest men ever to have beenelected as FRS.

By that time, he had started the workwhich was to win him a place in history.In 1812, he had been captivated by thethought that mathematical tables, suchas logarithms, could be calculatedmechanically instead of being laborious-ly computed by hand, as it were. During1813, he constructed a small mechani-cal calculator, similar in principle to thekind of thing that is nowadays sold as atoy, but able to calculate values to eightdecimal places. His work revealedseveral mistakes in earlier sets of tables,and led to a considerable improvement inthe accuracy of engineering calcula-tions.

The Industrial Revolution was accom-panied by a demand for some method ofcoping with more complicated calcula-tions, particularly for solving those pro-blems in mechanical engineering whichwould be very laborious to carry out bytraditional pen -and -paper methods. In1823 Babbage was commissioned bythe Government of the day to constructa mechanical calculator which wouldwork to 20 decimal places, a task whichhe completed despite the formidabledifficulties involved. The problem ofbacklash in gear trains, for example, wasone that plagued any attempt to usemechanical methods for precision equip-ment. Remember that in 1823 the ideaof using electricity for performingcalculations was a century too early. Itwas, after all, in the same decade thatMichael Faraday, demonstrating the firstdynamo, was asked what use couldpossibly be made of it! Don't laugh - theopponents of electricity, then, used ex-actly the same arguments as the op-ponents of nuclear power do now.

By 1828, Babbage's career was suf-ficiently distinguished to ensure hisappointment as Lucasian Professor ofMathematics at Cambridge, a postwhich he held with distinction until1839. It was in 1834 that he embarkedon a scheme which, though doomed tofailure by the inadequate technology ofthe time, was to prove his masterpiece.This was the 'analytical engine' - to us,us, a computer. His design owed a debt

a computer. His design owed a debt to anearlier engineer, Jacquard. Jacquardwas a designer of looms for weavingcomplicated patterns and he had deviseda system of loom control that can only bedescribed as programming. A set of pun-ched cards was fed into a controller andholes in the cards were detected by a setof rods which were then used to selectthe threads for the weaving. The Jac-quard system was fast and accurate,once the program was correctly punch-ed, and gave its users an enormous ad-vantage over their competitors, puttingthousands into work. Anotherdemonstration of the fact that successcomes by chasing new ideas, not bypropping up old ones.

Babbage reasoned that Jacquard'sloom system could also be used to con-trol a calculating machine and he startedwork on a specification, for which hereceived another Government grant. Hisspecification shows that he thoroughlyunderstood all the ideas that we nowdeem essential in computing. He realis-ed, for example, that there could beseveral types of inputs to the machine,which he called number codes (data, aswe now call it), direction codes to controlthe movement of numbers (addressing),and operation codes to determine whatwould be done with a number (opcodes).

His anticipation of modern computingmethods was complete. He made itclear, for example, that an essentialoperation of his analytical engine wouldbe the conditional transfer - what wenow call a branch or jump. The idea, thenas now, was that when the result of anoperation was well defined, like being

zero, negative, or positive, it could beused to decide what the next operationshould be, choosing betwen two or moreoptions. This, undoubtedly, is thefeature that distinguishes the program-mable calculator and the computer fromthe ordinary calculator and there is plen-ty of evidence to show that Babbagewas well aware of its importance.

He also seems to have followed upthe possibility that the results of acalculation could be used to changesome of the operating codes, so that theprogram could modify itself - evennowadays, this is considered as prettyadvanced programming. Babbage'sideas were never very well circulated,and few among his contemporaries hadany idea of what he was aiming at andhow important his work would be. Onenotable exception was Ada, CountessLovelace, daughter of the poet, LordByron. She goes down in history as thefirst computer programmer, because sheseems to have grasped Babbage'sdesigns well enough to write several pro-grams for the machine - even though itwas never completed.

All they needed was technology.There was simply no way thatmechanical methods using gears, withall the problems of backlash, could copewith such advanced designs and elec-tricity was still a curiosity. By 1842, theGovernment, which had beenremarkably patient, withdrew its sup-port and the Babbage computer wasnever completed.

Another attempt to make a Babbagemachine was made by a Swedishmathematician in Stockholm in 1855,but this too had to be abandoned. Bab-bage, disheartened, made no attemptsto follow up his own work and died in1871, in London, without having seenhis dream fulfilled.

His working life was one of continualproblem -solving and invention but fewpeople realise, to this day, the extent ofhis work. It was Babbage who compiledthe first actuarial tables (expectance oflife), on which all life insurance is basedand these tables still give a better pictureof our health than any other evidence.They show, for example, the remarkablerise in life expectancy over the past 100years. (If you think that the simple life is ahealthy one, look up the actuarial tablesfor the 1860s - they make chillingreading; the simple reason for the Vic-torian families of 10 to 20 children wasthe hope that one or two of them mightactually survive to adulthood.) Babbageshould be remembered,as the man whobrought these awful statistics to ourattention. He also invented the cow-catcher, used on US railway locomotivesin the pioneer days, and thespeedometer; but his pioneering efforton computers, rediscovered in 1937,are the greatest monument to his genius. HE

Hobby Electronics, May 1982 53

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GREENWELD443F Millbrook Road, Southampton, SO1 OHX

All prices include VAT at 15% - just add 50p post

LIE DETECTORNot a toy, this precision instrument was originallypart of an "Open University" course, used tomeasure a change in emotional balance, or as a liedetector. Full details of how to use it are given,and a circuit diagram. Supplied complete withprobes, leads and conductive jelly. Needs 2 4Y)batts. Overall size 155 x 100 x 100mm. Only£7.95 - worth that for the case and meter alone!!


PCBs FOR PEANUTS!!A bulk purchase of PCBs from several well-knowncomputer games including Battleships, Simonand Starbird enable us to offer these at incrediblelow prices:

COMPUTER BATTLESHIPSProbably one of the most popular electronicgames on the market. Unfortunately the designmakes it impractical to test the PCB as a workingmodel, although it may well function perfectly.Instead we have tested the sound chip, and sellthe board for its component value: SN76477sound IC; TMS1000 u -processor; Batt clips, R's,C's etc. Size 160 x 140mm. Only E1.50

Instruction book and circuit 30p extra

STARBIRDGives realistic engine sounds and flashing laserblasts - accelerating engine noise when moduleis pointed up, decelerating noise when pointeddown. Press contact to see flash and hear blast oflasers shooting. PCB tested and workingcomplete with speaker and hart clip. (needs PP31.PCB size 130 x 60rnin. Only £2.95

MICROVISION CARTRIDGESrhese are a small PCB with a micro -processorchip, designed to plug in to the rnicrovisionconsole. Only snag is, we don't have anyconsoles!! However, they can be used as anoscillator with 4 different freq. outputs simply byconnecting a battery and speaker. Tested andworking las an osc) with pin out data. PCB size

72 +x 60rnrnONLY 25o eachl!

'SIMON'The object of this game is to repeat correctly alonger and longer sequence of signals in 3different games. (Instructions included) PCBcontains chips, larnphoiders and lamps, and istested working, complete with speaker. NeedsPP3 and 2 x HP11. PCB size 130 x 130mm Only£3.95

LOGIC 5 PANELTested Logic 5 now sold out - but we have somePCB's with 10 LED's and chip on, but no keyboard.Not tested. 50p

OPTO/REGS/OP-AMPSFNA5220 2 digit T2- 7.-seg. display on PCB.CC. With data, £1.507-seg. displays: END360, 367, 501, all 50p;530, 847, 850, all E1.50.Regs, T03 case: 7924 120p, 7885 100p, 7808100p, 7912 100p, 78CB 230p. Others on B/L 13.Op -Amps, uA4136 130p; uA776 145p; uA777300p; uA318 245p.Isolators: FC0831, IL15, TIL118, all 60p.TIL311 Hexadecimal display with decoder, 0-9and A -F. With data, £3.50.

COMPONENT PACKSK503 150 wirewound resistors from 1W to12W, with a good range of values £1.75K505 20 assorted potentiometers, all typesincluding single ganged, rotary and slider

E1.70K511 200 small value poly, mica, ceramic capsfrom a few pF to .02uF. Excellent variety E1.20K514 100 silver mica caps from 5pF to a fewthousand pF. Tolerances from 1./0 to 10% £2

K520 Switch Pack, 20 different, rocker, slide,rotary, toggle, push, micro, etc. Only E2.0521 Heatshrink Pack, 5 different sizes each200mm, 50p

PANELSZ521 Panel with 16236 (2N34421 on smallheatsink, 2N2223 dual transistor, 2 BC108,diodes, caps, resistors, etc. 60p2482 Potted Oscillator Module works from1.20V, can be used as LED flasher 13V min.).Supplied with connection data, suitable FL C &LED ElZ527 Reed relay panel - contains 2 x 6Vreeds, 6 x 2S030 or 2S230, 6 x 400V rests Rs

50pZ529 Pack of ex -computer panels containing74 series iCs. Lots of different gates and com-plex logic. All ICs are marked with type no. orcode for which an identification sheet issupplied. 20 ICs £1; 100 ICs £4.A504 Black case 50 x 50 x 78mrn with octalbase. PCB inside has 24V reed relay, 200V 7ASCR, 4 x 5A 200V rests, etc. 60p

RELAYITRIAC PANELZ537 PCB 1C0 x 75mrn containing a wealth ofcomponents:2 x 12v DPCO min. relays, 2 x 47uf 16v tants,SC146E 10A 5000 triac, C112d 8A 4000 SCR, 555timer, 10 x IN4001 diodes, 2N5061, 2 x 3mm leds3 x 2N3704, also R's and C's.Amazing value!! - if bought separately partswould cost around E80 -- Price for the panel just£2.00.

1W AMP PANELSA011 Compact audio amp intended for recordplayer on panel 95,65mm including volcontrol and switch, complete with knobs.Apart from amp circuitry built aroundLM3BON or TBA820M. there is a speed controlcircuit using 5 transistors. 9V operation,connection data supplied ONLY E1.50.

TOROIDAL TRANSFORMER110mm dia. x 40mm deep. 110/240V pri., sec.18V 4A, 6.3V 1A, 240V 0.3A. Ideal for scopes.monitors, VDUs etc. Reduced to £5.95

1,000 RESISTORS, £2.50We've just purchased another 5 millionpreformed resistors, and can make a similaroffer to that made two years ago, at the sameprice!!! K523 - 1,000 mixed 1/4 and 1/2W 5%carbon film resistors, preformed for PCBmntg. Enormous range of preferred values.1,000 for £2.50; 5,000 E10; 20k E36.

200 ELECTROLYTICS, £4K524 Large variety of values/voltages, mostlycropped leads for PCB mntg. 1 10000E, 10-63V. All new full spec. components, notchuck -outs!! 200 E4; 1,000 E17.50.

FILAMENT DISPLAYSZ653 7 seg display 12.5mm high. Ideal for TTLoperation, taking 5V 8mA per seg. Std 14 DILpackage. Only El each, 4 for £3.00. Data supplied.

RELAY24 ac coil, but works well on 6V DC. 2 x 10A. c/ocontacts Ex -equip, only 60p

REGULATED PSU PANELExclusive Greenwald design, fully variable 0-28V & 20mA-2A. Board contains all compo-nents except pots and transformer. Only£7.75. Suitable transformer and pots £6. SendSAE for fuller details.

ELECTRO-DIALElectrical combination lock - for maximumsecurity - pick proof. 1 million combinations!!Dial is turned to the right to one number, left to asecond number, then right again to a thirdnumber. Only when this has been completed inthe correct sequence will the electrical contactsclose these can be used to operate a relay orsolenoid, Overall dia. 65 x 60mm deep Only f9.95Also available without combination Only E3.95

DEVELOPMENT PACKSThese packs of brand new top quality compo-nents are designed to give the constructor acomplete range so the right value is to handwhenever required. They also give a substan-tial saving over buying individual parts.0001. 50V ceramic plate capacitors, 5%, 10 ofeach value 22pF to 1,000pF, total 210. E4.60.K002 Extended range 22pF to 0.1. Values over1000pF are of a greater tolerance. 10 of eachvalue 22 27 33 39 47 56 68 82 100 120 150 180220 270 330 390 470 560 680 820 1000 15002200 3300 4700 6800 01 .015 .022 .033 .047 .1.PRICE: E7.66.K003 C280 or similar Polyester capacitors, 10each of the following: 01, .015, .022, .033,047, .068, .1, .15, .22, .33 and .47pf PRICE:6.40.K004 Mylar capacitors. Small size, verticalmounting 100V. 10 each of the following:.001, 0012, .0015, 0018, .0022, .0027, 0033,0039..0047, D056, .0068, .0082_01. Total 130capacitors. PRICE: £4.70.O 007 Electrolytic capacitors 25V workingsmall physical size axial or radial leads. 10each of the following: 1, 22, 4.7, 10, 22, 47,100oF. Total 70 capacitors. PRICE: £3.59.00011 Extended range, as above, also includ-ing 220, 470 and 1000oF all at 25V Total of 100capacitors. PRICE: E6.35.K021 CR25 resistors or similar, miniature 1/4watt carbon film 5%, as used in nearly allprotects. 10 of each value from 10 ohms to1M, E 1 2 series. Total 610 resistors, PRICE:E5.95.O 041 Zener diodes 4001nW 5%. 10 of each ofall the values from 2V7 to 36V. Total 280 zen-ners. PRICE: E15.95.0051 LEDs - pack of 60, comprising 10 eachred, green and yellow 3mm and 5mm, to-gether with clips. PRICE: 6.95.

CAPACITOR BARGAINS22000 1005 cans 77 x 35mm dia. 76p;10/E5.50.220uF 10V axial 5p; 100 E2.30; 1000 E16.4000 100uF 275V 102 x 44mm dia. 75p; 106.50.200uF 3504, 100 4 100 + 50uF 300V can. 75 x44mrn dia. 40p; 10/0; 100/020.100uF 25V axial 100/E3.

BECOME ARADIO AMATEURLearn how to become a radio amateur in contactwith the whole world. We give skilled preparation

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PCB HOLDERS PCB's may be rotated through 360 degrees and

locked at any angle Simple adjustment for different board sizes Foam pad available enabling a number of

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CNC 6PCB Capacity: 10" x 7"PCB's held by spring clipswhich also allow the holder tobe used as a small soldering jig.

PRICE: £13.80 INC. VAT

CNC 9PCB Capacity: 8" x 8"PCB's held by adjustable VeeClamps which eliminate the riskof damage to the face sides ofthe PCB and allow a highdegree of accessibility.PRICE: £15.95 INC. VAT

FOAM PADSize: 8" x 8" Clips onto Holder. PRICE: £5.50 INC. VAT



54 Hobby Electronics, May 1981

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LIGHT SEEKERThe warning tonemeans 'Shut thedoor, quickly'!IF you've ever worked with photographicmaterials, you'll know how important itis to ensure that there is no light presentwhen handling sensitised materials (ielight sensitive paper). This is not a pro-blem if you can be there all the time.However, if you need to leave the room, itwould be handly to have some means ofprotecting those valuable films.

One way of doing this is to use adevice which provides an audible toneupon exposure to light - the LightSeeker circuit does this admirably. Infact it will produce a pulsed warbling tonethat increases in frequency depending onthe intensity of light present.

The circuit is a simple combination oftwo unijunction relaxation oscillators,one modulating the other. Relaxationoscillators are the most common ap-plication of unijunction transistorsbecause very few external componentsare needed to get the thing oscillating (asingle resistor and capacitor will do). Inour design there are two oscillators; oneproduces around five pulses per secondand this modulates the second one,which is oscillating at a frequency ofabout 300 Hz rising to 4000 Hz whenbright light is shone on the LDR (lightdependent resistor). The frequency ofoscillation is determined by the resistorand capacitor networks connectedacross the supply lines and to the emitterof each transistor. For instance, the cir-cuit around Q2 is composed of R3,R4and C2. The resistor from b 1 (base one ofQ2) connects to ground via the currentlimiting resistor R4. The other two com-ponents effectively control the frequencyof oscillations. Each time C2 charges upvia R3, there is a point where the emitterof Q2 no longer acts as a high impedanceand the capacitor discharges throughR4. After discharging, b1 is zero voltsand so the emitter again appears as ahigh impedance. This exponentialcharge/discharge cycle repeats at a ratedetermined by the values of R3 and C2and produces a rounded sawtooth out-put. With two such oscillators, the seekerprovides an audible pulsed tone, whosefrequency is dependent on the resistanceof LDR1, which in turn depends on theamount of light present. Resistor R2 actsto couple both oscillators to a degreewhich allows Cl and C2 to fully completetheir respective charge/dischargecycles. Current consumption is about sixmilliamps, and so a PP3 should last quitesome time.


0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0o o() o 0 0 0 o() o 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0

e b2 b1O 0 0



O 0 0 0 0 0 0 0 0 oO 0O O 0 °


0 C2+ 0

R2 0T e n 0



O IR3 bi0 01

Q20 0 0 0 0

RESISTORS(All 3/4 W 5% carbon)R1R2R3R4

Parts List

4k 747k MISCELLANEOUS12k SW1 SPST on/off toggle switch56R LS1 speaker (64R or greater)

B1 PP3 9V Battery

B1 PP3

LDR1 ORP 61light dependent resistor

CAPACITORSCl 47 n polyester C280 PP3 clips, veroboard (11 strips, 11C2 10 u 10 V radial electrolytic holes), case, wire etc.

SEMICONDUCTORSQ1 , Q2 T IS 43 See Buylines (page 68) for availability of

unijunction transistor these components.


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BACKNUMBERSFebruary 1980Passion Meter, Win Indicator,Short Circuit Special, Kit ReviewSpecial, Into ElectronicsConstruction Part 1.

May 1980MiniClocks, 5080 Preamp, ModelRailway Track Cleaner, 5080Loudspeakers, LoudspeakerCrossover Design, RadioControlled Model Survey.

June 1980Microbe Radio Control System,Egg Timer, Two Watt Amplifier,Fog Horn, Short Circuits, LEDsand LED Displays.

July 1980Sound -Operated Flash Trigger,18 + 18 Car Stereo Booster,Hazard Flasher, Electronics inPhotography, ElectronicEspionage, Piezo Electricity.

August 1980EquiTone Car Equaliser, Pass -The -Loop Game, Gaztec GasDetector, OP -Amp Checker, In -Car Entertainment Survey,Introducing Microprocessors.

September 1980MicroMixer, Reaction Tester,Guitar Phaser, DevelopmentTimer, Teletext Explained, IntoDigital Electronics Part 1.

October 1980Kitchen Timer, Tug 'o' WarGame, Light Dimmer, FreezerAlarm, Intruder Alarm,Temperature -ControlledSoldering Iron.

January 1981Car Rev -Counter, BenchAmplifier, Sound -Into -LightConverter, Chuffer, ElectronicGames reviewed.

February 1981Heartbeat Monitor, High -Impedance Voltmeter, MediumWave Radio, Two -Tone TrainHorn, Audio Signal Generator.

March 1981Public Address Amplifier,Windscreen Wiper Controller,Bicycle Speedometer,Photographic Timer,Microcassettes.

August 1981Electronic Ignition,Thermometer, Electronic Organ(final part), RPM Meter, BenchPower Supply, Radio ControlSurvey, Into ElectronicComponents Part 1.

All of the 1980 issues, except January and April, are still available togetherwith the remaining issues from 1981.

April 1981Pre -Amplifier Part 1, SuperSiren, Guitar Tremolo, RussianRoulette Game, DoorbellMonitor, Anatomy of a SpaceShuttle.

May 1981Electronic Organ, Voice -Operated Switch, Infra -RedController, Pre -Amplifier Part 2,Audio Millivoltmeter.

June 1981Power Amplifier Part 1,Continuity Checker, EnvelopeGenerator, Early Radio, Gadgets,Games and Kits Supplement.

July 1981 21:-___---......4.-......Burglar Alarm, Doorbuzzer, --------!. .... ----Treble Booster, Electronic Aidsfor the Disabled, PowerAmplifier Part 2.

1-31-3Electronicso y



,,,...:17.,. g!arAla

a ..tiMr11,.... a

Boost rRooster "le,arkit

All backnumbers cost £1.25 each. For those of you who only want copies of articles, wedo offer a photocopying service. Each copy costs £1.25 and information as to its title andpublication date should be given. Ordering backnumbers and photocopies could hardly be

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56 Hobby Electronics, May 1982

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Into Radio


Part 2 of our new series introduces the most important buildingblock in radio circuits.

Ian Sinclair

IF YOU'VE DABBLED a bit in electronicsconstruction, you've probably found out,by now, how easy it is to make oscillatorcircuits such as multivibrators, whichgenerate square, or nearly square, waves.Generating sinewaves seems much moredifficult, at low frequencies at least. It allseems a bit odd, when everyone tells youhow important sinewaves are, that theyshould be so difficult!

In the early days, no-one had any dif-ficulty generating sinewaves. All you hadto do was to rotate a coil of wire betweenthe poles of a magnet and, behold, youhad a sinewave at the ends of the wire!Add a pair of slip -rings to make the con-nections to the rotating coil (Figure 1) andyou had an alternator. That's one of thereasons why the sinewave is important -you can generate it mechanically. In theearly days of radio, alternators like thiswere used, spinning at high speeds, togenerate the carrier waves for Morsesignals.

The use of alternators is fine so long aswe don't need really high frequencysignals, but is causes a bit of difficultywhen we want frequencies around1 MHz. A large coil revolving one milliontimes per second is a device no-one wouldwant to be close to, so other methods areneeded. The 'other methods' all end upwith that useful circuit, an inductor inparallel with a capacitor - otherwise call-

ed a parallel resonant circuit. Now let's beclear about one thing right away. Theparallel resonant circuit doesn't, by itself,generate sinewaves. What it does is togrow sinewaves. Grow them? It's asgood a description as any, for what hap-pens. If, somehow, you start a currentflowing in the inductor of a parallel reso-nant circuit then that current can't justkeep on flowing, because it charges upthe plates of the capacitor until thevoltage builds up enough to make the cur-rent first fall to zero, then reverse direc-tion. Then, the current builds up in the inthe opposite direction until, once again, ithas charged the capacitor to a voltage(the other way round now) which willmake the current reverse. This cyclecreates, (as naturally as the rotation of acoil between the poles of a magnet), asinewave. It can't keep going, though,any more than the alternator coil can keepspinning, without some outside help.That help, as far as the parallel resonantcircuit is concerned, is provided by a tran-sistor, a valve or whatever else we canuse to keep the current flowing.

There's the snag of course. We canmake a transistor pass current betweenits collector and its emitter by bringing thevoltage between the base and the emitterabove about OV55. (that's the magicfigure for a silicon transistor). Now thiscurrent is in one direction, from collector

to emitter if -the transistor we use is anNPN type, but the current won't flow theother way. If we connect our parallel reso-nant circuit to the collector circuit of anNPN transistor, then, we can't just startthe transistor conducting at any old time.The sinewave which is shaped by theparallel resonant circuit will have adefinite frequency, whose value dependson the values of inductance andcapacitance in the circuit. If we brieflymade the transistor conduct by applyinga short positive voltage to the base (apositive pulse), the parallel resonant cir-cuit would oscillate - but only for a shorttime. Certainly, it oscillates for a lot longerthan the time of the pulse at the base ofthe transistor (just as a bell rings for sometime after it has been struck), but theoscillations die out - just as the sound ofthe bell does. Because of the similarbehaviour, in fact, a circuit used in thisway is called a 'ring;ng' circuit (Figure 3).

The Positive AnswerTo keep the oscillation going, we have tomake the transistor conduct each timethe current through the coil is flowing inthe right direction - that is, from supplyto earth. When the current reverses, thetransistor has to shut off otherwise it willsimply take all the current which shouldbe going into charging the capacitor. The






Figure 1. The alternator principle. As thewire loop rotates between the poles ofthe magnet, a sinewave signal isgenerated. The frequency of thesinewave is equal to the number ofcomplete revolutions per second of thecoil.

lc) Id) el

Figure 2. Charge and discharge in aparallel resonant circuit; (a) the circuit;(b) the capacitor discharge (c) causescurrent to flow through the coil and thenthe recharging of the capacitor (d); theprocess then reverses, reversing thedirection of current through the coil (e).

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Into Radio




4 Figure 3. The waveform produced by a'ringing circuit'. A current pulseapplied to a parrallel resonant circuitcauses the charge/discharge cycle ofFigure 2, but eventually the currentdies away.

4 Figure 4. To produce continuousoscillation, the ringing circuit needs tobe 'stuck' by a succession of currentpulses, at exactly the right moments.This is achieved by a positive feedbackconnection, via the parallel resonantcircuit, arranged so that the amplifierconducts only when its output voltageis in phase with the voltage across thetuned circuit.

way we can ensure the base voltage ofthe transistor causes it to conduct at theright time is by feeding it with a bit of thesinewave generated by the parallel reso-nant circuit (Figure 4). This is a positivefeedback connection; just as the currentin the inductor starts to flow earthwards,the transistor starts to conduct, helpingthe current on its way and so generating avoltage which turns the transistor harderon, helping the action a bit more. Whenthe current reverses, the voltage at thebase of the transistor drops, cutting offthe current and allowing the inductor toget on with the job. The result is a

sinewave oscillator. Any sinewaveoscillator which uses a parallel resonantcircuit must include a circuit which feedssignal to the base of the transistor to pro-vide the current drive for the oscillations.Seems simple enough but in fact there area remarkable variety of sinewaveoscillators, many of which carry namesthat date back to the very early days ofradio.

A circuit for a sinewave oscillator - anold reliable favourite - is shown in Figure5. Two windings are needed; one (L1) ispart of the resonant circuit, while theother (L2) is a feedback winding whichswitches the base voltages of the tran-sistor up and down as needed. To makethe circuit self-starting, a little bit of bias isneeded to start the transistor conductingand this is provided by R1, decoupled byC1. If the oscillator is to operate at a high

frequency, L1 may consist of a few turnsof wire on a ferrite rod or a hollow former.L2 should have about 1/4 to 1/5 thenumber of turns of L1, as a rough rule -of -thumb. Lower frequency sinewaves canbe generated if L1 and L2 are windings ofa transformer with a ratio of around 1 : 5,using the larger winding for L1.Sinewaves of 20 kHz or less can begenerated when a capacitor of about Ou1is used to tune L1 , but the windings mustnot have a high resistance, otherwiseoscillation may not start.

Bright IdeaWhen you make or use a pair of coils inthis circuit, there's a 50:50 chance thatthe oscillator won't work - this isbecause you can seldom be sure aboutwhich way the currents flow in the win-dings. The easiest cure is the practical one- if it doesn't oscillate, reverse the con-nections to L2. But how do you know ifit's oscillating, without using a 'scope?One old-fashioned way to find out if a highfrequency oscillator is working is to use alow -wattage torch bulb, something usingless than 60 mA at around 2V5, connec-ting it to a few turns of a coil, as shown inFigure 6. Hold this lot near L1 and the bulbwill light as the coil picks up the frequencygenerated by the oscillator. Don't go tooclose, though, otherwise the load on theoscillator may be enough to stop it work-ing.

Another type of oscillator, called aHartley after its inventor, is shown inFigure 7. There are several varieties ofthis design, all recognisable by the use ofa tapped coil, and Figure 7 shows two ofthem. The one shown in Figure 7 (a) usesthe tap on the coil to connect the supplyline, and feedback is taken from the end ofthe coil opposite the collector connection.Because the collector and the base of thetransistor cannot be allowed to be at thesame DC voltage, a capacitor is used tocouple the feedback to the base. Thevalue of this coupling capacitor, C1, mustbe considerably greater than the value ofcapacitor C2, which is used to tune thecoil. Another variety of the Hartley circuitis shown in Figure 7(b), with the feed-back, this time, taken to the emitter of thetransistor. A resistor must be connectedbetween the emitter and the earthline, toavoid shorting out the signal or, as analternative, an inductor called a radio fre-quency choke (RFC) can be used. ResistorR1 and R2 are used to bias the transistor,so ensuring that the circuit will startoscillating when it is switched on. Thecapacitor, C1, is also important; it en-sures that no signal reaches the base ofthe transistor from the collector circuit.Any signal reaching the base will kill theoscillation, so that C1 is essential - it'decouples' the base.

One version of a circuit called a Col-pitts oscillator is shown in Figure 8. Thecoil, this time, is a single winding with no

41 Figure 5. An oscillator using a feedbackcoil, L2.

Figure 6. Detecting oscillation. Unlessthe oscillator is a powerful one, the bulbwill have to be a miniature low -voltage,low current type. These are not easilyobtainable, but an LED can besubstituted if more turns of the coil areused.





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Into Radio tapping - a convenient arrangement!The tapping is still there, though, in theform of capacitors C2 and C3. The tuning

of the parallel resonant circuit is carriedout by the combination of C2 and C3 inseries while the signal to feed to emitter is

al Ibl

Figure 7. Two versions of the Hartley oscillator; (a) feedback to base; (b)feedback to emitter.

Figure 8. One version of the Colpittsoscillator.

Figure 9. A crystal oscillator, one of alarge number of circuits using themethod of obtaining a precise and stablefrequency.

tapped by C2 and C3 acting as a potentialdivider. Once again, capacitor C 1 has theessential job of decoupling the base.

Crystal GazingThe best sinewaves, though, are shapednot be a conventional parallel resonantcircuit but by a quartz crystal, connectedas if it were an inductor. An example of acrystal oscillator which can generate avery precise and perfectly shapedsinewave, is shown in Figure 9.

It's not so difficult, then, is it? Well, no- not as long as you want high frequencyoscillations, anyway. But what happens ifyou want low frequency oscillations - afew kHz or less? Then, inductors get largeand expensive because of the large valuesyou would need for operating these fre-quencies. Worse still, all that wire makesfor high resistance, which causes theshape of the sinewave to distort. In factwe have to abandon parallel resonant cir-cuits at low frequencies - but that'sanother story!In radio circuits, however, the parallelresonant circuit is vital. It crops up,in oneform or another, in almost every 'block' ofevery radio. This month, we have had abroad look at parallel tuned circuits andhow they work - soon, we will get togrips with the theory, showing how towork out the resonant frequency andother important factors. Later we will goonto look at oscillator circuits in moredetail, together with the other basicblocks of radio circuitry.



This new style course will enableanyone to have a real understandingof electronics by a modern, practicaland visual method. No previousknowledge is required, no maths, andan absolute minimum of theory.

You learn the practical way in easysteps mastering all the essentials ofyour hobby or to start or further acareer in electronics or as a self-employed servicing engineer.

All the training can be carried out inthe comfort of your own home and atyour own pace. A tutor is available towhom you can write personally at anytime, for advice or help during yourwork. A Certificate is given at the endof every course.

You will do the following:Build a modern oscilloscopeRecognise and handle current electronic

components R ead,draw and understand circuit diagramsCarry out 40 experiments on basic

electronic circuits used in modernequipment

Build and use digital electronic circuitsand current solid state 'chips'

Learn how to test and service every typeof electronic device used in industry andcommerce today. Servicing of radio, T,V.,Hi-Fi and microprocessor/computerequipment.

NewJob? New Career?NewHobby?Get into Electronics Now!rnin Please send your brochure without any obligation to I am interested in: HE '5 821 I





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Hobby Electronics, May 1982 59

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ynadeCgs l\For Readersi

-PRINTED CIRCUIT BOARDS (PCBs) for HE projects have often represented anobstacle for our readers. Some of you, no doubt, make your own but our PCB Servicesaves you the trouble.NOW you can buy your PCBs direct from HE. All (non -copyright) PCBs will be availableautomatically from the HE PCB Service. Each board is produced from the same master asthat used for the published design and so each will be a true copy, finished to a high stan-dard.Apart from the PCBs for this month's projects, we are making available some of thepopular designs from earlier issues. See below for details. Please note that only boards forprojects listed below are available: if it isn't listed we can't supply it.

May 80 December 80 August 815080 Pre -amplifier £3.50 Stereo Power Meter £2.12 RPM Meter £1.33

Digital Speedo (set of two) £3.50 Thermometer £1.25June 80Fog Horn £1.40 January 81 September 81Egg Timer £1.58 Car Rev Counter £2.24 Power Pack £1.27

Reaction Tester Game £1.28July 80 February 81 'Diana' Metal Detector £2.4818 W + 18 W Car Stereo Heartbeat Monitor £1.90Booster (two required for Audio Signal Generator £1.85 October 81stereo) each £1.20 Combination Lock £1.99August 80Equitone Car Equaliser £1.79

March 81Steam Loco Whistle £1.99 November 81

Sound Torch (Set of Two) £3.98Pass The Loop Game £1.98 April 81September 80 Super Siren £1.48 December 81Auto Probe £1.25 Russian Roulette Game £1.20 Pedalboard Organ £4.48Guitar PhaserDevelopment TimerBench PSU

£1.48£1.35£2.20 May 81

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January 82Intelligent NiCad Charger £2.28

October 80Nobell DoorbellIntruder Alarm


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June 81

£3.48 February 82Relay DriverMast -Head Amp


Tug 0' War £1.99 Envelope Generator £1.40 March 82November 80

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60 Hobby Electronics, May 1982

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Feature II


THE FIRST ICs ever made were (mainly)digital types, and by far the majority of ICsmanufactured today are digital ICs. Thereason is that digital signals are peculiarlysuited for ICs, and it is much easier todesign complex digital ICs than equallycomplicated linear ICs.

A digital signal is an OFF or ON signal,without any state between. A squarewave is a type of digital signal, becausethe voltage is either at a positive ornegative peak; it spends only a negligibletime getting from one to the other. Nowwhy should this be important? Imagine fora moment a transistor with a load resistorof 500R, operated from a 5V supply.Suppose that the transistor is passing nocurrent. The power dissipated in the tran-sistor is then zero - no current means nowatts. If we now suppose that the tran-sistor is passing about 10 mA, then itscollector voltage will be low (saturated atOV2) and the power dissipated by thetransistor must also be low, only 0.2 x 10mW or 2 mW. If we had biased the tran-sistor to half the supply voltage, as we dowhen we want linear amplification, then itwould pass 5 mA at 2V5 and the powerdissipation would then be 5 x 2.5 mW(12.5 mW), a lot more than it dissipatesat full current. In a digital IC, no transistorsare biased 'half -way'; they are either fullyoff or fully on. In this way, each transistordissipates only a small amount of power,so large numbers of transistors can bepacked together in an IC without causingthe chip to run excessively hot.

Over the years of IC developmentthere have been many digital types pro-duced. By types, we mean the types of







Figure 10.1 Digital and analoguesignals. The difference is that the digitalsignals have only two important voltagelevels and the voltage switches veryrapidly between the two.

circuits that were used, rather than thejobs that the chips performed. For exam-ple, in old books you will find referencesto DTL, RTL and other long -forgottentypes of ICs - some of these are now socompletely forgotten that it is impossibleto even get pinout diagrams for them.The only types worth considering todayare TTL and CMOS chips; these are theones we shall look at this month.

The most important differences bet-ween these two types of digital ICs arethat TTL ICs are based on bipolar tran-sistors (PNP and NPN types) and CMOSare based on MOSFETs, TTL has low in-put resistance and can pass currents ofseveral milliamps, while CMOS has veryhigh input resistances and can pass onlysmall amouts of current, often less than amilliamp.

TTL TaleTTL digital ICs are intended to operatefrom a 5 V stabilised supply, whosevoltage must be held close to 5V at alltimes. Though the ICs will generally workin supply voltages as low as 4V5, correctoperation cannot be guaranteed unlessthe supply voltage is maintained withinthe limits of 4V75 to 5V25. Voltagesabove 5V5 are risky because some TTLcircuits will overheat at such levels, andmost of them will be damaged if they areoperated for any length of time at 6 V.

The reason for this fussiness aboutvoltage is that the input stage of a TTLdigital IC is a common -base transistor(Figure 10.2), whose base is connectedto the supply voltage through a resistor ofa few thousand ohms. This results in afeature of TTL circuits which many begin-ners find most confusing; the input of anyTTL circuit is the emitter lead of a tran-sistor. When this input is at logic level 11+ 5V for TTL), the input transistor is cutoff because both base and emitter are atthe same + 5V level and the collectorvoltage is also high, around + 5V. Whenthe input emitter is connected to logiclevel zero, which is earth, current will flowfrom base to emitter. The value of theresistor which connects the base to the+ 5V supply ensures that the transistor isthoroughly saturated and passing enoughbase current to make the collector -to -emitter path a very low -resistance one. Inthis condition, the collector voltage islow, and a current passes from the emit-ter to earth. The 'standard' TTL circuit willthen pass 1.6 mA from its input terminalto earth.

Unlike most transistor circuits, then, inwhich a current has to pass into the inputwhen it is taken positive, the TTL inputpasses current out from its input whenthe input is taken to zero volts. Any circuitwhich is connected to the input of a TTL

IC must therefore be able to pass this cur-rent - to sink this current, in the jargon ofthe TTL designers - when it forces theTTL input low. Because of this, anemitter -follower is not a good circuit forpassing signals to (driving) a TTL IC.When the emitter -follower is cut-off, theIC current will have to pass through theemitter resistor of the emitter -follower(Figure 10.4), and the voltage across thisresistor may be too high to guarantee thatthe IC will be at logic 0.

For example, if the IC can pass1.6 mA, and is guaranteed to behave cor-rectly if the logic 0 voltage is below OV8,then an emitter -follower with a 1 k emitterresistor is completely unacceptable as adriver. With 1.6 mA flowing through 1k,the voltage drop would be 1 V6, which is






Figure 10.2 The usual TTL input stage;(a) a common -base stage; multipleemitter contacts can be formed; (b) ifmore than one input is needed.







Figure 10.3 Why current passes outfrom input of a TTL IC at logic 0 input. r

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Figure 10.4 Using an emitter -follower todrive TTL is unsatisfactory.

much too high. It might work with a fewICs, but what counts is that it must workwith allICs.

A common -emitter circuit (Figure10.5), by contrast, is very much better.With the transistor cut-off, the loadresistor connected to + 5V will ensurethat the input of the IC goes to + 5V.When the transistor is switched on, thecollector -to -emitter circuit will easily pass1.6 mA to earth, with a voltage drop ofonly about OV2 if the transistor issaturated.

Digital circuits make use of TTL ICscc lnected together, so that we canexpect to find the output of one IC con-nected to an input of another. This ispossible only if the output of one IC cansink the input current of the next and,because the output of one IC may have to






Figure 10.5 Using a common -emitteramplifier circuit to drive TTL is muchbetter.




Figure 10.6 Fanout illustrated.


Figure 10.7 A typical TTL output stage

drive several inputs, the manufacturersusually arrange the output circuit so that itcan sink the current of ten intputs. If aninput needs to pass 1.6 mA, for example,the output must be able to sink a current(pass current to earth) of 16 mA if it is todrive ten inputs. This requirement is calleda 'fanout of ten'.

The usual output circuit uses two tran-sistors in series with a diode (Figure10.7), directly coupled to the collectorand emitter leads of a driver transistor.First, imagine that the base of the drivertransistor Q1 is cut-off. This will make theemitter voltage zero and the collectorvoltage high, so that current will passthrough R1 into the base of Q2, whichpasses current. This makes the outputvoltage high, though, because of the OV6difference between base voltage andemitter voltage of Q2, The emittervoltage of Q2 cannot rise above 4V4which is OV6 below 5V; the outputvoltage cannot rise above about 3V8because of the OV6 drop across diodeD1.

Now, say the base voltage of Q1 is

raised, so it is saturated. The collectorvoltage will be low, and the emittervoltage will rise, until Q3 is switched on.Because the base of Q3 is connecteddirectly to the emitter of Ql, the emittervoltage will not be able to rise higher thanOV6, and the collector voltage only will beabout OV2 higher, at about OV8. Thiswould, normally, be enough to switchQ2on, but the diode D1 prevents this (asilicon diode needs about OV6 to con-duct) so that the base voltage of Q2would have to be at least 1V2 above zeroto make Q2 conduct. Therefore, Q3 con-ducts and this will make the voltage at theoutput about OV2, well within theguranteed voltage for a zero logic level.

The low voltage is really important,because it is only the low voltage whichallows the input to pass current, whichthe output will have to sink. Because all ofthe transistors in the chip are made at thesame time, and are therefore similar, thecircuit can also pass current out from theoutput when the output voltage is high,around 3V8. This voltage cannot beguaranteed by the manufacturers,however, because if a lot of current ispassed, the high voltage can fall quitenoticeably, to less than 3V. For ex-perimental purposes, though, there is noobjection to using an output to drive anLED like the circuit of Figure 10.8 (therecommended method is to use an addi-tional IC, an inverter, in the circuit ofFigure 10.91.




/7 77

Figure 10.8 Using a TTL output tooperate an LED - simple method.

Figure 10 9 The recommended circuitfor driving LEDs.

This business of guaranteed voltagelevels is important, because it affectstrouble -shooting measurements. Youcannot expect to read + 5 V as the highvoltage at an output unless there is aresistor, called a 'pull-up' resistor, con-nected between the output and the + 5Vsupply line. Neither can you expect to finda true zero reading at an output which isdriving several inputs because there maybe enough current flowing to raise thevoltage to more than OV2. What you canexpect, though, is that the voltage in thehigh state, logic 1, will be above themanufacturer's guaranteed level (usually2V4) and that the voltage in the low statewill be below the manufacturer'sguaranteed level (0V4 typically).

Identity CrisisThere are several variations of TTL cir-cuits which are still with us. The originalrange of TTL circuits, pioneered by TexasInstruments and also manufactured bymany other suppliers, used code numbersstarting with 74. These ICs employed thetechniques described in this article, withinput currents of 1.6 mA and output sink-ing capability of 16 mA, so that a fanoutof ten could be achieved. For some com-puter applications, the speed of switchingof standard TTL circuits, which is around30 nanoseconds,is too slow, so a(smaller) range of high-speed TTL cir-cuits, the 'S' series was manufactured.These used higher currents flowing insidethe chips to achieve higher switchingspeeds. The chips carried numbers star-ting with 74S, so that a 7408 was a stan-





Figure 10.10 Using a pull-up resistor toensure the logic 1 voltage.

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Feature MI



Figure 10.11 The equivalent circuit ofan LS type of TTL gate, using Schottkydiodes, which have a very low forwardvoltage drop.

dard AND gate and the 74S08 would be ahigh-speed AND gate.

Later on, a new principle, the low -power Schottky circuit, was used tomake a range of TTL-type circuits whichcombined lower currents with high-speedoperation. These were distinguished bythe 74LS type numbers, and are now themain type of TTL ICs. The 74L S seriesuse input currents of 0.4 mA and outputcurrents of only 4 mA for the same fanoutof ten, but are as fast or faster than thestandard range. The standard 74 range isnow, in fact, no longer produced for newequipment, though stocks should last forsome time to come.

Gather No MOS?CMOS ICs use MOSFETs, with entirelydifferent circuitry. The circuits containboth N -channel and P -channel MOSFETs,arranged so that only one of the pair isever turned on at a particular time. Therange of power supply voltages that canbe used is much more flexible than theTTL + 5V - in general CMOS circuits canoperate with supplies in the range 3V to1 5V. Practically no current is needed tohold the inputs at either voltage level, highor low, and only very small currents canbe passed either out from, or into, the out-puts. Most CMOS circuits which we seehere are from the RCA series, bearingserial numbers from 4000 upwards, butNational Semiconductor manufacture arange with type numbers starting with74C and which, very usefully, follow thesame numbering system as TTL, so that a74C08 is a CMOS version of the74LS08.

Early CMOS chips suffered from areputation for being easily damaged bystatic and users were recommended totake remarkable precautions such as be-ing chained to a metal bench, with wriststraps, to ensure discharging of static.Nowadays, CMOS ICs can be handledwith few special precautions (unless youwork in a room where static is a problem).One of these is that CMOS chips shouldnever be plugged into or removed from acircuit which is switched on. Another isthat an earthed soldering iron should

always be used - never use an iron whichis not earthed, in any case!

By keeping any CMOS chip in its pack-ing until ready for use, using holders andplugging in the chip only when the circuitis nearly complete - and with the + veand - ve leads of the board shorted -you should have no problems. The mostimportant thing is that CMOS chipsshould be the last items put on the boardand that each input should have a resistorconnected either to earth or to the + vesupply line.

When you take a quick look at CMOSand TTL, you might find it difficult to seewhy CMOS has not completely supersed-ed TTL. CMOS can, after all, use a muchgreater range of power supply voltagesand pass much lower currents so thatbattery -operated circuits can be design-ed. What makes the difference is that a lotof circuits still require very fast operatingtimes, so CMOS circuits do not generallyfind much application in computing or indigital control equipment where speed isimportant. For most home -constructedcircuits, however, CMOS ICs or similar(PMOS, NMOS) are more likely to besed.





Figure 10.12 Circuit of a CMOSinverter, which uses both P - and N -channel MOSFETs.

Open And Shut Case?We've called this series Into ElectronicComponents and we've tried to stick tocomponents, and what they do, ratherthan become involved in circuit theory.Digital ICs, however, can't easily beseparated from circuitry because eachdigital IC is itself a complete circuit. So,we now have to spend some time on thetypes of circuits built in to digital ICs,whether TTL or CMOS.

All digital ICs can be grouped into twotypes: combinational or sequential. Thedifference between the types is impor-tant. Combinational circuits have severalinputs and one output, and each combina-tion of inputs will give pre -determinedoutput. For example, a chip which hastwo inputs can have four possible sets ofinput voltages; these are 0,0 ; 0,1 ; 1,0 ;and 1,1 (where 1 means high voltage and0 means low voltage). For each set of in-puts there will be a definite output voltage(0 or 11 and that chip can never produceany other result. The simplest combina-tional circuits are 'gates' and the easiestway of summarising their action is by us-ing what are called 'truth tables'. A truthtable is just a list of all the possible inputsto the gate, along with the output voltage(0 or 1) that is produced by each set of in-puts. Different types of gates have dif-


Supply VoltageFanoutOperating Temperature


4V75 -5V2510 max0 to 70 deg.0


4V- 12V50 or more-40 to BO deg.0


Input CurrentOutput Current (marlSwitching Time

40 uA (11,1.6 mA (0130 mA11 nS

10 pA0.25 mA300 nS

Figure 10.13 CMOS and TTLcompared.


1 00


1 1



O 0 0O 1

1 01 1







° B B


O 00 1


1 01 1 0




O 0O 1

1 01 1



07 0

1 0

1 1


Figure 10.14 Symbols and truth tablesfor gates.

ferent truth tables, and the types that areavialable in IC form are the AND, OR,NOT, X -OR (Exclusive -OR) NAND (NOT -AND) and NOR (NOT -OR). Their symbolsand truth tables, for two -input gates, areshown in Figure 10.14.

It would be possible to build any typeof gate circuit using only NAND or onlyNOR gates, but simpler circuits can usual-ly be achieved by using the full range ofgates though designers often prefer tomake use of one type of gate as much aspossible, just to simplify the parts list for acircuit.

A circuit for checking the action oflogic gates is shown in Figure 1 0.1 5. Thegate is placed on the Eurobreadboard andLEDs, along with current -limiting 1kresistors are used to monitor the output(or outputs). The inputs are provided byDIL changeover switches, obtainablefrom Electrovalue and other suppliers.The switches must be wired so that theinputs of the gate are always connectedeither to 1 or 0; inputs should never be left'floating', ie unconnected.

When several gates are connected,the truth table can be worked out trying allthe combinations of inputs and observingthe outputs (taking a lit LED as logic 11, asoutlined above. The alternative is to draw



Figure 10.15 Checking the action of agate or gate circuit by finding its truthtable experimentally

Hobby Electronics, May 1982 63

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Featurean extended truth table (Figure 10.16).Each connecting line from an output toanother input is labelled with a letter andthe outputs of the first set of gates arethen entered into the truth table. The nextset of gates can then be treated, using theoutputs of the first lot as inputs to thesecond, and so on until all the columns ofthe truth table have been filled in.

The other type of digital circuit is thesequential circuit, in which the sequenceof inputs determines the output, ratherthan the combination of inputs. A coun-ting flip-flop is a typical sequential circuit,and a sequence table for such a flip-flop isshown in Figure 10.17. The input con-sists of pulses which, in this example, areat regular intervals, qualifying for thename 'clock' pulses. The output of the cir-cuit switches over for each clock pulse,so that one pulse will switch the outputfrom 0 to 1 , the next will switch it from 1to 0. The action is rather like a ' push -on -push -off ' light switch.

There is another type of flip-flop, thelatching flip-flop, which uses the clockpulse to 'transfer' an input to the outputterminals. Looking at a typical example ofthis type, Figure 10.18, the voltage at theoutput is made equal to the voltage at theinput on each clock pulse and is kept atthe value until the next clock pulsearrives. All sequential circuits are basedon flip-flops, just as all combinational cir-cuits are based on gates.

Next month we move on from activecomponents and take a look attransducers - devices for transformingother forms of energy into electricalsignals.


:IgxyzciAo o 1

o 1 o

o 1 1

1 0 0

1 0 1

1 1 0

1 1 1


Figure 10.16 Finding the truth table of ad gate circuit by paperwork; (a) circuit;

(b -d) stages in filling in truth table.

ABCXYZQo 0 0 0 0o o 1 0 0o 1 o 0 00 1 1 0 1

1 0 0 0 01 0 1 0 01 1 0 1 01 1 1 1 1

Figure 10.17 A sequence table for counting flip-flop.




c I


A B C X V Z 0o o 0 0 0 0 1

o 0 1 0 0 0 1

0 1 0 0 0 0 1

o 1 1 0 1 1 01 0 0 0 0 0 1

1 0 1 0 0 0 1

1 1 0 1 0 1 01 1 1 1 1 1 0






0 00


Figure 10.18 The action of the edgetriggered latching flip-flop. The outputchanges only when the trailing edge ofthe clock pulse arrives, in this example.


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tia Hobby Electronics, May 1913Z

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Cable TrackerA D -I -Y Useful

THIS nifty little project is a must for thehandyman - even if your Do -It -Yourselfexperience is limited to the occasionalshelf! Our Cable Tracker will detect notonly cables, but also pipes and nailsburied in wood, thus saving the extrawork - and expense - that wouldresult if you should accidentally drill intoa central heating or water pipe. Safety isanother important aspect of this project,as it will allow you to avoid live cablesand gas pipes. Have you ever cut into apiece of wood with a saw and struck ahidden nail? Not much good for theblade, is it! Well, the HE Cable Trackerwill also detect nails in wood, to a depthof at least 1 cm.

The Cable Tracker is very simple touse - just run it over the wall, about1 cm from the surface, where yoususpect there may be a hidden pipe orcable. We have used an audible outputto indicate the presence of metal, whichallows you to watch where you'regoing, rather than having to look at theTracker.

The CircuitTransistors Q1 and Q2 are connected inan oscillator circuit using L1, C1 and C2as a high 'Q' tuned circuit resonant at100 kHz. A high 'Q' circuit is veryefficient, producing a generous outputat one specific frequency. If even asmall piece of metal is brought close tothe search coil 11_11, however, theinductance of the coil will be altered,reducing the 'Q' of the circuit and henceits output voltage. This change involtage is detected by the remainder ofthe circuit.

The oscillator output is full -waverectified by D2, 3, producing a steadyDC voltage across C5. This voltage isAC coupled (so only sudden changes inthe DC level will be passed) via C6 tothe non -inverting ( +1 input of IC1a. Theinverting input is held at a fixed level byZD1, R5 and R6 and, provided thevoltage across C5 remains constant, theoutput of IC1a is held at 3V9.

If the oscillator output falls, though,the change is amplified by IC1a andpassed to the inverting input of ICI b,which is wired as a comparator. Itcompares the voltage at the invertinginput with a reference voltage, from thesensitivity preset PRI. When the outputfrom IC 1 a drops below this preset level,ICI b output goes high and switches onthe audio oscillator, IC1c. This drivesthe piezo transducer with a frequency ofa few kHz, giving an audible warningthat metal is present near the detectorhead.










Figure 1. The Cable Tracker circuit diagram.







0S1 ,9V

R10100k R12


10 IC1c

R13ICS 220k



Hobby Electronics, May 1982 65

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Cable Tracker

Figure 2 (above). Fitting the pipe in tothe clips.

Figure 3 (below). Assembling thedetector head. Don't forget to thread thewires through the hole in the pipe, theninto the case. Grommets are used to sealboth ends of the pipe, as shown.

ConstructionAssembly of the Cable Tracker issimplified by the use of a printed circuitboard and a ready -wound coil.

Start with the 'detector head'. Usinga suitable adhesive, stick the clips to thewooden blocks and drill a hole (about5 mm) through one of the clip/blockassemblies. Position the pipe in theclips, as shc wn in Figure 2, and drillthrough the pipe, using the previouslydrilled hole as a guide. Now take thepipe out of the clips and glue theblock/clip assemblies in position on thecase - take care to put the one withthe hole in it at the top, as shown.When the glue has set, drill through thecase, again using the pre -drilled hole asa guide.

Other holes must be drilled near theresonator, to allow the sound out, andfor the on/off switch. Remember toleave plenty of room below the switchfor the assembled PCB.

Tape the coil in position on the ferriterod, about 1 cm along from the end,with the coil tabs towards the centre ofthe rod. Solder two 1 50 mm lengths ofwire to the coil and put a grommet onthe 'coil end' of the ferrite rod to seal itinto the pipe. Twist the coil wirestogether and push the ferrite rod downthe pipe, at the same time threading thewires through the hole in the pipe asshown in Figure 3. Push the pipe into itsmounting clips, remembering to threadthe wires through the pipe clip into thecase. Fitting the second grommet needspatience; lubricate it with washing-upliquid and gently push it into place usinga screwdriver.

Next the PCB is as§embled with thehelp of the PCB overlay, Figure 4. Startby soldering the terminal pins for the coilconnections. Insert the resistors roundand solder into place, then solder all thecapacitors - all types in this circuit cango either way round. The preset may

also be soldered in place. Take care toinsert and solder the diodes (the broadband is the cathcode end) andtransistors the correct way round.Solder the IC socket with pin number 1in the correct place, but leave out the ICuntil later.

Now solder the following; the blacklead (-ye) of the battery clip, the leadsfrom the resonator and a length of wireready to go to the switch. The PCB cannow be fastened to the bottom of thecase, using small pieces of double -sidedadhesive tape. The ceramic resonatorand battery can also be taped to thecase - these are a snug fit, so positionthem carefully before fixing. The batterymust also be fixed in position, to avoidinterference with the detection circuit.

Fix the switch and solder the leadfrom the PCB, and the red lead from thebattery clip, to it. Finally, solder theleads from the coil to the terminal pinson the PCB. The IC can now be inserted- take care, it must be the right wayround.

Setting UpConnect the battery and move to anarea away from the metal - thisincludes your watch and rings! Switchon; the circuit adjusts its sensitivity afterswitch on, so wait for approximately 10seconds. Using a non-metallic trim tool,turn the preset clockwise until a bleepsounds, then turn it back approximately1 /8th turn - this is the most sensitiveposition, but if you are using the trackerto find large objects you can lower thesensitivity. To use the tracker, hold thebox and pass the detector tube over thewall. The optimum distance is about1 cm away. Note that a very damp wallwill interfere with the detection circuit.

Now you can put up your shelves orpictures - with the confidence that youwill not need to rewire or replumb!

Figure 4. The PCB component overlay.The foil pattern is reproduced onpage 72.




Hobby Electronics, May 1982

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Cable Tracker NI

The circuit for the HE Cable Tracker canbe divided into six basic blocks (seediagram). The detector part of the circuitconsists of a tuned oscillator; wheneven a small piece of metal is broughtnear the search coil, the output from theoscillator falls significantly. This changeis amplified and compared with a presetreference level, set by the sensitivitycontrol. The comparator detects anysudden change in the output of theoscillator and will switch on the audiooscillator, which drives the transducer.



Figure 5 (above). End view of the pipe,with grommet in place.Figure 6 (left). Internal view of thecompleted Cable Tracker.

How It Works

100k HzTUNED









Parts ListRESISTORS(All '4 watt carbon film 5%)R2 R6 47kR3 1kR4, 5, 7, 9 2M2R8 22kR1, 10, 11, 12 100kR13 220k

POTENTIOMETERSRV1 22k min horizontal preset

CAPACITORSCl 3n3 polystyrene 63 VC2 470p ceramic(low temperature coeff ) or polystyreneC3 1 n ceramicC4 100n C280 polyesterC5 22n C280 polyesterC6 1 u multilayer

min polyester (100 V)C7, 8 4n7 ceramic

SEMICONDUCTORSQl, Q2 BC183 NPN transistorsD1, 2, 3 1 N41 48 signal diodesZD1 BZY88C 3V9 zener diodeIC 1 TL064CN quad BIFET

op -ampX1 Piezo sounder

(ceramic transducer)S1 min. slide switch SPST

MISCELLANEOUSL1 100 mm x 9.5 mm diaferrite rod with long wave coil type CTLPrinted circuit board; plastic case;plastic pipe Er clips; wood, grommets,and wire. 14 pin dil socket. PP3 batteryclip.

BUYLINES page 68

Hobby Electronics, May 1982


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VA Digital ThermometerTHERE ARE two things to note aboutthe thermometer project; first, it usestwo separate boards and, second, if youbuy our recommended case everythingfits inside exactly. So, you will have tocut and shape the boards carefully andalso make sure you mount them asshown in the diagrams.

The components are spaced on themain PCB to ensure that thetemperature around the sensor (IC1) isnot influenced by any of them - wiringit off the board is even better. Thiscomponent will probably be the mostdifficult to obtain, but you can get itfrom Crewe Allan Et Co. Ltd., of 51,Scrutton St., London.

The CMOS ICs (including the 7555)can be bought from Technomatic alongwith the seven -segment displays. Youwill generally find that Technomatic cansupply most of the semiconductors usedin our projects. So, if you want to saveon postage you know where to go!

Another source for semiconductors,and some hardware items, is WatfordElectronics. They stock the ZTX300transistors and the low voltage Zenerdiode; you will also find a range ofsuitable presets in their catalogue.

The only other parts that might behard to buy are all available direct fromBICC-Vero. They make and supply thecase with battery compartment, thetwo -digit LED bezel and the Veropins. Ifyou want further details of any of theirproducts, just drop them a line(enclosing an SAE).

The cost of components (excludingcase, PCBs and the optional bezel)should be under £13.

Light SeekerTHE Light Seeker is probably one of thesimplest, yet most versatile quickprojects, we've ever run. None of thecomponent values are particularlycritical, since oscillation occurs readily.Make sure you put the transistors in theright way round, though - they onlywork when base 1 is feeding the load.These unijunction transistors seem to begoing out of fashion, sad to say, butthey can still be bought fromElectrovalue (T1 S43) in their


catalogue), who also sell the 64 ohmspeaker - don't use a lowerimpedance, as the transistors willoverheat.

The light -dependent resistor isavailable from Maplin. We used theORP61 with a side -on sensitiveelement, though the ORP60 will workjust as well.

Due to the large tolerance ofelectrolytic capacitors, it may be worthbuying a pack of ten, say, and choosingthe one which gives a nice even'warble'. A good bargain on thesecommonly used capacitors can beobtained from Delta Tech Er Co. Ltd., of82, Naylor Road, London N20 OHN.

The cost of this Quick Project isaround £4.

Signal GeneratorTHIS DESIGN must be one of the easiestsignal generator circuits around, for thequality. It uses only a single IC! Most ofthe cost is in the switches and controlsand the only hard -to -obtain componentis the thermistor (RA53), which is soldby Maplin, or Watford's R53 type is asuitable equivalent.

The heart of the project, ICI(TDA2006), is available fromTechnomatic, where you can also getthe rotary switches (they call themwafer switches).

The resistors and potentiometers canbe purchased from Watford orElectrovalue, who will be able to supplyall the capacitors between them.

The case used on our prototype wasa plastic type with a single metal panelon the top. This was used because of itsneat appearance, but any small case canbe used. A glance through the WestHyde catalogue will give you some ideaof the wide range available.

The cost of this project, complete, isabout £7, depending on which case youchoose.

Echo-ReverbTHE HE Echo-Reverb is a single PCBproject, making constructionstraightforward if you follow theinstructions in the article carefully. Theonly component requiring specialhandling is the 4046 CMOS IC. Luckily,they are quite easy to obtain should youhave to replace one.

The remaining semiconductors arequite a mixed bunch, but not unusual.The voltage regulator, bridge rectifierand signal diodes are stocked byWatford, who are also one source forthe PCB mounting transformer. If youhappen to get a type that doesn't fit ourboard, it is sometimes possible tochange the position of the tags - butbe careful you don't break the thin wiresfrom the primary windings.

The delay line ICs (TDA1022) arequite expensive and it's worth shopping

around a bit for the cheapest. RapidElectronics offer them at a reasonableprice, with the added bonus of a quickturn around for mail order customers.

The 1000 microfarad capacitor,though not an unusual component, willhave to be chosen to fit on the PCB.Electrovalue supply the correct sizecapacitor, as well as the others in theproject.

Most of the hardware andswitches,etc are to be found in the Bi-Pak catalogue, along with a goodselection of knobs and the pots that gowith them.

We used quite an expensive case forthe prototype, to give it a professionalfinish. It also provided the necessaryscreening, being a totally enclosed metalbox. A suitable alternative may be foundin the range of instrument cases offeredby BICC-Vero. If you leave out the casein estimating your total outlay, it shouldcome to around £24.


Cable TrackerThe design feature of the Cable Trackeris its neat construction, enabling it to behand-held. Some of the componentswere specially selected for the projectand they may, therefore, be difficult toobtain. However, Magenta Electronicsare offering a full kit of parts, includingthe PCB, for £9.37 plus 45p postage.They will also supply any individualcomponents you can't get hold of,though most of them are generallyavailable from mail order suppliers.

A good bet for the capacitors isalways Electrovalue, but you mustensure that low temperature coefficienttypes are used where specified; analternative to this is to use polystyrenetypes.

The semiconductors, including thetransistors, diodes and IC, are availablefrom Watford, Rapid or Technomatic.

This leaves the piezo transducer andthe coil. Both these items were speciallyselected for the circuit, however, thetransducer can be replaced with anydevice of high impedance (a fewkilohms or so) and the coil can be anylong wave type, though you may losesome sensitivity. HE

68 Hobby Electronics, May 1982

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TECHNICAL TRAININGIN ELECTRONICS ANDTELECOMMUNICATIONSICS can provide the technical knowledge that is so essentialto your success.knowledge that will enable you to take advant-age of the many opportunities open to the trained man. Youstudy in your own home, in your own time and at your ownpace and if you are studying for an examination ICS guaranteecoaching until you are successful

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Hobby Electronics, May 19E 69

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ottHere is a small selection of thebooks available from HE'sbook service. New titles willappear each monthELEMENTS OF ELECTRONICSby F A WilsonThis series of books covers thebasics of electronics, in an easy tounderstand manner. The topics arewritten so that important conceptscan be grasped by the beginner andyet they can also provide an in-depthreference source for the practisingengineer.Book 1: THE SIMPLE ELECTRONIC CIR-CUITANDCOMPONENTS £2.25This book contains all the fundamentaltheory necessary to lead to a fullunderstanding of the simple electroniccircuit and its main components.Book 2: ALTERNATING CURRENTTHEORY £2.25Sinewaves, complex waveforms, timeconstants, reactance, resonance andother important aspects of AC arecovered.Book 3: SEMICONDUCTORTECHNOLOGY £2.25From simple atomic structure modelsthrough to complex integrated circuitsand the elements of computers.Book 4: MICROPROCESSINGSYSTEMSANDCIRCUITS £2.95Starting with simple computer models,this book takes the reader up to com-plete microprocessing systems andtheoretical circuits.Book 5: COMMUNICATIONS . . £2.95All aspects of communication systemssuch as channel bandwidth, transmis-sion systems and signal processing, arediscussed in this final book of the series.

A MICROPROCESSOR PRIMERby EA Parr £1.75Newcomers to electronics and com-puting tend to be overwhelmedwhen first confronted with literatureabout microprocessors. This bookhelps to alleviate the problem by re-counting the design of a simple com-puter in an easy to understand man-ner.

PRACTICAL COMPUTER EX-PERIMENTSby EA Parr £1.75Readers of this book will findthemselves involved in experimentswhich help to explain the inner work-ings of computers andmicroprocessors. All circuits and ex-periments use discrete logic circuitsto demonstrate such things as 'ad-ders', 'stores', 'arithmetic and logicunits' etc.

HOW TO MAKE WALKIE-TALKIESbyFGRayer £1.50Low power hand-held, or portable,radio transmitting/receiving equip-ment need not be complicated - asthis book shows. A book of great in-terest, not only to the licensedoperator, but also to those en-thusiasts without transmittinglicences who wish to participate as

listeners with the describedreceiving equipment.

ELECTRONIC PROJECTS FORBEGINNERSbyFGRayer £1.35The newcomer to electronics willfind this a very useful book. It con-tains a wide range of easily made pro-jects including component and wir-ing layouts. A number of projects canbe constructed without the use of asoldering iron.

POPULAR ELECTRONIC PROJECTSby R A Penfold £1.45A collection of circuits and projectsto interest most electronics con-structors, covering four popular mainareas: radio; audio; household pro-jects and test equipment.

INTERNATIONAL TRANSISTOREQUIVALENTS GUIDEby A Michaels £2.95Transistors from over 100 interna-tional manufacturers are tabulated inthis book in an easy to understand,cross-referenced format, to enablethe reader to quickly locateequivalent devices from an alter-native source. This book is an ex-tremely useful addition to the elec-tronics enthusiast's library.

To receive your books fill in the form below (or write the details on a sheet of paper) and send it, with your pay-ment, to the address given.Please wait 28 days for delivery. The offer applies to the UK only. Prices may be subject to change without notice.


I enclose a cheque/postal order made payable to ASP Ltd, for theamount shown below

ORI wish to pay by Barclaycard/Access. Please charge to myaccount number





Books Required

Add 75p p&p

Total enclosed £



Hobby Electronics, May 1982

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Price3'/z digit DPM kit 19.95LCI) 31/2 digit module 7.503'/z digit LCI) display & driver 11.004 0.5 inch LEI) s 4.0031/2 digit LED A/I) driver 6.00IL74 Optocoupler 0.70Optoelectronic photointerruptermodule 2.50IR emitter/phototransistor (pair)

0.75LCD 31/2 digit module &backlighting panel 13.0031/2 digit LCI) panel metermodule 19.95


TITAN DRILL AND STANDEngineering in miniature, the most powerful British, 12 VoltD.C. drill made, producing 10,000 R.P.M., 1740 C.M.P. comescomplete with tommy bar, alien key, steel chuck + 4 collars,size zero, 1/16, 3/32, 1/8. Specially designed for drilling P.C.Board, mild steel, and many other uses in electronics, othercrafts and hobbies.

Also availablespecially designedDura Life P.C.B.Drills, very hard anddurable, but notbrittle like Tungsten

Price £1.10




Titan Drill, complete4 collars, Cat No 0175

£10.28Dual purpose Stand

Cat No 0209£12.00

Hundreds of toolsand kits to choose

fromTransformer Rectifier

also available

Available from your local stockistsEXPO Prices are approximately 40% cheaper than all other makes.

Distributors, Wholesale, Retail and Export enquiries welcomeMany other Expo drills, tools, kits available

Send for your copy of latest catalogueand price list today

EXPO (DRILLS) LTD Unit 10, Sustanum WorksTitchfield, Hants Tel Titchfield 103294141752


products is now available in theU.K. and Ireland from RHEINBERGS

SCIENCES LIMITED. Over 2000 productsfor industry, education and the enthusiast.

Send 60p for your colour catalogue and you'llreceive El off your first order. Or visit our showroom

Magnifiers & MicroscopesLight/Infrared ProductsFibre OpticsMotors & PumpsPolarizing MaterialToolsSolar Energy

Optics/LasersMagnetsLaboratory EquipmentPhotographyEducational KitsDiffraction GratingsHolography

RHEINBERGS SCIENCES LIMITED, Dept. HE4Sovereign Way. Tonbridge, Kent TN9 1RN. Tel: 0732 357779

Your Leading OPTOELECTRONIC SupplierExcellent Quality - Very Good Prices - Broad Range

PriceLED'sPacks of 10red 3 mmgreen 3 mmyellow 3 mmred 5 mmgreen 5 mmyellow 5 mmElectrofluorescentbacklightingLCD panel



Contact us for other opto products All prices are exclusive of VAT.ELECTRONIC HOBBIES LTD Add P&P at 0.45 to total order

Roxwell Road, Chelmsford, Essex CM1 2LY Tel (0245) 62149

First the EuroBreadBoardNow the EuroSolderBoard


for the professionalIdeal

for the beginner

Design on a EuroBreadBoard - Instal on a EuroSolderBoard

First the EuroBreadBoardWill accept 0.3" and 0.6" pitch DIL IC's, Capacitors, Resistors,LED's, Transistors and components with up to .85mm dia leads.500 individual connections PLUS 4 integral Power Bus Strips alongall edges for minimum inter -connection lengths.All rows and columns numbered or lettered for exact locationindexing (ideal for educational projects)Long life, low resistance (<10m ohms) nickel silver contacts£6.20 each or £11.70 for 2

Now the EuroSolderBoardNew 100mm square, 1.6mm thick printed circuit board with pre -tinned tracks identically laid out, numbered and lettered to Euro-BreadBoard pattern.Four 2.5mm dia fixing holes.£2.00 for set of three ESB's

And don't forget the EuroSolderSuckerIdeal for tidying up messy solder joints or freeing multi -pin IC's, this195mm long, all metal, high suction desoldering tool has replaceableTeflon tip and enables removal of molten solder from all sizes ofpcb pads and track. Primed and released by thumb, it costs only£7.25 including VAT & PP

Snip out and post to David George Sales,James Carter Road, Mildenhall, Suffolk IP28 7DE

ImmoDavid George Sales, HE3James Carter Road, Mildenhall, Suffolk IP28 7DEPlease send me: -

1 EuroBreadBoardor 2 EuroBreadBoardsor 3 EuroSolderBoardsor 1 EuroSolderSucker

@ £ 6.20 0@ £11.70 0 Please

@ £ 2.00 0 Tick@ £ 7.25 0

All prices are applicable from Jan. 1st, 1981 and include VATand PP but add 15% for overseas orders.



Tel. NoPlease make cheques/P.O. payable to David George Salesand allow 10 days for cheque clearance and order processing

Hobby Electronics, May 198 71

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0110 0-0


S. DJ-


Below: The two foil patterns for the Digital Thermometer. Some of the tracks onthis board run quite close together, so make a thorough check of the undersidebefore switching on! The main board fits snugly inside the case and must be madeto fit - the small board's width must also be controlled, as there is a limited gapbeneath the top of the case.

Above: PCB foil pattern for the HECable Tracker; don't forget to drillthe three lead -out holes on the left.

Left: The HE Echo-Reverb PCB foilpattern. This board will take the mainsvoltage inputs at the bottom left, somake sure all the copper is etchedaway. The tracks contain sharp anglesat some places - be careful to checkthese have not been dissolved by theferric chloride.

meas CO




vaLt).:- c HE Th.

as gm!


+6V = J

72 Hobby Electronics, May 1982

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From National Semiconductor:Some Egs: 4011 15p 4013

4093 38p 4016174LS00 12p 74LS90LM380 75p TBA120

Full range list available + Quantity discounts.PCBs. The FOTOway - Brochure

NEW BARGAIN Packs:16 - TTL ICs 0.10 6 - Slide Switches 60p9 - CMOS ICs £1.10 100 - 1/8 Watt Resistors 60p10 - Croc Clips £0.60 30 - 1 Watt Resistors 60p5 - Ex Equipment Bds £1 5 - Large Elect Caps f18 - Knobs f1.20 4 - Rotary Switches £1.20

Post & packing 60p per order. FREE over £10ALL PRICES + VAT

PAT.H. Electronic Services Wholesale + Retail369 Alum Rock Road, Birmingham B8 3DR. Tel: 021 327 2339

Open 10 till 6 weekdays. 9.30 till 5 SaturdaysDistributors for: Antex - Altai - Babani - Hills - Servisol - PATH FOTOway --

PATH Box - Safyan - Vero Electronics 4- MUCH MUCH MORE . . .


34p 4017 42p69p 4543 134p33p 74LS386 28p90p TDA2640 216p

Ask for detailson Request.

fe)01014'i,141440iiiii 1001Litesold's new 'L' Series soldering iron - now at a bargain priceOutstanding performance. Lightweight. Easy to maintain.Elements are enclosed in Stainless Steel shafts,insulated with mica and ceramic. Non- seizeinterchangeable bits, choose from'copper' or 'long life'. A very specialtool at a very special 'direct' price.Just £5.58 for iron fitted with 3.2mmcopper bit. Just 52.40 for 3 sparecopper bits (1.6; 2.4; 4.7).A mere 54.38 forprofessional springstand! Or buy thelot for 111.12!and save 10%.All prices inc. VAT P.& P.Please allow 14 days delivery.Write today. Send Cheque/P.O. to Litesold, 97-99 Gloucester Road, Croydon CRO 2DN

or phone 01-689 0574 for Barclaycard/Access sales.


Send for my CATALOGUE ONLY 75p(plus 25p post/packing)

My VAT and postIpacking inclusive prices are thelowest. All below normal trade price - some at only

one tenth of manufacturers quantity trade.

See my prices on the following:


















TAPE, BOARDS and others.

Prices you would not believe before inflation!


ESTABLISHED 25 YEARS161 St. Johns Hill, Battersea, London SW11 1TQ

Open 11 am till 7 pm Tues. to Sat. Telephone: 01-223 5016

ANY SINGLE service sheet £1/ L.S.A.E.Largest stockists service/repair manuals.Named T.V. repair data £6.50 (with circuits£8.50). S.A.E. pricelists. Free publications,quotations. Aushe, 76 Churches, Larkhall,Lanarkshire.

DIGITAL WATCH BATTERIES any type£1.20 each. Send SAE or 15p with number orold battery to DISCLEC Y, 511 FulbridgeRoad, Werrington, Peterborough,

ADVERTISEMENTRATES Semi -Display (min 2 cms)1-3 insertions £6.00 per cm4-11 insertions £5.50 per cm12+ insertions £5.00 per cmLineage 21p per word (min 15 words)Box Nos. £2.00Closing date 2nd Friday of the monthpreceding publication date.All advertisements in this section must be prepaid.Advertisements are accepted subject to the terms andconditions printed on the advertisement rate card (availableon request)

Send your requirements to:


CLOSE ENCOUNTERS GROUP Personalintroductions and social events. Meet interest-ing, attractive people. Local, 051-931 2844 (24hours).

16K ZX81 SUPER INVADERS. Machinecode sophistication, 54 manoeuvring aliens,saucers, defence bases, laser firing, on-screen scoring. On cassette with graphicHANGMAN, SWAT, GOLF and BREAK-THRO'. £3.50. J. Prince, 29 Brook Ave.,Levenshulme, Manchester M19.

LOTS OF USEFUL ITEMS for constructors.Cases, transformers, P.C.B.'s etc. S.A.E. forlist. Mr. Hyde, 3 Rosebery Road, SmethwickWarley, West Midlands B66 3R7.

PRINTED CIRCUIT BOARDS 4'/2" x 6"copper clad resin paper. 10 for £4.00 includingU.K. post. Hill, 22-26 Bath Road, WorcesterWR6 3EL.

Hobby Electronics, May 19F 73

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IONISER KIT (MAINS OPERATED)This Negative Ion Generator gives you the power to saturate your home or office with millions of refreshing ions. Withoutfans or moving parts it puts out a pleasant breeze. A pure flow of ions pours out like water from a fountain, filling yourroom. The result? Your air feels fresh, pure, crisp and wonderfully refreshing.All parts, PCB and full instructions E12.50A suitable case including front panel, neon switch, etc E10.50

Price includes Post & VAT Barclaycard/Access welcome


44 WALLACE ROAD, LONDON N.1.TEL: 01-2261489

Hours: Mon -Fri 9-5 p.m. Sat 9-4.30 p.m.

BOOKS BOOKS BOOKS. "PracticalComputer Experiments BP78. Published byBabani. A highly recommended book for thenewcomer to electronics and computing.Explanations about analogue digital circuits,logic, memories, triggers, control systems,digital arithmetic, encoders, adders, shiftregisters, computer architecture, A/D,D/A, converters and more. Also experimentalcircuits to construct. £1.75 + 35p p&p.Despatch within 24 hours. ALPHA BOOKS,Registered Office, 18 Connaught Close,Hemel Hempstead, Herts HP2 7AB.

REPLACE ANY 996 type battery with the X -Cell Plus and improve equipment perform-ance. Incorporates all circuitry to allowautomatic recharging from almost any supplybetween 8-300 volts - Up to 3000 times.Fully Guaranteed. £19.95 (inc VAT + pap).Nitech (Northern) Ltd, Gallowfields TradingEstate, Richmond, N. Yorks.

INDIVIDUAL POSTAL TUITION in Math-ematics. R.D. Calver Ph.D., 6 Albany Road,Bexhill -on -Sea, E. Sussex.

ELECTRONICS component shop inMAIDSTONE, KENT! Thyronics controlsystems, 8 Sandling Road, Maidstone, Kent.Maidstone 675354.


100 watt superb treblelbau overdrive, 12 months' guarantee. Unbeatable at£50; 50 wan £44; 200 watt £88; 100 watt twin channel sep. treble/

bass par channel 685 60 wan 652; 200 watt 618; 100 wan four-ohannei sap.treblelbass per channel 675; 200 watt 698; slaves 100 watt 631; 200 watt II

£60; 250 wan 170; 500 wan 6140; fuzz boxes, great sound, £12; bass fuzz£12.90; overdriven Nu with treble and bass boosters, £28; 100 watt combo,superb sound, overdrive, sturdy construction, castors, unbeatable, £100;twin channel, 6115; bass combo £118; speakers 15in 1D0 watt £36; 12in 100watt £24; 60 wan £18; microphone shure unglyn 8 628.

Send chegued4.0, to: WILLIAMSON AMPLIFICATION62 Therncliffe Avenue, Ouirintield, Cheshire. Tel 061-308 2064

AMAZING ELECTRONICS PLANS. Lasers;Super -powered Cutting Rifle, Pistol, LightShow. Ultrasonic Force Fields, Pocket De-fence Weaponry, Giant Tesla, Satellite TVPyrotechnics, 150 more projects. Cata-logue95p. - From Plancentre, 16 Mill Grove,Bilbrook, Codsall, Wolverhampton.

DIGITAL DESIGN DECK PLANS for bread-boarding prototype - experimental circuits.Educational aid. £3. From R. Caunt, 16Baggley Crescent, Mansfield, Notts.NG19 7DS.

AERIAL AMPLIFIERS Improve weaktelevision reception. Price £6.70. S.A.E. forleaflets. Electronic Mailorder, Ramsbottom,Lancashire BLO 9AGH .

BURGLAR ALARM equipment. RingBradford 10274) 308920 for our catalogue, orcall at our large showrooms opposite OdsalStadium. C.W.A.S. Ltd.

BUILT TRANSMITTERS £2.90. Receive onFM. Range 150yds. Refund Guarantee.(Unlicensable). P. Faherty, 37 College Dr.,Ruislip.

NEW 1982 ACE COMPONENT CATA-LOGUE Let your problems be our business.Be certain; have your components deliveredquickly and, efficiently and get that projectworking. Send 30p now for the easy to use1982 Catalogue to: Ace Mailtronix, DeptH.E.3A, Commercial Street, Batley,W. Yorks WF17 5HJ.

PRINTED CIRCUITS. Make your own sim-ply, cheaply and quickly! Golden Fotolac!light-sensitive lacquer -_now greatly im-proved and very much faster. Aerosolcans with full instructions, £2.25. De-veloper 35p. Ferric Chloride 55o. Clearacetate sheet for master 14p. Copper -cladfibreglass board, approx. 1mm thick£1.75 sq. ft. Post/packing 75p. WhiteHouse Electronics, Castle Drive, PraaSands, Penzance, Cornwall.

PARAPHYSICS JOURNAL (Russian trans-lations); Psychotronic Generators, Kirliano-graphy, gravity lasers, telekinesis. Details:S.A.E. 4 x 9" Paralab, Downton, Wilts.

CENTURION BURGLAR ALARM EQUIP-MENT Send S.A.E. for Free list or aCheque/PO for £5.95 for our Special Offer ofa Full Sized Decoy Bell Cover, to: Centurion,Dept HE, 265 Wakefield Rd., Huddersfield,W. Yorkshire. Access & Barclaycard.Telephone orders on 0484-35527.









British design & manufacture.Tested & fully guaranteed.

Ex -stock delivery

16-95 PInocst & PackuAT 60p

Ideal for the experimenterCOMPLETE WITH DATA

HEIVRY5404 EDGWARE ROAD LONDON W2 1ED..................

£ 1 2.9 9 + vATI: k1 T A Single P.C.B. on which to mount the L.C.D.,

" voltmeter I.C. and other components (ALLIII SUPPLIED) to build a 200mV full scale 3% digit DVM which is powered from a single 9V battery (not

supplied). A data sheet supplied with each kitillustrates useful circuits enabling you to use this DVMas a basis for further projects.

We can only hold this price while current stocks last, so order NOW and be sure. Cheques and P.O.'s to Measuretech Instruments (Electronics)

Ltd., P.O. Box 31, Aldershot, Hants. GU 12 ERZ [MAIL ORDER ONLY] ALLOW 28 DAYS DELIVERY


:DVM LIGHTNINGDO YOU NEED: Electronic components, Tools, Test Equipment, Cases, Cabinetsand Hardware Etc. IN A HURRY777777777????7777777777777777777177777777777THEN YOU NEED:LIGHTNING Electronic ComponentsWHY7777?

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LIGHTNING ELECTRONIC COMPONENTS84 Birchmoor Road, Birchmoor, Tamworth, Staffs B78 1AB

(NOTE New Address)

74 Hobby Electronics, May 19F

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MULLARD SPEAKER KIT40 watt R.M.S. 8ohm DESIGNEDBY MULLARD SPECIALIST TEAMIN BELGIUM comprising a Mullard8" woofer with foam rolledsurround, Mullard 3" high powerdome tweeter and a cleverlydesigned B.K. Electronics crossovercombining spring loaded loud-speaker terminals and recessed

_ mounting panel. Supplied completewith assembly and cabinet details.Recommended cabinet size 240 x216 x 445mm.

PRICE £13.90 + £1.50 P&P per kit

typr, keys

STEREO CASSETTE TAPE DECK MODULE.Comprising of a top panel and tape mechan-ism coupled to a record/play back printedboard assembly. Supplied as one completeunit for horizontal installation into cabinet orconsole of own choice. These units are brandnew, ready built and tested.Features: Three digit tape counter. Auto -stop. Six piano type keys, record, rewind,fast forward, play, stop and eject. Automaticrecord level control. Main inputs plussecondary inputs for stereo microphones.Input Sensitivity: 100mV to 2V Input Im-pedance: 68K. Output level: 400mV to bothleft and right hand channels. Output Im-pedance: 10K. Signal to noise ratio: 45dB.Wow and flutter: 0.1%. Power Supply re-quirements: 18V DC at 300mA. Connections:The left and right hand stereo inputs andoutputs are via individual screened leads, allterminated with phono plugs (phono socketsprovided). Dimensions: Top panel 51/2in x11 r/xin. Clearance required under top panel21/4in. Supplied complete with circuit dia-gram and connecting diagram. Attractiveblack and silver finish.Price £26.70 + £2.50 postage and packing.Supplementary parts for 18V D.C. power supply(transformer, bridge rectifier and smoothingcapacitor) £3.50.

NEW RANGE QUALITY POWER LOUD-SPEAKERS (15", 12" and 8"1. Theseloudspeakers are ideal for both hi-fi anddisco applications. Both the 12'. and 15"units have heavy duty die-cast chassisand aluminium centre domes. All threeunits have white speaker cones and arefitted with attractive cast aluminium!ground finish) fixing escutcheons.Specification and Price; -

15'' 100 watt R.M.S. Impedance 8ohm59 oz. magnet, 2- aluminium voice coil.Resonant Frequency 20Hz. FrequencyResponse to 2.5KHz. Sensitivity 97dB.Price E32 each, £2.50 Packing and Car-riage each.

12" 100 watt R.M.S. Impedance 8 ohm, 50 oz magnet 2 a mum voice coil.Resonant Frequency 25Hz. Frequency Response to 4KHz. Sensitivity 95dB. PriceE23.70 each. £2.50 Packing and Carriage each.

8" 50 watt R.M.S. Impedance 8 ohms, 20 oz. 1'/" aluminium voice coil, ResonantFrequency 40Hz, Frequency Response to 6KHz, Sensitivity 92dB. Also available withblack cone fitted with black metal protective grill. Price: White cone £8.90 each. Blackconelgrill E9.50 each. P & P f1.25 each.

PIEZO ELECTRIC TWEETERS - MOTOROLAJoin the Piezo revolution. The low dynamic mass (no voice coil) of aPiezo tweeter produces an improved transient response with a lowerdistortion level than ordinary dynamic tweeters. As a crossover is notrequired these units can be added to existing speaker systems of upto 100 watts (more if 2 put in series). FREE EXPLANATORY LEAFLETSSUPPLIED WITH EACH TWEETER.






TYPE 'A' IKSN2036A) 3" round with protectivewire mesh, ideal for bookshelf and mediumsized Hi-fi speakers. Price £3.45 each.

TYPE 'B' IKSN1005A) 3 V," .super horn. Forgeneral purpose speakers, disco and etc. Price £4.35 each.

TYPE 'C' IKSN6016A) 2" x 5" wide dispersionhorn. For quality Hi-fi systems and qualitydiscos etc. Price £5.45 each.

TYPE 'D' IKSN1025A) 2" -r 6" wide dispersionhorn. Upper frequency response retainedextending down to mid range 12KHzI. Suitablefor high quality Hi-fi systems and qualitydiscos. Price £6.90 each.

TYPE 'E' (KSN1038A) 3K" horn tweeter withattractive silver finish trim. Suitable for Hi-fimonitor systems etc. Price £4.35 each.

TYPE 'F' IKSN1057A) Cased version of type'E'. Free standing satellite tweeter. Perfectadd on tweeter for conventional loudspeakersystems. Price £10.75 each.U.K. post free for SAE for Piezo leaflets).

PRICE £63.25UK Post FREE




five ranges*Up to 330MHz (Harmonics)

*Slow motion tuning

"Internal modulation 1KHz

'External mod. input facility

*Power supply 240v AC

"Size - 150.250.130mm

"Prise - £63.25 - UK.Post Free

1K.WATT SLIDE DIMMER Controls loads up to 1KW Compact size

430" x13 "x 2 V0"


Easy snap in fixing throughpanel/cabinet cut out

Insulated plastic case Full wave control using 8amp

triac Conforms to 8S800 Suitable for both resistance

and inductive loadsInnumerable applications inindustry, the home, and discos/theatres etc.

Price: £11.70 each+ 50p P&P. (Any quantity)

BSR P256'TURNTABLEP256 turntable chassis S shaped tone arrn Belt driven Aluminium platter Precision calibrated counter balance Anti -skate (bias device) Damped cueing lever 240 volt AC operation (Hz) Cut -ourtemplate supplied Completely manual arm.This deck has a completely manual arm and isdesigned primarily for disco and studio usdwhere all the advantages of a manual arm are

required.Price: £28.50 4- £2.50 P&P


Vu Meter

Matching 3 -way loudspeakers'and crossoverBuild a quality 60watt RMS system 8ohms

Build a quality 60 watt R.M.S. system.

* 10" Woofer* 3- Tweeter* 5- Mid Range

* 3 -way crossover

Fitted with attractive cast aluminium fixing es-cutcheons and mesh protective golls which areremovable enabling a unique choice of cabinetstyling. Can be mounted directly on to bafflewith or without conventional speaker fabrics.All three units have aluminium centre domesand rolled foam surround. Crossover com-bines spring -loaded loudspeaker terminals andrecessed mounting panelPrice £22.00 per kit + £2.50 postage and pack -Inn. Available separately. prices on request.

12" 80 watt R.M.S. loudspeaker.A superb general purpose twin cone loud-speaker. 50 oz. magnet. 2' aluminiumvoice coil. Rolled surround. Resonant fre-quency 25Hz. Frequency response to13KHz. Sensitivity 95dB. Impedance 8ohm.Attractive blue cone with aluminiumcentre dome.Price £17.99 each + £2.50 P&P.


BK ELECTRONICSPrompt Deliveries

VAT inclusive


Audio Equipment

Test Equipment






General purpose full range loudspeaker, idealfor mini systems etc.

Rolled fabric surround Twin cone Bohmimpedance 15 watt RMS 1" voice coil

013oz magnet Frequency range 50/15000Hz

Price: £6.90 each + 75p P&P

100 WATT R.M.S.Power Amplifier Modules with integral toroidaltransformer power supply and heat sink.Supplied as one complete built and testedunit, Can be fitted in minutes. Auxilliarystabilised supply and drive circuit incorporatedto power an L.E.D. V.u. meter, available as anoptional extra.SPECIFICATION:Max. output power 100 watts R.M.S. IOMP1001Loads: (Open and short circuit proof) 4-16 ohms

Frequency Response20Hz-25KHz :3 3dBSensitivity for 100 watts 500mV at 10KT.H.D. 00.1%

Size: 360 x 115 x 80rnmPrices: OMP 100W £29.99 62.00 P&P

V.u. Meter £6.50

37 Whitehouse Meadows, Eastwood, Leigh -on -Sea, Essex SS9 5TY* SAE for current lists. * Official orders welcome. * All prices include VAT. * Mail order only. * All items packed (where

applicable) in special energy absorbing PU foam. Callers welcome by prior appointment, please phone 0702-527572.


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k w -



and fitting instructions

are includedtogether

with circuit qualitydHtcornponents


used throughout.

424i; 080



NOW AVAILABLE IN KIT FORMS X1000Electronic Ignition

Inductive Discharge Extended coil energy

storage circuit Contact breaker driven Three position changeover switch Over 65 components to assemble Patented clip -to -coil fitting Fits all 12v neg. earth vehicles


SX2000Electronic Ignition

The brandleading systemon the market today

Unique Reactive Discharge Combined Inductive and

Capacitive Discharge Contact breaker driven Three position changeover switch Over130 components to assemble Patented clip -to -coil fitting Fits all 12v neg. earth vehicles



f fti ,

IMP t lc -

-..-r` I


r '

-4.4 91'-6*--

A ACDE F G .61 I.-AlAT -80Electronic Car Security System

Arms doors, boot, bonnet and has security loop to protectfog/spot lamps, radio/tape, CB equipment

Programmable personal code entry system Armed and disarmed from outside vehicle using a special

magnetic key fob against a windscreen sensor pad adhered tothe inside of the screen Fits all 12V neg earth vehicles

Over 250 components to assemble


". 26.10.3



MAGIDICEElectronic Dice

Not an auto item but great funfor the family

Total random selection Triggered by waving of hand

over dice Bleeps and flashes during a 4 second

tumble sequence Throw displayed for 10 seconds Auto display of last throw 1 second in 5 Muting and Off switch on base Hours of continuous use from PP7 battery Over 100 components to assemble Supplied in superb presentation gift box

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TX2002Electronic Ignition

The ultimate system Switchablecontactless. Three position switch with

Auxiliary back-up inductive circuit. Reactive Discharge. Combined capacitive

and inductive. Extended coil energy storagecircuit. Magnetic contactless distributor trigger -head. Distributor triggerhead adaptors included.

Can also be triggered by existing contact breakers. Die cast waterproof case with clip -to -coil fitting Fits

majority of 4 and 6 cylinder 12v neg. earth vehicles. Over 150 components to assemble

VOYAGER Car Drive Computer A most sophisticated accessory. Utilises a single chip maskprogrammed microprocessor incorporating a unique programmedesigned by EDA Sparkrite Ltd. Affords 12 functions centredon Fuel, Speed, Distance and Time. Visual and Audible alarmswarning of Excess Speed, Frost/Ice, Lights -left -on. Facility tooperate LOG and TRIP functions independently or synchronously. Large 10mm high 400ft-L fluorescent display with autointensity. Unique speed and fuel transducers giving aprogrammed accuracy of + or -1%. Large LOG & TRIP'memories. 2,000 miles. 180 gallons. 100 hours. Full Imperialand Metric calibrations. Over 300 components to assemble.A real challenge for the electronics enthusiast!

' 6 1

-6. l8 b' 8 4',

Alf EDA-SPARKRITE products and designs are fully covered by one or more World Patents




SX 1000 £12.95 £25.90SX 2000 £19.95 £39.90TX 2002 £29.95 £59.90AT. 80 £29.95 £59.90

VOYAGER £59.95 £119.90MAGIDICE £9.95 £19.90PRICES INC. VAT.1


III MI - III OMStreet, Walsall, West Midlands, WS1 3DE England. Tel: (0922) 614791