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NEXT MONTHINTO THE NINETIES

WITH PE!

We ring in Christmas and the NewDecade with our bumper bundle ofproject ideas, including a keypadoperated Eeprom Programmer that's fartoo good to miss. And, especially for theearly starters, we begin a brand newseries on Basic Electronics, telling youhow and why electronics works! Plus, ofcourse, our usual feast of other greatfeatures.

* WELCOME THE NEWDECADE WITH

OUR JANUARY 1990ISSUE

* ON SALE FROM FRIDAYDECEMBER 1ST

* YOU CAN'T BEAT OURVALUE

* OR OUR CELEBRATEDOFFERINGS!

PRACTICAL ELECTRONICSVOL 25 NO 12 DECEMBER 1989

CONTENTSCOMPETITION RESULTSMAPLINOSCOPY WINNERS! 45All is revealed - the winners and the answers to the competition withMaplin 20MHz oscilloscope as prizes!

CONSTRUCTIONAL PROJECTSVIDEO AGC STABILISER by Robert Penfold 12Copy -protected video tapes may not satisfactorily play back on some videomachines. Using an agc stabilising interface might help cure the problem.

MONO -STEREO ECHO STATION - PART TWO by John Becker.27Taking up where we left off in Oct 89, we conclude the construction andsetting -up of this very versatile audio effects modifier.

THERMAL CONVERSION VOLTMETER by Rod Cooper 35A companion to the true rms ammeter of June 89, this ingenious voltmeter isspecifically designed for use in power electronics.

GATED TIMING by Andrew Armstrong 38As an answer to this month's Ask PE query, Andrew describes another way inwhich timing periods can be extended, using selectable digital gates.

MINI METRONOME by John Becker 48A flashy led and audio additive circuit doubly marks accented beatsfor musicians, and is this month's Easi-build project.

SPECIAL FEATURESREPAIRING THE AL80 by Vivian Capel 17The manufacturers of the old-time favourite power amp may be obsolete, butthe module needn't be!

HF RADIO - PART FIVE by Mike Sanders 21In the penultimate article in the series, special refinements to receiversare examined, including agc and squelch considerations.

A PARTY TO REMEMBER by John Becker 30The Party's over but the memories (and the hangovers?) linger on.Nostalgic reunions, spirited chats, a shining award, an odd -ode and a tourround the Steam Museum suitably commemorated our 25th Birthday.

UCT MEASUREMENTS - PART TWO by Anthony H. Smith 41Concluding tf e. authoritative advice on how to get the best out ofuniversal counter timer measurements.

REGULAR FEATURESEDITORIAL by John Becker - Cop out 9

LEADING EDGE by Barry Fox - Deaf video phones 8

INDUSTRY NOTEBOOK by Tom Ivall - "Sorry, wrong person!" 57

SPACEWATCH by Dr Patrick Moore - Neptune and Venus 46

PRODUCT FEATURESANNUAL INDEX -January to December 1989 60

NEWS AND MARKETPLACE - what's new, where and when 4

PCB SERVICE - professional PCBs for PE Projects 44

BOOKMARK - Your Ed browses through some new books 50

ARMCHAIR BOOKSHOP - haven for practical bookworms 58ADVERTISERS' INDEX - locating favourite stockists 62

PE FURTHERS TECHNOLOGY'S FUTURE - BE PART OF IT!

N EJW`--S

NEW AMSTRADFAX FACTS

On display at the PC Show inSeptember was a really elegant -

looking personal fax machine fromAmstrad, their FX9600T, selling at£599 plus vat.

One nice touch is that the machinehas an in -build full function telephonehandset allowing hands -free diallingas well as a 100 -number library ofcommonly used fax and phonenumbers. The sheet feeder takes up to20 sheets at a time (if you've everused as fax you'll know the advantage

of this facility), and there is a built-incutter which automatically cuts thereceived sheets to their originallengths.

Alan Sugar, Amstrad's chairmanis obviously proud of the last twofeatures, saying, "We have lookedcarefully at fax machines already soldin this price sector and have notedthat they failed to incorporate (these)two very important features. Our£600 machine includes them andmany others that you could onlyexpect to find on a model costingnearly twice as much."

The other key features that thisfax incorporates are : CCITT Group 3compatibility; user-friendly

interactive lcd display with 20 digitsby two lines; local copying facilityenabling desktop copying oforiginals; separate fax and phoneautodialling allowing up to 50 faxnumbers and a further 50 phonenumbers to be stored; automaticredial; special 16 -shade halftonefeature ensuring clear transmissionsof photos and graphics; standardbuilt-in parallel port offering highquality Epson and IBM compatibleprinter option, and the sending of faxdocuments from a word processorwithout using paper. There is anadditional feature that enablespersonal signatures to be scanned andstored in memory (can't helpthinking, though, I wouldn't wantmine stored like this foi fear offraudulent use).

For the business operator themachine offers a choice of usingeither BT or Mercury communicationlines. Amstrad say this machine is thefirst fax to be Mercury compatible.Finally, the machine has a batteryback-up, a paper -out alarm, andconfirms that the transmission hasbeen received.

"Personal computers are now acommonplace sight on people's desksand in their homes", observes AlanSugar, "and 1 consider that beforelong the same will be true of our faxmachine."

For further information on theFX9600T, contact Amstrad at 169Kings Road, Brentwood, Essex,CM14 9BB. Tel : 01-240 2520.

ANONYMOUSENCLOSURE

OK Industries have dubbed theirPacTec range 'the anonymous

enclosures' because, says the managingdirector James Doman, of their simple,gimmik-free, functional extemaldesign. Hundreds of thousands ofthese plastic boxes house hundreds ofmanufacturers' electronic circuits, butthe brand remains anonymous.

The latest instrument enclosurekits, made from impact resistant abs inmoulded -through colours, are simpleboth to 'design into' and assemble.Internally, moulded -in bosses, cardguides and panel guides cut assemblytime and production costs, and thecases can be supplied in kit andproduction quantities off -the -shelf.Eliminating tooling costs, the PacTecenclosures can, nevertheless, becustomised with specific customerpanel designs.

For further information contact:OK Industries UK Ltd., Barton FarmIndustrial Estate, Chickenhall Lane,Eastleigh, Hants SO5 5RR. Tel: 0703619841.

CATALOGUE

Continuing our alphabetical browsethrough advertisers' literature

Maplin have sent another copy of their catalogue. There's no doubtthat this bumper cat, of over 500 pages, is a must for any electronicsenthusiast, whether a raw beginner, or fully-fledged expert. Fromcomponents, kits and pcbs to tools, test gear and transformers, plusmasses more, this book covers the ranges. For this catalogue, contactMaplin Electronics, PO Box 3, Rayleigh, Essex, SS6 2BR. Tel : 0707554161. Maplin also have a professional supplies division too. I can'taltogether go along with the claim that the catalogue associated withthis division is the 'Electronic Component Buyers Encyclopaedia' but ithas a good range of products, from batteries through ics to xenontubes. Maplin Professional Supplies are at PO Box 777, Rayleigh,Essex, SS6 8LU. Tel : 0707 552961.

What an amazing organisation Mauritron are! Over 20 A4 pages inwhich they highlight their stocks of service manuals, repair and dataguides (even covering vintage wireless sets), and magazines. Manycomputing and electronic mag back issues are listed, including PE!

Mauritron also offer to do their best to obtain service manuals forunusual or obsolete pieces of equipment. They also publish theMauritron Review which, they say, is a unique publication catering foranyone who ever uses, repairs or sells any electronic/electricalequipment. Other services are offered by this enterprising company aswell. Mauritron Technical Services, 8 Cherry Tree Road, Chinnor,Oxon, OX9 4QY. Tel : 0844 51694.

Number One Systems say that their theme is to "make ComputerAided Design affordable". It is also apparent from the literaturethey've sent that they make cad easy as well. Among their manysoftware packages they cover pcb design (including one specifically forhobbyists), pen and photo plotting, a schematic symbol library for 7400ttl series, several versions of ac linear circuit analysis, and a textprocessor for programmers. Additionally, they have a range ofhardware, including pen plotters and pens. They also have theQ.A.Robot, a fully automatic, computer controlled, non -contact,optical inspection system for industrial applications. Number OneSystems Ltd, Harding Way, Somersham Road, St Ives, Cambs, PE174WR. Tel : 0480 61778.

Omni Electronics have sent their current catalogue, though theysay that a new one will be issued towards the end of the year. Don'twait though, for their current cat has a great many items suited to thehobbyist constructor. They have a very wide selection of active andpassive components, including semiconductors, resistors, capacitors,pots, knobs, stepper motors, uhf modulators, meters, etc. Of specialinterest to those who make their own pcbs are the items of pcbequipment and materials, including ferric chloride. OmniElectronics, 174 Dalkeith Road, Edinburgh, EH16 5DX. Tel : 031-6672611

Phonosonics' specialise in the supply of kits for publishedprojects, many of them published in PE. Their latest catalogue has hadmany more kits added to the range, and covers projects as varied ascomputer add-ons, sound effects, music modifiers, test gear,environmental monitors, and even a few fun designs! Send a stampedaddressed envelope to Phonosonics, Dept PEC, 8 Finucane Drive,Orpington, Kent, BR5 4ED. Tel : 0689 37821.

4 PRACTICAL ELECTRONICS DECEMBER 1989

NTHE ABC OFACTIVEBOOKS

News has come in of an intendedproduct that could well represent

the next significant wave incomputing technology.

The Active Book Company isdeveloping a pocket computerwithout a keyboard. The BookComputer will be about the size of apaperback book, and so fit into aninside jacket pocket. You will interactwith the active book by using a penwhich, in conceptual terms, writes on'electronic paper'. The 'paper' will infact be a liquid crystal display plus atransparent graphics tablet, and thepen an electronic stylus emitting asmall electromagnetic field. Thepaper is arranged as pages in a book,with chapters and page numbers.Having the illusion of a book, ABCsay that we can draw on theknowledge that people already haveabout books, their organisation intochapters, contents pages, indexes, andso on.

It is intended that the book shouldnaturally fit into the existingcomputer environment and becompatible with both UNIX and MS-DOS operating systems. IBM PCscan be used as base stations withwhich active books communicatewherever they are in range of acomms link, typically within a tenmetre radius.

The Active Book Co wasconceived by Dr Hermann Hauser,pictured above, a founder of AcornComputers in the late 70s, anddeveloper of the BBC micro. For thelast three years he has been VicePresident of Research of the Olivetti

Group. With his link with Acorn, itsprobably not surprising that the heartof the active book is an Acornproduct, the ARM, or Acorn RiscMachine. Using Risc architecture, themicroprocessor is capable of veryhigh speeds of operation since it hasfewer instruction command sets withwhich to contend.

ABC expect that the book willcost less than £1000, and willadditionally serve as a paperless faxmachine. They also intend that it shallbe a dictating machine, using anintegral microphone, storing thespoken message in the computer'sdigital memory. ABC haven'tdisclosed the book's memory size,but in the region of at least 100 pagesof text is likely, though couldultimately be as high as 300 pages.

The launch date of the active bookis somewhat dependent on theavailability of additional financialinvestment, but could be in about ayear's time. If you've got a few quidto invest, you could well reap afortune from the expected success ofthis new development.

The Active Book Company Ltdare at 4a Market Hill, Cambridge,CB2 3NJ. Tel : 0223 355144.

PE AT PC SHOWur sister magazine, Program Now,

...e was on full display at September'sPC Show at Earls Court and had aroaring success encouraging newreaders and advertisers, as well aswelcoming familiar friends.

Determined not to leave all theglory to our sister, PE prominentlyfound a position on this year's enlargedstand. We'd just received the 25thAnniversary Issue from the printers andso were pleased and proud to have that

on display, together with enlargedcolour posters of its cover and that ofthe very first issue from Nov 64.Behind the desk in the photo DavidHewett and David Bonner attend tointerested customers. On the right andignoring the photographer, PN EditorNigel Stuckey rests his elbow whilediscussing in depth programming.

Making a guest appearance (!) onthe stand on the Friday, I was pleased tochat with several PE readers, including

If you are organising any event to do withelectronics, big or small, drop us a line, weshall be glad to include it here.

Please note : Some events listed here may be trade or restrictedcategory only. Also, we cannot guarantee information accuracy, socheck details with the organisers before setting out.

Nov 7-11. Productronica. 8th International Trade Fair for Electronicsproduction. Munich Trade Fair Centre. 01-948 5166.

Nov 14-16. Total Solutions. Incorporating Drives, Motors, Controls,Interface, Data Acquisition, Telemetry UK, Coil Winding. NECBirmingham. 0799 26699.

Dec 3. 8th 'Aerial' Christmas Vintage Wireless stall sale and swap -meet, Clarence House, Bristol. Entrance by advance ticket, bookingforms from (sae please): Doris Roe, 7 Ashdown Road, Portishead,Bristol BS20 8DP.

1990

Mar 7-8. Laboratory 90. G-Mex Centre, Manchester. 0799 26699.

Mar 9-10. London Amateur Radio Show. Picketts Lock Centre,Edmonton, North London. Advance ticket sales and trade enquiriesto The Secretary. LARS, 126 Mount Pleasant Lane, Bricket Wood,Herts AL2 3XD. 0923 678770.

Mar 28-29. Laboratory, Science & Technology Show. KelseyKerridge Sports Hall, Cambridge. 0799 26699.

Apr 9-11. Cable and satellite exhibition and conference. Olympia,London. 01-486 1951.

Apr 4-5. Drives, Motors, Controls. New Century Hall, Manchester.0799 26699.

Apr 24-26. British Electronics Week. Olympia, London. 079926699.

Jun 26-28. Infrared Technology. Wembley Conference Centre. 079926699.

some who had been devotees for all 25years. I was particular delighted to bewarmly congratulated by two readerswho said their choice of a career inelectronics was directly attributable toPE's inspiration over the years. It wasalso amusing to be greeted by someonewho recognised me from my 'digitised'(as he put it) picture on the editorialpage! It turned out that he writes forWireless World and now wishes to joinmy illustrious band of authors. Greatplaces, are Shows!

PC SHOWPRAISESPUBLISHER

Show News, the daily newspublication produced for exhibitors

at the PC Show, devoted a short featureto our Publisher, Angelo. It told how

Angelo had become the originator notonly of Personal Computer Worldmagazine, but also of the PC Showitself.

Both ideas were conceived in theTroubadour coffee house, in theBrompton Road, London. "This quaintestablishment", reported Show News,"brought together Meyer Solomon, apoet, programmer and linguist, andAngelo Zgorelec, a newsagent,magazine distributor and computerenthusiast. Between them, they dreamtup PCW.

"Angelo became publisher andMeyer the editor. Their first workingpremises were a comer bench in theTroubadour and a telephone boxoutside."

The first issue of PCW appeared onFebruary 8th 1978.

Sometime, when Angelo isn'tlooking, perhaps I'll tell a fuller tale ofhis interesting publishing career; you'dbe fascinated!

PRACTICAL ELECTRONICS DECEMBER 1989 5

..,NESSTRAFFICKINGTOWER POWER

INMARSATPOWER -UP

Inmarsat's third generation satellitesystem will have enough power

and capacity to offer a full range ofmulti -customer, worldwide mobilecommunications services using small.low-cost terminals.

The third generation satellitenetwork will be highly flexible,capable of dynamically reallocatingpower and bandwidth on a globalscale. This will allow Inmarsat toprovide special coverage facilities tocope with particular service needs oremergencies around the world. Theproposals are for three or fourdedicated spacecraft with options forup to a total of nine.

The total EIRP (effectiveisotropically radiated power) of thesatellites will be 48 dBW, nearly 10times the effective capacity of theInmarsat II satellites, due to belaunched in 1990 and 1991, or 30times the capacity of the satellitescurrently in operation. The liftoffmass is expected to be in the 1,800 -2,500 kilogram range, and thesatellites will feature a minimum offour spot beams for mobilecommunications in key service areasin addition to global coverage.

"Inmarsat's third generationsatellites will have enough capacity tomeet existing and new mobileservices for projected demand intothe 21st century," said Dr AhmadGhais, Inmarsat's Director ofEngineering and Operations.

For further information, pleasecontact: Elizabeth Hess, Inmarsat, 40Melton Street, London NW1 2EQ.Tel: 01-387 9089.

STARGENERATOR

SG4160B is a compact portable rfsignal generator now available

from Alpha Electronics. A widefrequency range from 100kHz to150MHz can be both internally andexternally modulated. This very lowcost instrument is ideal for checkingand aligning IF circuits and tuners inam/fm audio and video systems.Many applications are found inservice and manufacturing industriesas well as in education and with thehobbyist.

The six frequency ranges of thisstable, solid state generator are clearlymarked on a large dial and go from100 to 320kHz, 300 to 1100kHz, Ito3MHz, 10 to 35MHz and 32 to150MHz. Accuracy is ± 3 per centwith frequencies up to 450MHz

Traffle jams are no problem forthis British Telecom technician as

he steers his car 40 feet above theSuffolk countryside.

The vehicle is not a modern-dayChitty Chitty Bang Bang. It's beingused in a test programme as part of aquality project sponsored by BTMobile Communications to check theefficiency of various types ofcellphone aerial.

Parked on its test-bed in the sky,the car can be easily manoeuvred.This enables radio signals from anearby transmitter to be picked up atvarious angles by the vehicle's aerial,which is connected to monitoringequipment in the tower.

Roger Stuckey, who is in chargeof the antenna test ranges at Britishtelecom's Martlesham ResearchLaboratories, explained: "It's not agimmick, but serious research aimedat achieving a true representation ofantenna performance.

"Mounting the aerial aboveground eliminates possibleinterference to the radio signal, suchas from surrounding buildings ornearby vehicles. The car itself, a scrapvehicle, creates a true-to-life situationof how the aerial would be located."

available on the third harmonic.Crystal oscillator facilities allowcrystals from 1 to I5MHz of the HC -6U holder type to be plugged into thefront panel. Output is controlled via ahigh/low attenuator and fine levelcontrol. Internal amplitudemodulation is at 1kHz which is alsoavailable as an audio output. Externalmodulating frequencies are from50Hz to 20kHz.

Housed in an attractive case with acarrying handle, model SG4160Bmeasures just 150 x 250 x 130mmand weighs approximately 2.5kg. ACpower requirements are 115/230V50/60Hz at approximately 3VA. Thisuseful rf generator is fully guaranteedfor one year and is available ex -stockat £79 plus vat.

For further information contactAlpha Electronics Ltd, Unit 5,Linstock Trading Estate, Wigan Road,Atherton, Manchester M29 OQA. Tel:0942 873434.

PROOP'SNEW SNOOPA t the beginning of September,

Messers Proops opened theirwarehouse to the public.

The 10,000 sq ft warehouse,stocking a wide range of new andsurplus items from solar panels toback magnets, from pcbs to valves,will be open six days a week,offering the professional andenthusiast alike an Aladdin's cave ofsurplus high and low -techcomponents, suitable for a widerange of projects and activities, at afraction of the cost.

Managing Director SydneyProops, who was part founder ofProops of Tottenham Court Road,says that the warehouse had alreadybecome a magnet for engineers,modellers, electronic enthusiasts,diy, research and developmentengineers and even design, theatreand film people looking for items forsets, and all he is now doing is torespond to the demand.

Sydney Proops said, "We havehad so many people calling on thewarehouse since the closure of theTottenham Court Road Proops thatwe have decided to open thewarehouse to the public. We believethat it is unique in the London areaand possibly in the UK."

The warehouse is open from 9.00am to 5.30 pm five days a week, andfrom 10.00 am until 2.00 pm onSaturdays. It can be reached by publictransport; the nearest tube is CamdenTown, the nearest railway station isalso Camden Town and the busservice is the 214 which runs every15 minutes Mondays to Fridays.

Proops of Tottenham Court Roadopened in the early 1950s and closedsome nine months ago with'bereavement' notices in thespecialist press and on LBC radio.During its 30 years of trading, theshop established a reputation secondto none among professionals andhobbyists alike (including yourEditor) for the quality of its high andlow tech surplus materials andcomponents.

Proop's warehouse is atHeybridge Estate, Castle Road,Camden Town, London. Tel: 01-2671718

CENTENNIALUNDERTAKING

et another birthday to celebrate1 - this year sees the 100th

anniversary of the Strowgertelephone exchange. It was in 1889that Kansas City undertaker AlmonStrowger patented the idea forautomatic switching. He wasspurred on to invent the systemafter discovering his local telephoneoperator was married to hisbusiness rival - and was connectingpotential customers to him!


N

SOARLY NEEDEDThe new ultra -compact Soar model3060 is a uniquely designed,

personal digital multimeter thatincorporates a 3200 count, 3 1/2 digitread out, a 32 segment analog bargraph, a variety of advanced featuresand can be operated with one hand.

This dmm is ideal for quick, on -site applications and measures 51 x106 x 10mm, weighs 100 grams andoffers high speed auto or manualranging. It delivers high speedsampling on the 32 segment bargraph display and is fully protectedon all ranges to 450V. Virtually"drop -proof', the model includes anautomatic power off feature whichcontributes to extra long battery lifeand lowrpower consumption. Thepocket -sized dmm also tests diodesand has a useful continuity beeperfunction.

The meter employs the dual slopeintegration method for measuring acand dc voltages up to 450V, andresistances from 300 ohms to30Mohms. It is supplied completewith a soft, protective carrying caseto guard against accidental drops.Using standard LR44 or SR44"button cell" batteries, it has bothover -range and low battery indicators.Accessories include an instructionmanual, test lead, protective case andtwo batteries.

Ideal for the technician, engineeror enthusiast, this new mini-dmmserves as an excellent "carry -along"instrument that belongs in every fieldkit.

For further details contact:Solex International, 95 Main Street,Broughton Astley, Leics LE9 6RE.Tel: 0455 283486.

POWERANALYSER

Now freely available fromNorthern Design is a technical

data sheet for their new Power andEnergy Analyser. Accurate, portableand easy to use, the PEA 200digitally indicates Power as Wh,

Volts, Amps, Power Factor andEnergy as Wh or kWh. Ideal for bothsingle and 3 phase systems itmeasures from 1mA to 2000 Amps.Unaffected by waveform distortionthe fundamental frequency can befrom d c to 400Hz.

For further informationcontact: Ian Hutchinson, NorthernDesign (Electronics) Ltd., 228 BoltonRoad, Bradford, West Yorkshire, BD3OQW. Tel: 0274 729533.

CHIP COUNTThis month's choice of newly introduced chips for the spotlight.

NEW 555 TIMER

One of the most universally used chips in timing circuitshas for many years been the good old 555. You'll beinterested to know that another version has been added to theexisting range of variations.

Philips has introduced two cmos timers to the range, the74HC5555 and 74HCT5555, which offer substantialadvantages over the popular bipolar 555 timer in terms ofpower consumption, increased precision and economy ofexternal components. The latter are due to advances in thetimer design.

In the new types, a frequency divider is driven by anoscillator and thus virtually eliminates timing variations dueto threshold voltage variations. As the frequency dividerdivides by a programmed number ranging from 2 to 214, lowvalues of C and R can be used. Using a low value ofcapacitance can lead to substantial savings in both space andcost. In contrast, greater timing periods can be accomplishedwith fewer problems associated with capacitor leakage.

Facilities are provided for three oscillator arrangements.The internal oscillator can be used with either an external RCor crystal. Alternatively, an external oscillator can be used.

The 74HC5555 operates with normal cmos voltage levels,whilst the 74HCT5555 operates with ttl voltage level inputs.Both devices are comparable to, but improvements on, the10343, which is available from other manufacturing sources.A related device from Philips is the 1CM7555C, which is acmos version of the standard popular 555 timer.

PCF8578 AND PC8579 LCD DRIVERS

The Philips family of I2C-bus lcd drivers has beenextended to include a new cmos chip set. The chip set isdesigned to drive medium -rate multiplex dot matrix lcds andconsumes less than one twentieth the current of competingproducts, typically 20 microamps per chip. The set, whichcomprises types PCF8578 and PCF8579, will drive fullgraphics or character displays using between 1:8 and 1:32multiplex and can be configured to drive from 256 up to amaximum of 40,960 dots.

The PCF8578 is an lcd row -and -column controller/driverwith forty outputs which are typically used for driving 5 -by -8dot matrix displays, although any format is possible. Standingalone this chip can drive up to 384 dots in variouscombinations of rows and columns, depending on themultiplex rate. The is can be cascaded with one or morePCF8579s for horizontally larger displays. The maximumconfiguration is 32 by 1280 dots.

The PCF8578 provides clock and display synchronisationfor the PCF8579. It can work with up to 32 cascadedPCF8579s, either as a row/column driver, or as a row driverwith up to 32 row outputs.

Overall control is exercised by a microcontroller, whichcommunicates with the ics via a two-line bidirectional I2C-bus. The microcontroller thus controls the device's operation,and determines parameters such as the multiplex rate,blanking and loading modes. The address and bus lines ofboth chips are ttl and cmos compatible.

For more information on the chips highlighted abovecontact Philips Components Ltd, Mullard House, TorringtonPlace, London WC1E 7HD. Tel : 01-580 6633.


e all lose some of our hearingwith age. The top end rolls offfrom 20 kHz at birth to 10 kHz

in later years. Sensitivity to quiet soundsgoes down too. But age alone does notusually cause complete deafness. Somepeople are born profoundly deaf, others losetheir hearing through illness or exposure toexcessively loud sound over a long periodof time.

There are 50,000 people in Britain whorely on sign language for communication,one fifth of the deaf population. Thesepeople cannot communicate by phone.Conventional videophones can send andreceive only still pictures so are no use forsign language conversation.

The new ISDN (Integrated ServicesDigital Network) works in the digitaldomain and can carry 64 kilobit/secondpulse trains, which is enough to conveymoving colour video. There is someblurring on motion but ISDN would allowsign language communication. However itwill be many years before domesticsubscribers get ISDN connections - if ever.

LEADINGThe new system takes advantage of the

fact that some areas of the picture, forinstance corners, seldom contain motion.So there is no need to keep on transmittingthe same digital description of the samepicture part. The system transmits onlyinformation about changes in the picture,caused by motion, such as facialexpressions and hand signs.

Processing circuitry in the modifiedvideophone divides each full picture into64 rectangular sections, in eight rows ofeight. It then examines each section andcompares it with a record of the lastpicture which has been temporarily storedin a solid state memory. When the picturecontent in a section remains the same, thetransmitting videophone sends just onedigital control bit down the line, indicatingthat the receiving videophone can pull thatwhole rectangle from its memory of thelast picture, and display it unchanged.

When the processor senses that thepicture content in a rectangle has changed,it splits the area into 16 smaller rectangles,four rows of four, and examines each

DEAF VIDEO PHONESBritish Telecom has worked with the

University of Essex to develop a new kind ofvideo telephone which allows the deaf to"talk" by phone. It sends moving videopictures down conventional phone lines. Thepictures look like black -on -white cartoonimages but they convey sign language"characters". Facial characteristics are clearenough to allow callers to recognise eachother',"

People who have used the system saythey would prefer to watch a full colourimage, but BT has found that monochromecartoon outlines are as accurate at conveyingsign language messages as broadcast qualitytv pictures. Although the hardware is stillonly in prototype form, BT believes that itwill lead to a system which sells for "theprice of a good quality video recorder".

The first working model has already beensuccessfully used by one profoundly deafcouple between their home near Ipswic=h andthe headquarters of the Suffolk DeafAssociation. The main advantage of thesystem is that it does not require deaf peopleto learn the new discipline of typing text intoa keyboard terminal.

BT's published description of the systemwas so garbled and superficial that mostpress reports failed to explain how thetechnology works. This is a pity because thesystem is clever and quite simple - once youhave found the right person inside BT toexplain it.

As a starting point, researchers at BT'slaboratories at Martlesham and theUniversity of Essex, took a videophone ofthe type already developed for ISDN use.

The videophone has a screen, 6 cmacross, which displays a tv image made up

BY BARRY FOXWinner of the

UK Technology Award

Briefly covered inlast month's newspage, the inventionthat will help the

deaf communicateby phone isdiscussed in

greater detail.

from 64 horizontal scanning lines. Each lineis made up from 64 individual picture pointsor pixels. For an illusion of smooth motion,ten full pictures per second must betransmitted. The videophone thus needs totransmit information describing around40,000 separate picture points per second(64 x 64 x 10).

If the technique used by fax machines isemployed, whereby one digital bit switcheseach pixel between black and white, the datarate becomes 40 kb/s. This is well within thecapabil;ity of a 64 kb/s ISDN line. Butexisting phone lines were designed to carryanalogue speech, have a bandwidth of 3 kHzand can only carry data at a maximum of14.4 kb/s. Often crackly lines reduce the topspeed to 9.6 kb/s.

separately. Where there has been nochange in a small rectangle, thetransmitting phone sends one digital bitand the receiving videophone displays thatarea from memory. Where motion isdetected in a rectangle, the processor splitsthe area again into even smallerrectangles. Again it sends one bit for eacharea where no motion is detected andagain it subdivides any rectangle in whichmotion is sensed. This sub -division goeson until the system is examiningindividual pixels.

Because the amount of motion in thepicture dictates the amount of digital bitsneeded, and because the amount of motionwill always be changing, the data ratecontinually changes. On average thesystem reduces the number of bits by afactor of four, which bringscommunication within the capability ofanalogue phone lines.

Where there is so much motion overlarge areas of the screen that the systemcannot reduce the data rate to the required14.4 or 9.6 kilo bit/s, the processorautomatically starts playing another trick.It reduces the number of full pictures sentfrom 10 per second to 8 or even 6. Themotion on screen starts to look jerky andover -active callers are obliged to reducethe speed at which they make signs. So thesystem is self regulating.

In the future colour images will be sentover ISDN lines, with twelve bits used todescribe each picture point, eight formonochrome detail and four for colour.The data rate will be reduced by the sametechnique of sensing motion and sendingonly essential information.


R ecently, an electronicsteacher wrote to me abouta couple of PE designs

which had interested the enthusi-astic pupils.

On receipt of the pcbs from thePCB Service the teacher hadfound that obtaining componentswas a tedious job due to nowbeing tied to pin spacings. "Inmany cases", comments theteacher, "hole spacing and com-ponent sizes available from myusual supplier looked very silly.There are also some componentswhich I am having difficulty locat-ing. Please can authors be moreprecise on the purchasing sourcefor components? I think sayingthat parts can be obtained fromadvertisers in the journal is some-thing of a 'cop out' and loses youcirculation quite unnecessarily."

It is not the authors who areresponsible for the exclusion ofcomponent sources, it is strictpublishing policy. There are twooccasional exceptions to this. Thefirst is when a particular compo-nent is only available from onesupplier, who is then named asthe source for that part. The sec-ond is an extension to the first,when an advertiser has instigatedthe project design and is the solesource of some parts, in whichcase he may be quoted as the kitsource.

When items are believed to bereadily available, we do not quote

PRACTICAL

ELECTRONICS

a source. It would be highly offen-sive to source A if we quotesource B when A stocks the partsas well. Nor can we quote both Aand B, because in reality the listprobably runs from A to Z. Weknow that this may mean that areader has to shop around forparts, but in many cases it only atakes a phone call or letter to afew suppliers to locate the parts.We also know that many readershave catalogues from manysources - it doesn't cost a lot tosend off for them, and a lot ofthem are free. Anyone who isdoing electronics as a serioushobby or subject is being unfair tothemselves if they don't have a

reasonably sized cataloguelibrary.

If a reader has difficulty locat-ing a component of the same sizeas that used by the author, thereare three main choices. One is tobuild the circuit on Veroboard orsimilar instead of a pcb. The sec-ond is to make the board oneself,modifying the track to accommo-date the different sizes. The third,and easiest, is to carefullyreshape the component leads or,if they are not suitable for this, tosolder stout wires to them andthen carefully bend the wires toshape.

It's worth while rememberingthat electronics is a pursuit thatrequires initiative and an analyti-cal ability. The ability to think outalternative solutions to physicalproblems is just as important asthe ability to find electronic, logi-cal or mathematical solutions.Doing electronics is not like play-ing with plastic plug-in bricks.Certainly there are some thingsthat can be achieved by doing lit-tle more than plug one bit toanother, but the person who real-ly wants to pursue electronics asa serious activity must not expecteverything to be available on aplate. PE does its best to providea very full plate of information,but readers may at times have toprovide their own salt and pep-per!

THE EDITOR

Editor:John Becker

Sub -Editor:Helen Armstrong

Technical Illustrator:Derek Gooding

Advertisement Executives:David Bonner, Richard Caplis

Office Manager:Louise Hewett

Production Manager:David Hewett

Publisher:Angelo Zgorelec

Editorial and Advertising Address:Practical Electronics,Infra House, 193 Uxbridge Road,London W12 9RATel: 01-743 8888Telecom Gold: 87: SQQ567Fax: 01-743 3062

AdvertisementsAll correspondence relating to advertise-

ments, including classified ads, should beaddressed to: The advertisement depart-ment, Practical Electronics, at the aboveaddress and telephone number.

Readers' EnquiriesAll editorial correspondence should beaddressed to the editor and any lettersrequiring a reply should be accompanied bya stamped addressed envelope, orequivalent payment.We regret that lengthy technical enquiriescannot be answered over the phone.

Subscription Address:Practical Electronics, Subscription Dept.,P.O. Box 500, Leicester LE99 OAA.

Annual Subscription Rates:U.K. £15.00 Overseas £18.00

Cover IllustrationScience Photo Library

Intra Press 1989. Copyright in all drawings,photographs and articles published inPRACTICAL ELECTRONICS is fully protected,and reproduction or imitations in whole or inpart are expressly forbidden. All reasonableprecautions are taken by PRACTICALELECTRONICS to ensure that the advice anddata given to readers is reliable. We cannot,however, guarantee it, and we cannot acceptlegal responsibility for it. Prices quoted arethose current as we go to press. All material isaccepted for publication on the express under-standing that the contributor has the authority topermit us to do so. ISSN 0032-6372

Published on I st Friday of each month by lntrapress, 193 Uxbridge Road, London WI2 9RA. Printed in England by Andover Press Ltd. Andover, Hants. Distributed by Quadrant Publishing Services 01-661 8526.PRACTICAL ELECTRONICS is sold subject to the following conditions, namely that it shall not. without the written consent of the Publishers first having been given.be lent, resold, hired out or otherwisedisposed of by way of Trade at more than the recommended selling price shown on the cover, and that it shall not be lent, resold, hired out or otherwise disposed of in a mutilated condition or in any unauthorisedcover by way of Trade or affixed to or as part of any publication or advertising. literary or pictorial matter whatsoever.


For fast delivery telephoneour order on 01.205 9558

using VISA Access Card TECHNOMATICY(

Techno House 468 Church Lane, London NW9 8T0.

Tel: 01-205 9558 Fax: 01-205 0190

Orders welcome from(overnment depts &ducational establishments

All prices ex VATPrices are subject tochange without noticePlease add carriage(a) £8.00 (Courier))b) £3.50(c) £1.50(d) £1.00

rchimedes computer stemsAll Archimedes systems are fitted with new RISC OSsystems.

ModelA3000310410/1420/1440/1

Basic£649£899£1199£1699£2499

PC Emulator £60Carriage £12/system IS/computer*Colour monitor can be Acorn AKF I I or PhilipsCM8833

Colour*£849£1099£1399£1899£2699

Technomatic Special DealTo get you going on any of the aboveArchimedes systems you purchase from us,we will contribute 10% of its cost towardsany additional hardware or softwarepurchase you make from us or to pay forextended finance if you require the facility.

The following MultiScan Monitors can be suppliedat special prices when purchased with a computer:TAXAN 770+ £419(a) MTS9600 £375(a)

CM168616" £1499(a)(for the extra HiRes modes on 400 series)

Extended FinanceWe can offer extended finance for 11

months under our special offer detailedabove or for 24/36 month period at 12.75%(approx APR 26%). Please send for details.Technomatic is a licensed credit broker.

R140 UNIX SYSTEMTechnomatic is an authorised dealer.

Pse send for details

Techno 410/1 Upgrade Specials3Mb RAM + 49Mb HD (fitted) £8403Mb RAM + 40Mb HD (fitted) £7201Mb RAM + 20Mb HD (fitted) £325(All hard drives are with fast access times)These prices apply only when purchased with Arc 410/1(Your discount entitlement can be used towards paying forany of the above kits)

EXPANSION SYSTEMSRISCOS kit (305/310/440) £29(b)1Mb RAM upgrade (410) £149(c)3Mb RAM upgrade (410) £429(b)2Mb upgrade (420) £290(b)1Mb RAM upgrade A3000 £199(b)3.5" Int Drive Upgrade(please specify 305/310 or 410)

£118(b)

External Drive Adaptor £30(c)External 5.25" Drive with psu (40/80T) £90(b)Acorn 20 Mb Upgrade (305/310) £449(a)Techno 20Mb upgrade kit (410) £199(a)Techno 40Mb upgrade kit (410) £349(a)Techno 49Mb upgrade kit (410) £465(a)Techno Ext HD upgrade (300 & 400 series) POA

£37(c)£39(c)£8(d)

£51(c)

Acorn Backplane (2 slots)TechnoLog Backplane (4 slots)Fan for TechnoLogAcorn ROM poduleWe have a large range of expansion cards in stockplease send for details.

NIULTI I/O Podule

rchimedes, soFTWAR

LANGUAGES & PROG . "POOLSANSI C/ISO PASCAL/FORTRAN 77PROLOG X/LISPTWIN EditorArchimedes AssemblerSoftware Dev. Toolbox

each £95(c)each £175(b)

£27(d)£185(b)£185(b)

RISC BASIC Compiler (SV) £85(c)Chares Toolkit £32(d) Clares Toolkit+ £41(d)

WORD PROCESSORS1st Word Plus £79(c) Graphics Writer £27(d)Pipedream £89(c) P. Spellchecker £43(d)Pipedream Ver 3 (inc Spelling checker) £129(c)

SPREADSHEETSLogistix £95(c) Sigmasheet £57(c)

ACCOUNT SYSTEMMinerva's Suite* each £54(d)(Order Processing/Sales Ledger/Stock Manager/Purch. Ledger/Nom. Ledger)*Requires System Delta plusHome Accounts £41(d)

GRAPHICS/ART/CADPro Artisan £137(b) Artisan £30(d)Artisan Support Disc £17(d)Atelier £90(b) Auto Sketch £55(c)Gamma Plot £52(d) Render Bender £59(b)Presenter £24(d) SVARC-PCB £167(b)SV Solid CAD £41(d) SV Super Dump £20(d)Sv Real Time Solids Modeller £75(c)

NEW PRODUCTStechnoSCAN Scanner 200dpi f.:159(b)

Genius Digitising Tablet £249

GAMESHoverbod/Missile Control each £12(d)Orion/Freddie's Folly/Jet Fighter each £12(d)Fugitive Quest/Rise in Crime/Overload each £26(d)Zarch £15(d) Conqueror £24(d)Enthar Seven £29(d) Minotaur £9(d)Clares 3 Pack £12(d) Viking £17(d)Thundermonk £12(d) Corruption £20(d)Pacmaria £16(d) Terramex £16(d)

DATABASESSystem Delta PlusSystem Delta Plus Prog Ref ManualReporter*/Mailshot*School AdministratorAlphabase*Requires System Delta plus

£57(c)£25(d)

each £33(d)£125(b)£37(d)

MISCELLANEOUSPC EmulatorHEARSAY. Comms PackageMAYA Terminal EmulatorControl (setup utility)(inc VT52, ANSI, Tektronix 4034, Kermit protocols)

Arcomm £29(d) Presenter II

£79(d)£55(d)£29(d)£13(d)

£POA

EMULATED PACKAGESVIEW/ViewsheetNiewstoreInterword/IntersheetWordwisc

each £47(d)each £35(d)

£20(d)

MASTER SERIESAMBI5 Master 128 £439(a)The Master package includes: View Word processorand Viewsheet spreadsheet software and EITHEREducational/Fun software bundle withSTARdataBASE package OR A 200K 40T DS singledrive TS200. Please specify your choice.ADC06 Turbo 65C102 Module £115(b)

Technomatic is a Acorn Authorised EconetReferral Centre. We carry a full range ofEconet Accessories in stock.

DISC DRIVES400K/640K per mechanism (all drives)5.25" Double sded (40/80T Sw):TS400 SinglePS400 Single with psuTD800 DualPD800 Dual with psuPD800P Dual with psu plinth mounted3.5" Double SidedTS351 (5.25")PS351 (5.25" Case + psu)TD352 DualPD352 Dual with psuCombo Drives 5.25" + 3.5" & psuPD853PD853P plinth mounted

£80(b)£90(b)

£160(a)£170(a)£185(a)

£69(b)£95(b)

£126(a)£139(a)

£170(a)£190(a)

WINCHESTER DRIVESP3OHD 30Mb Hard Drives £410(a).Other sizes available

PRINTERSEPSON

LX800 £145)a) L0500FX850 £285(a) LQ850

£225(a)£405(a)

STARLCIO £149(a) LCIO Colour

LC24-10 £269(a)£199(a)

NATIONAL PANASONICKXPI081 incl master lead £139(a)KXP1124 £269(a)Integrex Inkjet ColourHitachi 672XD A3 Plotter

£515(a)£479(a)

PLOTTERSHitachi 672XD A3 4 pen plotter Special £409(a)Roland DXY880A A3 Flatbed 8 penplotter £495(a)

I-This advertisement can only show an exampleof the range of products stocked byTchnomatic. So send for our latest free BBCcatalogue providing detailed information andprices on BBC Computer Systems,Peripherals, Software and Books.

NAME

ADDRESS

A low cost card provides: Video Digitiser, SoundSampler, a full RS232 interface, 3 ROM skts &Joystick interface within RISCOS environment.Details on request. £1 17(b)

Ex -SERVICE EQUIPMENTOur Service department has a number of discdrives, monitors and printers available for sale atreduced prices. Please send for detailed list.

Post CodeReturn to Technomatic Ltd, Techno House,468 Church Lane, London NW9 8TQ.

L PE11/89j

TTLs

Please mention Practical Electronics when contacting advertisersLINEAR ICs CPU's RAMS CRYSTALS INTERFACES ICS

74

7400 0.307401 0.307402 0.307403 0.307404 0.367405 0.307406 0.407407 0.407408 0.307409 0.307410 0.307411 0.307412 0.307413 0.507414 0.707416 0.367417 0.407420 0.307421 0.607422 0.367423 0.367425 0.407426 0.407427 0.327428 0.437430 0.3137432 0.367433 0.307437 0.307438 0.407439 0.407440 0.407441 0.907442A 0.707443A 1.007444

1 0.700.1

70457446A 1.0074474 1.007448 1.007450 0.367451 0.367453 0.387454 0.387460 0.567470 0.507472 0.457473 0.457474 0.507475 0.607476 0.467480 0.667481 1.8074834 1.057484A 1.257485 1.107486 0.427489 2.107490A 0.567491 0.707492A 0.7074934 0.567494 1.107495A 0.607496 0.607497 2.9074100 1.9074107 0.5074109 0.7574110 0.7574111 0.5674116 1.7074118 1.1074119 1.7074120 1.0074121 0.5674122 0.7074123 0.8074125 0.6574126 0.5674128 0.5674132 0.7574136 0.7074141 0.9074142 2.5074143 2.7074144 2.7074145 1.1074147 1.7074148 1.4074150 1.7574151A 0.7074153 0.8074154- 1.4074155 0.8074156 0.9074157 0.8074159 2.2574160 1.1074161 0.8074162 1.1074163 1.1074164 1.2074165 1.1074166 1.4074167 4,0074170 2.0074172 41074173 1.4074174 1.1074175 1.0574176 1.0074178 1.5074179 1.5074180 1.0074181 3.4074182 1.4074184 1.8374185A 1.11074190 1.3074191 1.3074192 1.1074193 1.1574194 1.1074195 0.8074196 1.)374197 1.10741% 2.2074199 22074221 1.1074251 1.0074259 1.5074265 0.8074273 2.0074276 1.4074278 1.7074279 0.9074283 1.0674285 3.2074290 0.9074293 0.9074298 1 8074351 200743654 0 80743664 0 8074367A 0.80743684 0.7074376 16074390 1.10

74393744907439374490

1.201.101.201.40

7415 SERIES

74L0007400174L002741003741.00474100574100874100974151074101174101274LS1374LS14741_015741520741021741022741024741026741_027741028741530741S32741033741037741S3874104074104274104774104874105174155474105574107347410744741S75741_07601741078741083A74108574158674109074159174LS9274109374109587410967410107741010974101127410113741011474101227410123741012474151257415126741513274101337415136741513874151397410145741014774151487415151741015274101537415154741515574151567410157741015874101604741.51614741516247415163474101647410165A741016647415168741516974151707410173A7410174741017574LS181741_018374101907410191741519274101937410194A741.0195A74101967410197741522174102407415241741_52427415243741_0244741024574102477410248741024974102517410253741525674102574741 0268A7410259741026074102617410266741027374152797410280741528374102931410292

0.240.240.240.240.240.240.240 240.240.240.240.340.500.240.240.240.240.500.240.240.240.240.240.240.240.240.240.500.800.900.240.240.240300.350.450.360.420.700.750.350.480900.350.540.750.900.400.400.450.450.450.700.801.400.500.500.650.500.450.550.550.951.751.400.652.000.651.600.650.650.500.650.750.750.750.750.751.301.501.301.00'1.401.000.750.752.001.900.750.750.800.800.750.750.800.800.900.900.900.900.900.700.901.101 101.100.750.750.9078.

0.701.200.751.200.601.250.701.900.800.80

14.007410293 0.807410295 1.407410297 14.007410298 1.007410299 2.207410231 3.707410323 3.007410324 3.20741034.9 2.007410352 1.207410353 1.207410356 2.1074LS363 1.8014L0364 1.807410365 0.507410366 0.507410367 0.527415368 0.507010373 0.707415374 0.707415375 0.757410377 1.307415378 0.95

74LS379 1 301415381 4.5074L 5390 0.607415393 1.0074153954 1 007410399 1 407415445 1.807415465 1.207415467 1.807415540 1.007410541 1.007415608 7.007415610 25.007410612 25.007410624 3.507415626 2.257415628 2.257415629 1.257415640 2.007410640 1 3.007415641 1.507415642 2.507410642 I 3.007415643 2.507415643 1 3.007415644 3.507415645 2.007410645 1 4.007415668 0.907415669 0.907410670 1.707410682 2.507410684 1 .741_0687741_068874C92274C92374C92574C926

3.503.50car6.506.506.50

4000 SE RIES

40014002400640074008400940104011401240134014401540164017401840194020402140224023402440254026402740284029403040314032403340344035403640374038403940404041404240434044404540464047404840494050405140524053405440554056405940604063406640684069407040714072407340754076407740784081406240854086408940934094409540964097409840994501 0.364502 0.564503 0.364504 0.964505 3.604506 0.904507/4030 0.364538 1.204510 0.564511 0.564512 0.564513 1.504514 1.10451545164517451845194520452145224526452745284529453145324534

0.240.250.700.250.600.450.600.240.250.360.600.700.360.560.600.600.800.600.700.300.480.240.900.400.600.750.351251.001.252.500.702.501.101.002.500.600.550.500 600 601 000600600.550.360.350.650.600.600.800.800.854.000.700.850.400.250.240.240.240.240.240.240.650.250.260.240.250.600.751.200.350.900.900.902.700.750.90

1.100.552.200.480.320.601.150.800.700.800.651.000.750.66

453645384539454145434551455345554556455745604 56645684569457245834584458547241441114412144161441914490144951450014599221002210122102400144008540097400984010040101401024010340104401054010640107401084010940110401144014740163401734017440175401924019340194402444024540257403734037480C958019780C98

2.500.750.750.900701.002.400360.502.401 401.402.401.700.450.900.480.601.507.507.503.002.604.204.506.502.003.507.007.000.481.200.360.401.501.251.502.001.201.500.480.553.201.202.252.252.801.031.201.001.001.001.001.001.501.501.801.801.800.750750.75

SPECIALOFFER

30%DISCOUNT

ONTTL/CMOS/

REGULATOR

PRIFFECRI26 pin 7.0040 pin 800

407581 12.00ADC0808 11 90AM7910DC 25.00ANI03 200AYI 5050 1.00AY3 8010 4.50473 8910 4.50AY3 8912 5.00CA30194 1.00CA3028A 1.10CA3046 0 70CA3059 3.20CA3060 3.50CA3080E 0.70CA3086 0.60CA3089E 2.50CA309040 3.75CA3130E 0.90CA31307 1 30CA3140E 0.45CA3140T 1 00CA3146 225CA3160E 1 50CA316IE 2.00CA3162E 600CA3189E 2.70CA3240E 1.50CA3280G 3.0007002 6.00DAC1408 8 3.00DAC0800 3.03DAC0808 3.00DG308 3.00HA1366 1.90C17106 6.75CL7611 0.95CL7650 4.00CL7660 2.50CL8038 4.00CM7555 0.90CM7556 1.40

LC7120 3.00LC7130 3.00LC7431 3.50LC7137 3.501E347 1.201E351 0.601E353 0.901E355 0.90LF356N 1.10LF357 1 00LMIOCLrl 4.501M3014 0.30LM307 0.451M308CN 0.751M310 2.25LM311 0.60LM3I8 1.501M319 1.80LM324 0.45LM334Z 1.15LM335Z 1.30LM336 1 60LM339 0.40LM348 0.60LM349CH 3.60LM358P 0.50LM377 3.00LM380N 8 1.50LM380N 1.50LM381N 3.00LM382 2.00LM383 3.25LM384 220LM386N 1 1.00L M387 2 70LM391 1.80L M392N 1.10L M393 085LM394CH 4.00LM709 0.35LM710 0481M711 1.00

VOLTAGE REGULATORS1A FIXED VOLTAGE PLASTIC 70220

- VE5 V 7805 0.45 7905 0.506 V 7806 0.50 7906 0.508 V 7808 0.50 7908 0.50

12 V 7812 0.45 7912 0.5015 V 7815 0.50 /915 0.5018 V 7818 0.50 7918 0.5024 V 7024 0.50 7924 0.50

TA FIXED VOLTAGE PLASTIC 0092

5 V 78106 0.30 5 V 78115 0.306 V 78106 0.30 5 V 79105 0.468 V 78108 0.30 12 V 79112 0.502 V 78112 0.30 15 v 17115 0.50

OTHER SWITCHINGREGULATORS REGULATORS

I

LM309K 1 40 117660 250LM323K 2.50 1S1G43524 3.00

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VARIABLE REGULATORSLM305AH 2.50 LM123N 0.50LM317T 1.20 78HGKC 6.50LM317K 2.40 7EIGLAC 2.25LM337T 2.25 79HGKC 6.75LM350T 4.00 79GUIC 2.50LM 3'.K 1 i 79MGT2C 1.40

OPTO ISOLATORSD -I 711112 0.70

11074 2.20 TIL113 0.70MCT26 1.00 TIL116 0.70MCS2400 1.90 6N 137 3.60M0C3020 1.50 6N139 1.75711111 0.70

DISPLAYSFND357 1.00 MAN6610 2.00FND500 TIL730 1.00 NSB588I 5.70END507 TIL729 1.00 TIL311 6.50MAN74 01704 1.00 711729 1 00MAN71 D1707 1.00 TIL730 1 00MAN3640 1.75 MAN8910 1 50MAN4640 2.00

DISPLAY DRIVERS9368 4.50 ULN2003 0.90

3.5033;?,

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0.902.90

LM3914 3.50 ULN2802 1.90LM3915 350 ULN2803 1.80LM3916 3.50 U1N288/4 1.90UDN6118 3.20 75491 0.70

DN6184 3.20 75492

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OTHERS0.501.802.801.60

OR P12ORP60ORP61SFH205

1.201.201.201 00

L M723 0.60LM725CN 4.00LM733 0.65LM741 0.22LM747 0.70L M748 0.30LM1011 4.80LM1014 1 50LM1801 100LM1830 2.50LM1871 3.00LM1872 3.00LM1886 6.001M1889 4.50LM2917 3.00LM3302 0.90LM3900 0.80L. M3909 1.00LM3911 1.80LM3914 150LM3915 3.40LM3916 1401M13600 1.50M515131 2.30M515161 4.50MB3712 2.00MCI310 1.50MC1413 0.75MC1458 0.45MC14951_ 3.00MC1496 0.70MC3340 2.00MC3401 0.70MC3403 0.65MF10CN 4.00MK50240 9.00M1920 5.00ML922 4.00MM66221A 300NE531 1.20NE555 0.22NE556 0.60NE564 4.00NE565 1 20NE566 150NE567 1.25NE570 4.00NE571 3.00NE592 0.90NE5532P 1.50NE5533P 1.60NE5534P 1.20NE5534AP 1.50OP 07EP 3.50P11024 5.00RC4136 0.55RC4151 2.00RC4195 1.50RC4558 0.55S50240 9.00SAA 1900 16.00SFF96364 8.0001490 3.00SN76033N 3.00SN76489 4.00SN76495 4.00SN76660 1.20SP0256412 7.00TA7120 1.0TA 130 1.40747204 1.50TA 7205 0.90T A 7222 1.50747310 1 50784231 1.20TBA800 0.80TBA810 0.90TBA820 080TBA820M 0.75TBA920 2.00

2.25

TC9109 5.00TC4940 1.75TDA1010 2.251041022 4.50TDA1024 1.1070411705 3.00TDA2002 3.257042003 1.907042004 2.40T 042006 120T DA2020 3.20TDA2030 2.507043810 7.50TDA7000 3.507E41002 7.0071061CP 0.40TL062 0.60T1064 0.90T1071 0.4011072 0.7011074 1.10T1081 0.3511082 0.5511083 0.75TL084 1.00TL094 2.00UA759 3.20UA2240 1.50UCN58014 6.00ULN2003A 0.75ULN2004A 0.75ULN2068 2.90ULN2802 1.90ULN2803 1.80ULN2804 1.90UPC575 2.75UPC592H 2.00UPCI156H 3.00UPC1185H 5.00XR210 4.00XR2206 4.00XR2207 3.75092211 5.75X92216 6.75092240 1.20ZN404 1.00ZN4I4 0.80ZN4I9P 1 75ZN423E 1 30ZN424E 1 30ZN425E8 3.50ZN426E8 3.00ZN427E8 6.00ZN428E8 4.50ZN429E8 2.25ZN447E 9.00ZN448 7.50ZN449E 3.00ZN450E 7.50ZN459CP 3.00ZN1034E 2.00ZN1040 6.60ZNA134H 23.00ZNA234E 9.50

68809 10.00681309E 12.008035 3.508039 4.2080C39 70080804 7.5080854 3.00800854 9.008086 22.008087-5 (708087-8 C9680287.8 11558028710 C175

17.5012.00

8748.. 12.00TMS9980 14.50TMS9995 18.00280 2.50Z804 2.902808 5.5070008 7.50

11111015M1802C9 6.5026504 10.506502 4.5065CO2.2MHz 12.0065024 6.506502B 8.006800 2.5068026803.2680968096

3.0012.006.50

10.00

*ATTENTION*ALL PRICES ARE

SUBJECT TO CHANGEWITHOUT NOTICE

SCRS LOW PROFILE OIL SOCKETS BY TEXAS

UPPORT DEVICES

1A 50V 70pSA 400V 50p134 600V 14001841004200pC106D 45pMCRIO' 36p11144 36p2N3525 130p204444 140p205060.4 40pZENERS

400MW 9p1W 15p

TRIACSPlastic3A 400V 60p6A 4006 70p64 5006 90p84 4006 75p84 5006 95p1245006105p164500V 130p1 28000 130p11206D SOP112260 7

DISCRETEDISPLAYS

LEDS125RED TIL209 012GREEN711211YELLOW111212 0.20Reel LEDs111 G 71 0.30COO 1131Colour 1 0010 LEDBalgrapliRed 2.25Green 2.25

111220TIL222TIL2261113271178111318TI181TIL100

0.15

0.150.180.220.550.561.201.200_75

Pr . : p14 p.n 10p 20 pm 19p 28 pin 25P16 p.n Ilp 22 p.n 22p 400, XV

2651 12.00281630 20.003242 8.003245 4.506520 3.006522 3.506522A 5.506532 4.8065514 5.506821 1.8068621 3.006829 12.506840 3.7568840 6.006850 1.8068850 3.006852 2.506854 65068854 8006875 5.008154 8.508155 3.808156 3.80

SP32.25

2.208216 1.608224 P.O.A.8226 4258228 5.508243 2.608250 12.0082514 3.258253C5 3.5082554C 5 3.208256 18.008257C5 4.008259C 5 4.008275 29.038279C-5 4.80

g2i3.00

4.008284 4.608287 1808288D 6.508755A0 1 2 9 16.00TM54500 14.00TMS9901 5.00TMS9902 5.00TMS9911 18.00TMS9914 14.00Z8OPIO 2.50

2.75Z8OCTC 2.50Z8OACTC 2.75Z8ODART 6.50

28040497 7.00Z80E/MA 7.0028040MA 7.502804510 7.002808P10 5.00Z80BCTC 5.00Z80BDART 9.00

2016 150 4.002101 4.0021078 5.002111435 4.002.14 1.50

2114 3 4.821475004116 15 2.004116-20 1.504164-15 I71/ 3.004164.15 1.50

4532 20 2.5041256 15 3.0051015501 4.005514.'5114 4.005516 4.005517 4.0050256-15 4.00311611.-3 3.00626419-15 2.609810 2.00745189 1.8074S201 3.50740289 22593415 6.0093425 6.00

ROMS PROMS

2812224510180030180403074018874028774028874038782023820123825129

4.002.502.002.001.802.251.802.251.501 501 50

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ON

TTL

CMOS

REGULATORS

CRTTR4 R

32 768 kHz 1.001 00 MHz 2.701.8432 MHz 2252.00 MHz 2.252.45760 MHz IS/

2.002.5 MHz 2.503 12 MHz 1.753 276 MHz 1.503 5795 MHz 1.00400 MHz 1.400 194 MHz 1.500 43 MHz 1.034 608 MHz 2.504.9152 MHz 2.005.000 MHz 1.505 068 1.756.00 MHz 1.406.144 MHz 1.40700 MHz 1.507 168 MHz 1.75800 MHz 1.508 867 MHz 1.7510.00 MHz 1.7510.50 MHz 2.5010.70 M711 1.5011 00 MHz 3.0012.00 MHz 1.5014.00 MHz 1.7514.318 MHz 1.6014 756MHz 2.5015 00 MHz 2.0016 00 MHz 2.0017 734 MHz 1.5018 00 MHz 1.7018 432 MHz 1.5019 969 MHz 1.5020.000 MHz 1.7524.000 MHz 1.5048.00 MHz 1.75116 MU, 2.50PX01000 12.00

DISCCONTROLLERS

6843765A8272FD1771FD1791FD1793FD1797WD1770WD2793WD2797WD1691WD2143

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RO 3 2513UC8 50

RO 3 2513LC8 50

REAL TIMECLOCK

MC6818P 4.00MM58174AN 9.90MSM5832R 53.50

BAUDRATEGENERATOR

I 7.50COM8116 6.5047028 7.50

CR T502/ 18.00CRT6646 9.00EF9364 8 00EF9365 25.00EF9366 25.00EF9367 36.00EF9369 12.00MC6845 6.50MC6845SP 6.50MC6847 6.50SF F96364 8.00TMS9918 15.00

ENCODER

AY 5 2376 11 50741922 5 0014C923 6.00

UHFMODULATORS6 MHz 3.758 MHz 4.50SOUND EtVISION12 MHz 12.00

40561) 20.00A07581 15.00ADC0808 11.90AM25510 3.504M251525213.50AM251525383.50AM261531 1.20AM261.032 1.20AM7910DC 250007002 6.00DAC80 CBI V

28.00DM8131 600DP8304 3.50DS3691 3.50058830 1.40O 08831 1.50DS8832 1.50DS8833 2.25D S8836 1.50DS8838 2.25MC1488 0.60MC 1489 0.60MC3446 2.50MC3459 4.50MC3470 4.75MC3480 8.50MC3486 2.25M13487 2.25MC4024 5.50M1.4044 5.50MC6883 16.00MC14411 7.50MC14412 7.50ULN2003 0.7575107 0.9075108 0.9075109 12075110 0.9075112 1.6075113 1.2075114 1.4075115 1.4075121 1.4075122 1.4075150P 1.2075154 1.20/5159 2.2075160 5.0075161 6.5075162 7.5075172 4.0075182 1.5075188 0.6075189 0.6015365 1.5075451 0.5075452 0.5075453 0.7075454 0.70/5480 1.50/5491 0.6575492 0.658126 1.208128 1.208195 1.208796 1.208197 1.208T98 120811595 1.40811096 1.40811097 1.40811098 1.40881.0120 4.509602 3.0096364 1.609637AP 1.609638 1.90ZN447E 9 50

TELETEXTDECODER

SAA502054450300445041SA45050

600700

16.00

UARTSAY 3 I015P 3.00AY 5 10I3P 3.00COMBO') 3.00M 450

WIRE WRAP SOCKE 1565 TEXAS

n 30p 18 pm 50p 24 pin 75014 n 42p 20 pin 60p 28 Pin 100016 n 45p 22 pin 65p 40 pin 1300

Turned PinLow Profile 8 pin 20p

4 pm 25p16 pin18 Pin

30p35p

20 irm24 pm

40p55p

28 pin40 pm

65p90p

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30% DISCOUNTON TTL/CMOS/REGULATORSDRAMS SIPS4164 - 12 2.00 256K x 9 - 124164 - 15 1.50 256K x 9 - 1041256 - 12 3.5014256 - 15 3.00 SRAM41464 - 12 4.00 6264LP - 121MB RAM - 8 15.00 62256 - 121MB RAM - 10 14.00 2732256K x 4 - 10 17.00 2764

27128 - 12.5VSIMMS 27128 - 21V1M x 9 - 10 155.00 272561M x 9- 8 159.00 27512

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TECHNOMATIC LTDTECHNO HOUSE, 468 CHURCH LANE, LONDON NW9 8TQ

(Tel 01 205 9558, Fax 01 205 0190, Telex 922800)305 EDGWARE ROAD, LONDON W2, Tel: 01 723 0233

PLEASE ADD 50p p&p & 15% VAT(Export: no VAT, p&p at Cost)

Orders from Government Depts. & Collegesetc. welcome.

Detailed Price List on request. L

Stock items are normally by return of post.

PRACTICAL ELEC.. tRONICS DECEMBER 1989 11

Ifilhere have been a number of systemsover the years which have beendesigned to combat video and audio

"pirating". The success of such systemsseems, so far at any rate, to have beenstrictly limited. The problem is in finding asystem that renders a video tape (orwhatever) uncopyable without making itunplayable as well! Also, there mustobviously be no easy way of defeating thesystem.

SYNC DOCTOR

Early attempts at copy protection forvideo tapes centred around doctoring of thesynchronisation pulses. They seem to havebeen rather unsuccessful, with no idealcompromise being possible. Doctoring thesynchronisation pulses sufficiently to maketapes uncopyable also tended to make themunplayable on some video recorders. Usinga more mild form of doctoring eliminatedthis problem, but meant that the tapes couldbe copied using some recorders. Largesums of money were reported to have beenlost by companies who had to exchangedoctored tapes that proved to be unusableby their purchasers.

who owns equipment which gives problemswhen playing back some of the latest films!There is a way around the problem whichinvolves the use of so-called agcstabilisation. This usually involvesremoving some or all of the offendingpulses via an out -board unit rather than anytampering with the agc circuits of the videoequipment.

The unit described here is designed to"wipe" the initial part of each video frame(ie the part where the Teletext signal goesin a broadcast television signal) and shouldhelp if used with equipment that hasproblems playing back copy protectedtapes. It processes the composite videosignal, and is designed to fit between therecorder and a monitor, or the monitor input

VIDEO AGCSTABILISER

Modern techniques are moresophisticated, and in general are successful.Rather than altering the synchronisationpulses in some way, the current trend is toadd extra pulses to the recorded signal.These are not designed to fool thesynchronisation circuits of the recorder atall, but are aimed at confusing the agccircuits. The effect of copying a tape that isprotected in this way is said to be periods ofvery high and very low picture brightness,making the copy totally unusable. Thissystem still relies on recording equipmentresponding to the signal from the tape inone way, while playback equipmentresponds to it in another. With a widevariety of video equipment using differenttechniques in widespread use, there areinevitably some people who run intoproblems (and presumably some peoplewho have recorders that are "blind" to thedoctoring techniques).

AGC STABILISATION

It would seem likely that anyone whobuys or hires a video tape that gives lessthan good results when played on fullyworking video equipment is legally entitledto a refund. After all, any doctored or addedpulses are not part of the VHS standard.This is probably cold comfort to anyone

BY ROBERT PENFOLD

Some videorecorders can't

replay copy-

protected tapescorrectly. This

signal -cleaner re-addresses the

problem.

of a television which has this facility. Itshould be possible to feed the output of thisprocessor into a suitable modulator unit sothat it can be used with a television set thatdoes not have a monitor input. However, asyet I have not tried this.

BLANKED OUT

The function the, unit must provide isquite simple, and merely involves blankingout the initial section of each frame (or thevery top part of the screen if you prefer tolook at it this way). Matters are complicatedby the fact that video signals are quitecomplex. There is the positive going picturemodulation signal, the negative

INPUT CLIPPINGAMPLIFIER

LOWPASSFILTER

ATTENUATOR

CLIPPINGAMPLIFIER

D.C.

RESTORE

MON OSTABLE

ELECTRONICSWITCH

VIDEOAMPLIFIER

OUT

Fig.1. Block diagram forthe agc stabiliser


VIDEO PROJECT

Fig.2. Circuitdiagram forthe agcstabiliser

Cl R3no 100 u

SKIIN

D2

33k

C2470n

ItRI

100

R4

22k

R2 R5

330 33k

TR1

C3 R822u

R7

10k

3k9

C5 TR1In

R6 En C4

5k6 15nD1

10

14 a

I C 1

7 9 12

C622n

11

2

R92k2

VRI

47k

RIO680k

D3

IC1 . 4047BE

mm C71101330n

D4

C810u

'11

12

IS I

IC2 pIn 14

111

19

R133M3

C9330n

R144k7

TR4

TR3

R16

2k2

TR5

R122k7

D5 IC2 pIn 7 R15 R174k7 4k7

IC2 = 4016BE TRI,4 BC559 TR2,3,5 BC549

R19 CIO SK268 100u OUT

R18390

+12V

OVD1 - D5 1N4148

synchronisation pulses, and the colour burstsignal somewhere in between. Theimportant point to note when blanking partof a video signal is that the synchronisationsignal must not be blanked. It must be leftlargely intact or problems with. picture roll,a shifted picture, or even a complete loss ofsynchronisation will result. Fig.1 shows theblock diagram for the agc stabiliser, andhelps to explain the way in which thesynchronisation problem is overcome.

The main signal path is through anattenuator, a dc restoration circuit, anelectronic switch, and a video amplifier.This is the route that the signal takes for thevast majority of the time, and it only takesthe alternative route during the briefblanking periods. With the electronicswitch (which is a changeover type) set tothe opposite position, the signal path isthrough the video stripper, another dc

restoration stage, the electronic switch, andthe video amplifier. The video stripper is aform of diode clipping circuit, and its effectis to clip off the positive going video signalwhile leaving the negative goingsynchronisation pulses largely unaffected.These pulses are attenuated slightly by thestripper circuit, and it is for this reason thatthe unprocessed signal is taken to theelectronic switch via an attenuator. Thisensures that the two signals have wellmatched amplitudes when they reach theelectronic switch.

By switching over to the secondarysignal path during the blanking periods, themodulation signal (complete with extrapulses) is removed while thesynchronisation signal is maintained. Thissystem works well in practice, but there is apotential problem caused by a lack ofsymmetry in the circuits ahead of the

Fig.3. The mains power supply circuit.

SlaON/OFF

L 240

240V A.C.

MAINS

SUPPLY

N

SlbON/OFF

T1IC3 = uA78L12

D6,7 = 1N4002

OUT

C13100n

+12V

OUTPUT

0V

electronic switch. This results in the biaslevels of the two signals tending to driftapart so that there are large dc jumps as theelectronic switch chops from one signal tothe other. It is not possible to preset the biaslevels so that the signals are kept preciselyin step. They might start out at suitablymatched bias levels, but changes in theintensity of the modulation signal wouldsoon have them drifting apart again, and byquite substantial amounts. The dcrestoration circuits overcome this problemby keeping both signals with their negativepeaks at the 0 volt supply level. This alsoensures that both signals are kept atvoltages that can be handled properly bythe electronic switch. The video amplifierat the output of the unit compensates for thelosses through the other circuits in thesignal path. It also provides the circuit withthe correct output impedance (75 ohms).

The rest of the unit is devoted toproducing the blanking signal. Thesynchronisation pulses consist of framepulses at a frequency of 50Hz, and linepulses at a frequency of 15.625kHz. Thereare actually only twenty five completepictures per second, with two interlacedframes making up each complete picture.This system gives reduced picture flicker.The blanking pulse can be provided by amonostable multivibrator triggered on theleading (negative going) edge of each framesynchronisation pulse. However, thesepulses must first be filtered out from theother elements of the signal, particularly theline synchronisation pulses.

First the signal is severely clipped, andthis effectively removes the picture


modulation signal. A lowpass filter thensubstantially attenuates the linesynchronisation signals. Apart from beingat' a higher repetition frequency, thesepulses are also much shorter than the framesynchronisation pulses (about 51.ts asopposed to 160[ts in fact). Quite simplelowpass filtering can therefore be veryeffective at reducing the higher frequencypulses while leaving the longer and muchlower frequency ones largely intact.Another clipping amplifier processes thefiltered pulses to produce a strong signalthat can drive the monostable properly.

CIRCUIT OPERATION

The main circuit of the unit appears inFig.2, but the mains power supply circuit isshown separately in Fig.3.

Taking the main signal path first; theinput attenuator is formed by RI and R2,and these feed into the dc restoration circuitformed by C7, D4, and R11. The electronicswitch is a cmos 4016BE (IC2) which is aquad spst type. In this case only twoswitches are required, and these are drivenin anti -phase with their outputs connectedtogether so that the required changeoveraction is obtained. There is no need to addan inverter ahead of one of the controlinputs because the monostable has anti -phase (Q and not -Q) outputs.

The video amplifier is a three stage dccoupled circuit based on TR3 to TR5. Thishas two common emitter amplifiers (TR3and TR4) followed by an emitter followeroutput stage (TR5). Although commonemitter amplifiers normally have a veryhigh voltage gain, in this circuit the voltagegaiii is kept down to the required level ofabout 6dB (two times) by the use ofmassive amounts of local negativefeedback. This feedback is provided by theunbypassed emitter resistors (R15 andR16). A useful byproduct of this negativefeedback is that it gives the amplifier awide bandwidth of several megahertz.Remember that we are dealing with videosignals, and that these contain frequencieswell into the megahertz range. On the faceof it there is no reason for including thesecond common emitter stage. It is essentialthough, as there is a phase inversionthrough each common emitter stage, andthe second stage is needed in order to re -invert the signal back to the correct phase.Unlike audio signals, the correct phasing ofvideo signals is crucial. The picturemodulation must be positive going and thesynchronisation signals must be negativepulses. TR5 gives the amplifier a lowoutput impedance, and R19 trims the outputimpedance to give the correct figure of(near enough) 75 ohms.

D2 acts as the basis of the video strippercircuit. The bias provided by R10 controlshow severely the signal is clipped, and thevalue of this component has been selectedto give a good compromise betweenattenuation of the modulation signal and

Fig.4. PCB component layout

Cl R3 R4

C3R7 C5 R8

OTRI.4aC2 D I

TR2R5 R6 C4 C9

R9C6

fir\VR1

3C 3C 3C 0 3C 3C 3C 3

R10 R 1 6R I 3 TR5

R2 =1. C7

>fa 0-1:1:11113-

D 2 ©COTR3 TR4 _ir C 1 2

O D5 D4

C8

(441 R18 0

°R 1 7

R I 9C 3R 10

ICI

CC

c-)11.)

333333

T115 0 15

I I

3

C I 1

F S I

( E )SK 1

R12 R 1 1 ( E )

SK 2

Reservation of the synchronisation pulses.The output of this circuit feeds into anotherstandard dc restoration circuit and then intoits section of the electronic switch.

TR I operates in the clipping amplifier atthe input of the frame synchronisation filtercircuit. This is followed by a simple singlepole lowpass filter using R7 and C4. C5couples the filtered pulses to the clippingamplifier based on TR2. This drives theinput of the monostable, which is a negativeedge triggered circuit based on a cmos4047BE astable/monostable (IC1). Theoutput pulse duration is set by C6, R9, andVR1. The latter gives an adjustment rangeof approximately 0.12ms to about 2.7ms.This enables the unit to blank a largenumber of lines at the top of the screen, andin practice VR1 is adjusted so that the mainpicture signal is not significantly affected.Only the unused lines need to besuppressed. As explained previously, theFig.5. PCB track layout

monostable has Q and not -Q outputs whichprovide the anti -phase control signalsneeded by the electronic switch.

A reasonably smooth and stable 12 voltsupply is needed, and this is provided by asimple mains power supply circuit. This haspush-pull rectification and a smallmonolithic voltage regulator (IC3) to give awell smoothed output. The currentconsumption of the main circuit is onlyabout 15 milliamps, and a small voltageregulator is well able to handle this.

CONSTRUCTION

Most of the components fit on to theprinted circuit board, details of which areshown in Fig.4. Both the dil integratedcircuits are cmos types, and consequentlyrequire the normal anti -static handlingprecautions to be observed. Pin 3 of the

*-7 ft077-0114

jertfll 0

0 Foi Wet'14 PRACTICAL ELECTRONICS DECEMBER 1989

VIDEO PROJECT

4047BE apparently has no static protectioncircuit, and this device therefore needs to behatidled with extra care. Sockets should beused for both devices, but they should notbe fitted into the holders until the board andall the wiring have been completed. Untilthen they should be left in their anti -static

COMPONENTS

RESISTORS

R1R2R3, R5R4R6R7R8R9, R16R10R11R12

10033033k (2 off)22k5k610k3k92k2 (2 off)680k100k2k7

R13 3M3R14, R15, R17 4k7 (3 off)R18 390R19 68All 0.25 watt 5% carbon

POTENTIOMETER

VR1 47k sub -min hor preset

CAPACITORS

Cl, C10

C2

C3C4

C5 ,

C6

C7, C9

C8C11C12, C13

100µ 16V radial elect(2 off)470n polyester 7.5mmpitch22µ 16V radial elect15n polyester 7.5mmpitchIn polyester 7.5mmpitch22n polyester 7.5mmpitch330n polyester 7.5mmpitch (2 off)101A 25V radial elect1000µ 25V radial elect100n ceramic (2 off)

SEMICONDUCTORS

IC1IC2IC3TR1, TR4TR2, TR3, TR5D1 to D5D6, D7

4047BE4016BEuA78L12BC559 (2 off)BC549 (3 off)1N4148 (5 off)1N4002 (2 off)

MISCELLANEOUS

SKI, SK2 Phono socket, (2 off)T1 15 - 0 - 15 volt 100mA

mains transformer(see text)

FS1 100mA 20mm anti -surgeSi Rotary mains switchPrinted circuit board, instrument caseabout 150 x 100 x 50mm, pair of 20mmfuseclips, 14pin dil is holder (2 off).control knob, mains lead and plug, wire,solder, pins, etc.

packaging. The cmos 4066BE analogueswitch can be used in place of the 4016BEin most circuits, and both devices seem towork perfectly well in this circuit.

The fuse is mounted on the board via apair or 20 millimetre fuse -clips. It isadvisable to use an anti -surge fuse ratherthan the more usual "quick -blow" variety.Otherwise there is a strong risk that the fusewill "blow" each time the unit is switchedon, due to the large initial charge current intoCI I. Note that the capacitors are all printedcircuit mounting types, and that the "axial"variety might not fit into the layout verywell. It is advisable to use polyestercapacitors of the correct (7.5 millimetre)pitch, as types having a different pitch couldeasily be damaged if you try to persuadethem to fit on to this board. Fit pins to theboard at the points where connections to off -board components will be made, and do notoverlook the three link wires.

A metal instrument case havingdimensions of about 150 by 100 by 50millimetres will accommodate all thecomponents, and there are several cases ofabout this size to choose from. Note though,that as this is a mains powered project thecase must be a type that can only be openedwith the aid of a screwdriver or other tool.Types with "clip -on" panels or covers areunsuitable for safety reasons. The on/offswitch is mounted well towards the righthand end of the front panel, with the twosockets fitted to its left. I used phono socketsfor both SK1 and SK2, but these canobviously be any type that will match yourvideo equipment. A hole for the mains leadmust be drilled in the rear panel, well towardsthe left hand end of the panel (as viewedfrom the rear). This hole should be fitted witha grommet to protect the cable.

The printed circuit board is mounted onthe base panel of the case, with the usualstand-offs or spacers being used to hold theunderside of the board clear of the metalcasing. Mount the board as far to the left ofthe unit as possible so that there is sufficientspace left for the mains transformer to fit toits right. You will probably have difficulty inobtaining a 15 - 0 - 15 volt mainstransformer, but a twin 15 volt type should bereadily obtainable and is perfectly suitable. Itshould be connected in the manner shown inthe wiring diagram of Fig.6. The transformer

Fig.6. Power supply wiringonly needs to have a secondary current ratingof about 50 milliamps, but the smallest typeyou are likely to find is one having a 100 to200 milliamp rating. A type having a higherrating is quite suitable provided there issufficient space for it in the case (I had nodifficulty in using a 200 milliamp type). Themetal case of the unit must (for safetyreasons) be earthed to the mains earth lead. Asoldertag on one of T1 's mounting boltsprovides a convenient way of making thisconnection.

ADJUSTMENT

With the unit switched on and connectedbetween a video source and a monitor, thepicture should appear on the monitor with nosignificant change in picture quality beingapparent. By adjusting VR1 you should beable to blank a small section at the top of thescreen, and vary the amount of the picturethat is blanked. Simply adjust VR1 to blankas much of the screen as possible withoutremoving any of the main picture lines.

The value of RIO controls the degree ofblanking in the top part of the picture. Thespecified value should be perfectlysatisfactory, but if necessary you can obtainmore complete blanking by using a highervalue here, or a stronger synchronisationsignal (but with lower attenuation of thevideo modulation signal) by using a

somewhat lower value. PE

Interior view ofunit housing

it11

showingpositioning ofpcb andtransformer. ii

,

.s`1 _41

. - 4

i L./


AIR IONISERSBy means of points raised to a very high voltage, ionisers re -structure the airyou breathe, turning ordinary air molecules into potent negative ions. Theeffects of breathing in these ions can be quite startling. Almost everybody

reports that it makes them feel good, and there is now strong evidence that itcan also improve your concentration, make you more healthy and alert, makeyou sleep better, and even raise your 10.

THE MISTRAL AIR I>IONISERThe ultimate air ioniser. The Mistral has variableion drive, built-in ion counter and enough power todrive five multi -point emitters with ease. Its ninemain drive stages, five secondary drives and fourbooster stages give an immense 15 billion ions perminute output - enough to fill the largest room ina matter of seconds.

The parts set contains everything you need tobuild the Mistral: components, PCB, case, emitterand full instructions. If you're keen to increase theoutput still further, there's an optional eight -pointinternal emitter set to give extra ionising capability,and an almost silent piezo-electric ion fan to drivethe ions away from the emitter and into the room.

MISTRAL IONISER PARTS SET £32.66

INTERNAL EMITTER PARTS SET

(optional) £3.22

ION FAN (optional) £11.27

THE DIRECT -IONThe ideal bedside ioniser. If you're keen to see what all the fuss isabout, and to experience the ion effect for yourself, this is the one togo for. The Direct -Ion parts set contains PCB, 66 components, case,mains lead, and even the components for the tester. Don't forget theexperiments: there's the smoke trick, triffids, the living emitter, andmore. And full constructional details too, of course.

DIRECT -ION PARTS SET £14.72

THE 0 -IONCheck out the ion levels around your house. The 0 -Ion will measurethe output of any ioniser, test the air to see where the ions areconcentrating, help you set up fans and position your ioniser for besteffect, and generally tell you anything you want to know about ionlevels in the air. The readout is in the form of a bar graph which movesup and down as the 0 -Ion sniffs the air in different parts of the room.Readings up to 1010 ions per second, positive or negative.

0 -ION COMPLETE PARTS SET £21.16

IONISEREXPERIMENTS

* The Vanishing SmokeTrick

Light up a cigarette and gently puff smokeinto a glass jar until the air inside is a thick,grey smog. Carefully invert the jar over theioniser so that the emitter is inside. Withinseconds the smoke will vanish! This is oneof the best demonstrations of an ioniser'sair cleaning action and with a large jar theeffect is quite dramatic.

* Triffids

Connect a length of wire from the ioniseremitter to the soil in the pot of ahouseplant. One with sharp, pointy leavesis best. Hold your hand close to the plantand the leaves will reach out to touch you!In the dark you may see a faint blue glowaround the leaf tips - this works better withsome plants than with others, so try several

different types. The plants don't object tothis treatment at all, by the way, and oftenseem to thrive on it.

* The Electric Handshake

Wear rubber soled shoes. Touch theioniser emitter for a few seconds until yourbody is thoroughly charged up. When yourhair stands on end, that's just aboutenough. Then give everyone you meet ajolly electric handshake. Just think, youcould lose all your friends in a singleevening! (A meaner trick still is to chargeup a glass of water or a pint of beer. Evenyour family won't speak to you after that!)

KIRLIAN CAMERABioplamic fields, auras, or just plain corona discharge? No matter how you explain them, the effects are strange andspectacular. Can you really photograph the missing portion of a torn leaf? Can you really see energy radiating fromyour finger tips? Most researchers would answer 'yes' to both questions.

Our Kirlian photography set contains everything you need to turn the Mistral into a Kirlian camera, your bedroomor spare room into a darkroom, and to expose, develop and print Kirlian photographs (photographs made with highvoltage electricity instead of light). The set includes exposure bed, safelight bulb, developing and fixing chemicals,trays, imaging paper and full instructions. A Mistral ioniser parts set is also required.

KIRLIAN CAMERA SET £19.78

D)UVEDifDa,Tel: (0600) 3715 LIMITED

SALES DEPT., ROOM 108, FOUNDERS HOUSE, REDBROOK, MONMOUTH, GWENT.

ORDERINGAll prices include VAT

UK orders: please add £1.15 postage and packing.

Eire and overseas: please deduct VAT and add

£5.00 carriage and insurance.

ACCESSPhone 0600 3715 for immediate

attention to your Access order.

MAINTENANCE FEATURE /10

he AL80 amplifier module is apopular unit that has been on themarket since early 1978. It joined a

Bi-Pak range consisting of the AL30 andAL60, 10 and 25 -watt units which had beengoing for some years previously.

There was nothing fancy about the designfrom the hi-fi viewpoint, and there was nooutput limiting or short-circuit protection;the circuit was just a straightforward fairlyconventional power amplifier. It delivered35 -watts rms into a range of impedancesfrom 16 ohms down to 3 ohms, and operatedat single -pole supply voltages from 40-60V,with a below 0.1% harmonic distortionrating (typically 0.06%).

Lower powers were obtained with thelower supply voltages and higher outputimpedance loads; the output could thus betailored for the intended use. The maximumrating of 35 watts was sufficient to enablethe module to be used for a wide range ofapplications, and the impedance range addedto its versatility.

Apart from these, a major attraction foramateur constructions and professionalcustom-built equipment alike was its size. Itwas just 4 inches by 2.5 inches including aflat metal plate which served as a heat sink.For power requirements below 10 watts, no

Technologyresolutely

marches on, butold-time favourites

need not betrodden down todust, as VivianCapel proves.

The device was thus contact -cooled incontrast with most power chips and modulesthat are air-cooled and so need to be mountedoutside the equipment casing to ensure anadequate air flow. This was a big advantagewhich simplified construction, and combinedwith its small size made the module an idealchoice for many custom-built systems.

Public address amplifiers for small hallsis one application for which many AL8Oshave been used. In fact many thousands ofthese modules have been produced andinstalled in a wide range of equipment in theyears since their introduction.

The more observant readers will have

hoped some manufacturer will fill beforevery long), this creates a problem for thosehaving AL8Os in use that have developedfaults or may do in the future.

Although they are repairable, there has uptill now been no published circuit diagram,service information or parts list. The hithertoready availability of the modules, theirmoderate cost and the ease of fitting, hasmeant that replacing and discarding thedefective module has been the most practicalcourse when a fault occurred.

The following information has thus beenprepared and is presented to aid the repairand servicing of faulty modules.

CIRCUITDESCRIPTION

The input is via C2 to the base of TR1which is a pnp device connected across theoutput circuit that is at half -supply voltage,and earth. Its supply is thereby modulatedwith the output voltage so providing negativefeedback. Capacitors C3 across the input andC6 limit the hf response to prevent hfoscillation. The base bias is decoupled by RIand C4.

REPAIRING THE AL80other heat sinking was needed, but for themaximum 35 watts, the plate had to bebolted on to a 16 swg aluminium sheet of atleast 45 square inches. Bolting it to a metalchassis or equipment case gave ample heatsinking in most cases.

noticed that I have been using the past tensein this description. The reason is that,unfortunately, the makers have ceasedtrading, and the units are no longer available.Apart from a gap in the range of power unitsavailable for custom -building, (which it is

33p

10C2

2-2p

C3MN 470p

R1

39k

R2

82k

C,2-2p

12k

TR7 AND TR8 MAY BEREPLACED WITH 80278

1.7p

1

R12 IC93900

114

R13

11,2182001

RIO

811

2k2

TR4.BC 17113

R15

TR5BC1.1.1

R16

8170110 0 RI

R3

100k

TR1BC 212(BC307)

25011

C5

12k

c.6ono 82p

R5

33k

TR2BCI82

R8 ic7Sk6 22P

R9

CIOam 22n

TR3BC639

111

LINK

R17

11

TR60C461

R18

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TR7BD711IBD709/

0 OUT

1 C11

TR8B0711(B07091

27n

8195-511

1:13:10:1

0

The output from TR1 is directly coupledto TR2 which is a npn transistor configuredas an emitter follower. It thus has a gain ofless than unity, but gives a good drive currentto TR3 across which C10 gives further hflimiting.

TR3 drives the bases of the power drivertransistors TR5 and TR6. These are acomplementary pair that are connected inseries across the supply via their emitter andcollector resistors. The current through themand hence also through the output pair towhich they are directly coupled, depends onthe potential difference between their bases.This is regulated by TR4 which is mountedon the heat sink. Any increase intemperature, increases its current whichreduces the potential difference between thebases of TR5 and TR6, thereby reducingtheir current and the current through theoutput pair.

So there is a degree of long-term self -regulation, with heat sink temperature risesreducing output current. This does notprotect against heavy overloads such aswhen the output is short-circuited, as thenthe output devices could be destroyed beforethe heat -sink temperature rose significantly.

The output pair are not complementarybut are both npn transistors. They have noemitter resistors, and the output capacitor isnot included. This is omitted to save space

Left: Circuit diagram of the AL80amplifier module.


Component layout on the AL80 printed circuit board.

on the panel and usually can be easilyaccommodated as an axial component wiredbetween the output on the module and theoutput socket on the amplifier. In addition,the value can thus be selected to suit theload.

C11 and R19 provide further hf roll -off tocomplement the capacitors fitted in earlierstages.

FAULTS

The unit has proved to be reliable withfew faults developing. This no doubt is

partly due to its uncomplicated designconsisting of just eight transistors and elevencapacitors of which only six are electrolytics.Most of the troubles that have occurred havebeen due to external causes such asoverloading the output stage. When used as apublic-address amplifier, this is likely due tonon -technical persons adding extra speakersthereby reducing the load impedance belowthe minimum specified.

When this happens, as there is nooverload protection, the output transistorsobject and finally go s/c. The visible effect isthat the two resistors R15 and R17disintegrate in a puff of smoke. Onexamination of the circuit though, it can beseen that these are not in the output stage atall but are the emitter resistors of the drivers.So why should they go?

The answer is because when the outputtransistors go short-circuit, the drivers startfeeding the load through the s/c outputdevices, and behave as if they were theoutput transistors. Of course, they are notman enough to feed a load that higher -powered output transistors couldn't manageso they too go s/c. Hence the burnt -outemitter resistors. (Moral: never get ideasabove your station.)

The repair thus involves replacing boththe output pair and the driver transistors

along with the 1 ohm emitter resistors. Asnag here is that BD709 or BD711transistors, both of which have been used forthe output in the AL80, are not generallyavailable. A suitable substitute whichrequires no circuit changes is the BD278.The BC441 and BC461 drivers and all theother transistors should be no problem toobtain.

MATCHED PAIRS

If possible, try to get a matched pairfor output and drivers. Some supplierswill match up a pair though unfortunatelymost will not. If you have a transistortester that measures Hfe (and such aninstrument is very useful for all seriouswork involving transistors), it is worthbuying two or even three pairs of each,and getting the closest match possible.Often a couple of reasonably matchedpairs can thus be achieved, therebyproviding a back-up pair for the future.At least a grossly mismatched pair will beavoided.

As with all directly -coupledamplifiers, systematic stage -by -stagefault-finding is not feasible because afault in one stage can upset the dcworking conditions in all the other stages,so all can thus exhibit fault symptoms.Transistors are the most likely culpritsfollowed by electrolytic capacitors.Whatever the fault then, the best plan isto remove each transistor in turn and testit. Testing in situ can be misleading dueto the presence of shunt circuits.

In the case of a s/c output pair, checkthe impedance of the load. If it seems tobe high enough, that is over 3 ohms, thefault could be due to an earlier transistorfailure. Should TR4 go o/c for example,the forward bias through the drivers is

OUTPUT CAPACITOR VALUES

Output load 20 Hz 40 Hz 60 Hz 80 Hz

3-4 ohms 1,000 tIF 470g 330g 220 µF8 ohms 470µF 220tIF 1500 100 g

16 ohms 220µF 100pf 681.1.F 47 pf

Capacitor values to nearest preferred value, to give ±2 dB response to indicated frequencyat given loads.

HEAT SINK REQUIREMENTS

Output power Load Impedance Supply Voltage Heat Sink Area

7.5W 16 ohm 35V 8in210.0W 16 ohm 40V 10 in212.5W 16 ohm 45V 20 in215.0W 8 ohm 35V 15 in217.5W 16 ohm 55V 35 in220.0W 8 ohm 40V 20 in225.0W 8 ohm 45V 30 in235.0W 8 ohm 55V 45 in237.5W 3 ohm 35V 40 in2

Heat sink area depends not only on output power but on supply voltage too because thisgoverns the quiescent current through the output transistors. Heat sink is of 16 gaugealuminium.


MAINTENANCE FEATURE /increased thereby increasing their current,and thus also the current through theoutp4t stage.

ELECTROLYTICS

If no fault is found with any of thetransistors, next, check the electrolytics.All have voltages, across them, so checkfor these. The absence of a readingsuggests a s/c capacitor, but measure theassociated resistors before replacing it incase one has gone o/c. If there are voltagesacross the capacitors, bridge a replacementacross each in turn, remembering toobserve polarity. If this clears the fault, thecapacitor is o/c.

GENERAL

When two modules are used at highpower, such as a stereo pair or a separatespeaker and hearing -aid loop supply in apublic-address system, ensure that there isadequate heat sinking. It should be doublethat of a single module. If the chassis area isless than the required amount, extra sinkingcan be obtained by mounting the modules onan aluminium strip or bar which itself isbolted to the chassis. (A suitable offcut maybe obtained from an aluminium windowmaker.) The bar will also lift the print boardclear of the chassis and so prevent thepossibility of any part shorting to the chassis.If the clearance is small, the chassis area

under the print should be covered withplastic tape.

As much as possible of the integral heatsink should be in contact with the chassis orbar. It should not be supported on nuts andwashers on the fixing bolts in order to givechassis clearance. A little heat -sink pastesmeared between the module and the bar,and the bar and the chassis will help toincrease heat conductivity.

The negative of the output and driverstages is isolated from that of the rest of thecircuit on the board, possibly to enable aquick check of the quiescent current, andalso permit the earlier stages to be poweredfor testing without running the output stage.The two, (points 1 and 2 on the board),should be linked for normal use.

MFANG-TASTIC

y dentist tells me he can spotelectrical or electronic engineers inany crowd - by the strange slots worn

in their teeth! Fangs, he observes, should not beused as wire -strippers! No doubt manicuristscould make a similar observation regardingfinger -nails. Hands up and open mouths all thosewho dare to say you're not guilty.

I can't, for one! What else are nature'snatural tools for, if not for wire -stripping? Well,years ago I did invest twenty or thirty quid on asophisticated stripper (!) but it brokeeventually, and I never got round to replacingit. Until now.

In the post came a news release from OKIndustries. "OK's new ST -500 adjustable wirestripper", said the first paragraph, "is aprecision tool which quickly and accuratelystrips 20-30 awg (0.25 to 0.8mm) wire, itshardened blades tackling all types of wireinsulation, including Teflon."

So I rang Mr Dornan at OK and corralledhim into sending me a free sample to try out.Which I have. Yes, as news release para twosays, it's "effortless to use and easy to set up,and the lightweight, squeeze action tool hasan adjustable wire stop to ensure consistentstrip lengths."

If it weren't already too late, my dentistwould be delighted, but for me fangs ain'twhat they used to be. There's new hope forthe claws though!

You could save your gnashers and nails(for about a tenner) by contacting Mr J.Doman at OK Industries UK Ltd, BartonFarm Industrial Estate, Chickenhall Lane,Eastleigh. Hants, S05 5RR. Tel: 0703619841.

OUTNOW!

1989Lrl ci. -Tni71111r.L

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ELMASET INSTRUMENT CASE300 x 133 x 217mm deep £10 ea (£2.20)

REGULATORSLM317T PLASTIC T0220 variable f1LM317 METAL £2.207812 METAL 12V 1A £17805/12/15/24V plastic 35p 100+20p 1000+ 15p7905/12/15/24 plastic 35p 100+20p 1000+ 15pCA3085 T099 variable reg 2/£1LM338 5A VARIABLE £5

COMPUTER ICSBBC MICRO PARTSVIDEO ULA 201647 £10ea 10+ £86845 CRT £56522 PIA £3DM88LS120 £4.50AV310150 UART £2.50IM6402 UART £38086 processor equipment £2USED 41256-15 £3.00USED 4164-15 ex -equipment £19 x 41256-15 SIMM MODULE NEW £258x 4164 SIP MODULE NEW £8HD 146818 CLOCK IC £22864 EEPROM £327128A 250nS EPROM USED £2 NEW £2.30FLOPPY DISC CONTROLLER CHIPS 1771 £10 eaFLOPPY DISC CONTROLLER CHIPS 1772 £16 ea68008 PROCESSOR EX-EOPT £5ALL USED EPROMS ERASED AND BLANK CHECKEDCAN BE PROGRAMMED IF DESIRED.2716-45 USED £2 100/£12732-45 USED £2 100/£12764-30 USED £2 100/£1.6027C256-30 USED £327C512 USED £3.501702 EPROM EX EQPT £52114 EX EQPR 60p 4116 EX EQPT 70p6264-15 8k static ram £3.504416 RAM £3.50USED 4416-15 RAM £2USED 41464-15 £7ZN427E-8 £3.50ZN428E-8 £3.50

CRYSTAL OSCILLATOR1.8342 MHz £1 each

CRYSTALS2.77 MHz/4.000 MHz/4.9152MHz 20MHz 49.504 MHz

TRANSISTORSBC107, BCY70 PREFORMED LEADS

full spec £1 £4/100 £30/1000BC557, BC548B 30/£1 £3.50100

POWER TRANSISTORS0C35 (Marked CV7084)POWER FET IRF9531 8A 60V2SC1520 sim BF259TIP141/2 £1 ea TIP112/125/42BTIP35B TIP35C

£12/£1

3/£1 100/£222/£1

£1.50SE9301 100y 10A DARL. SIM TIP1212N3055 EX EQPT TESTEDPLASTIC 3055 OR 2955 equiv 50p2N3773 NPN 25A 160V £1.80

QUARTZ HALOGEN LAMPSA1/216 24V 150 WATTS £2.25H1 12V 50W (CAR SPOT) £1.50

14 WAY AMP ZIF SOCKET 2/elTEXTOOL single in line 32 way. Can be ganged (couplingsupplied) for use with any dual in line devices.14 WAY AMP ZIF SOCKET 2/£1

CAPACITORS COMPUTER GRADE3300uF 350V SIC SAFCO FELSIC 037 £6(£1.50)2200uF 160V SIC SAFCO FELSIC C038 £4(£1.20)

TURNS COUNTING DIALS all for 0.25" shaft

10 turn dial 21 mm dia. fits 3mm spindle C210 turn digital dial (3 digits) for 3mm or 6mm shaft £3.5010 turn clock face dial for 6mm spindle C4

MISCELLANEOUSASSORTED SWITCHES, ROCKER, SLIDE, PUSH .. £2.50SLOPING FRONT PLASTIC CASE 225 x 215 x 76mm76 mm WITH ALI FRONT PANEL 200 x 130mm ... C4.00(£1)HUMIDITY SWITCH ADJUSTABLE £2WIRE ENDED FUSES 0.25A 30/E1NEW ULTRASONIC TRANSDUCERS 40kHz £2/pair12 CORE CABLE 7/0.2mm OVERALL SCREEN..£1/3metresPOWERFUL SMALL CYLINDRICAL MAGNETSOP AMP LM1OCLN £2.90BNC 50 OHM SCREENED CHASSIS SOCKET 3/£1BNC TO CROC CLIPS LEAD 1 metre £1MOULDED INDUCTOR 47001

size of a 1 watt film resistor 5/£1

2/£14/£1

100/£3510/£16

TO -220 HEAT SINK sim RS 403-162 10/£2.50SMALL MICROWAVE DIODES AEI DC1028A 2/£1D.I.L. SWITCHES 10 WAY £1 8 WAY 80p 4/5/6 WAY 50p180 volt 1 watt ZENERS also 12v & 75v 20/£1PLASTIC EQUIPMENT CASE 9 x 6 x 1.25 in. WITH FRONT ANDREAR PANELS CONTAINING PCB WITH EPROM 2764-30 ANDICS 7417 LS30 LS32 LS74 LS367 LM311 7805 REG, 9 WAY DPLUG, PUSH BUTTON SWITCH, DIN SOCKET £1.90VNIOLM 60v 1/2A 5ohm TO -92 mosfet 4/£1 100/£20MIN GLASS NEONS 10/£1RELAY 5v 2 pole changeover looks like RS 355-741 markedSTC 47WBO5T Cl eaMINIATURE CO -AX FREE PLUG RS 456-071 2/£1MINIATURE CO -AX FREE SKT. RS 456-273 2/£1.50DIL REED RELAY 2 POLE n/o CONTACTS £1PCB WITH 2N2646 UNIJUNCTION with 12v 4 POLE RELAY El400m 0.5w thick film resistors (yes four hundred megohms) 4/E1STRAIN GAUGES 40 ohm Foil type polyester backed balco gridalloy .. £1.50 ea 10+ £1ELECTRET MICROPHONE INSERT £0.90Linear Hall effect IC Micro Switch no 613 SS4 sim RS 304-267

£2.50 100+ £1 .50HALL EFFECT IC UGS3040 + MAGNET E1OSCILLOSCOPE PROBE SWITCHED X1 X10 £10CHEAP PHONO PLUGS 100/£21000/C181 pole 12 way rotary switch 4/elAUDIO ICS LM380 LM386 Cl ea555 TIMER 5/E1 741 OP AMP 5/£1ZN414 AM RADIO CHIP 80pCOAX PLUGS nice ones 4/£1COAX BACK TO BACK JOINERS 3/£14 x 4 MEMBRANE KEYBOARD £1.5015.000uF 40V £2.50 (£1.25)INDUCTOR 20uH 1.5A 5/£1NEW BT PLUG + LEAD £1.501.25" PANEL FUSEHOLDERS 3/E1CHROMED STEEL HINGES 14.5 x 1" OPEN Cl each12v 1.2w small wire ended lamps fit AUDI VW TR7 SAABVOLVO12V MES LAMPSSTEREO CASSETTE HEADMONO CASS.HEAD £1 ERASE HEADTHERMAL CUT OUTS 50 77 85 120°CTHERMAL FUSE 121°C 240V 15ATRANSISTOR MOUNTING PADS TO-5/TO-18TO -3 TRANSISTOR COVERSSTICK ON CABINET FEETPCB PINS FIT 0.1" VEROTO -220 micas + bushes 10/50p 100/£2TO -3 micas + bushes 15/E1PTFE min screened cable 10m/£ 1Large heat shrink sleeving pack £2CERAMIC FILTERS 6M/9M/10.7M 50p 100/£20MAINS LEAD WITH MOULDED 13A PLUG AND IEC SOCKET

£1.50IEC chassis plug rfi filter 10A £3Potentiomenters short spindles values 2k5 10k 25k lm2M5 lin 5/£1500k lin 500k log 4/£140Khz ULTRASONIC TRANSDUCERS EX-EQPT NO DATA

£1/prPLESSEY INVERTER TRANSFORMER 50 CYCLES11.5 0 11.5V to 240v 200VA C6 (£3)

DIODES AND RECTIFIERS1N4148 100/£1.501N4004/SD4 1 A 300V 100/£31N5401 3A 100V 10/£1BA158 1 A 400V fast recovery 100/£3BA159 1 A 1000V fast recovery 100/E4120V 35A STUD 65pBY127 1200V 1.2A 10/C1BY254 800V 3A 8/C1BY255 1300V 3A 6/£16A 100V SIMILAR MR751 4/£11A 800V BRIDGE RECTIFIER 4/£14A 100V BRIDGE 3/£16A 100V BRIDGE 2/£18A 200V BRIDGE 2/£1.3510A 200V BRIDGE £1.5025A 200V BRIDGE £2 10/C1825A 400V BRIDGE £2.50 10/e22

SCRSPULSE TRANSFORMERS 1:1+1 £1.252P4M EQUIV C106D 3/£1MCR72-6 10A 600V SCR £135A 600V STUD SCR £2T1CV106D 800mA 400V SCR 3/£1 100/£15MEU21 PROG. UNIJUNCTION 3/C1

TRIACS DIACS 4/£1BT137-600 8A TO -220 2/£1BT138-600 12A TO -220 70pNEC TRIAC ACO8F 8A 600V TO220 5/£2 100/C30TXAL225 8A 400V 5mA GATE 2/£1 100/£35TRAL2230D 30A 400V ISOLATED STUD £4 each

KEYTRONICTEL. 0279.505543FAX. 0279-757656

P 0 BOX 634BISHOPS STORTFORD

HERTFORDSHIRE CM23 2RX

10/£110/£1

£250p

£1 ea5/£1

£3/100010/£130/£1

200/£1

CONNECTORSD25 IDC SOCKET FUJITSU £234 way card edge IDC CONNECTOR (disk drive type)

£1.25CENTRONICS 36 WAY IDC PLUG £2.50CENTRONICS 36 WAY IDC SKT £4.00BBC TO CENTRONICS PRINTER LEAD 1.5M £3.50CENTRONICS 36 WAY PLUG SOLDER TYPE £4USED CENTRONICS 36W PLUG+SKT £3

USED D CONNECTORS price per pairD9 60p, D15 £1 .50, D25 £2, D37 £2, D50 £3.50 covers 50p ea.

WIRE WOUND RESISTORSW21 or sim 2.5W 10 of one value ClR10 OR15 OR22 2R0 4R7 5R0 5R6 8R2 10R 12R 15R 18R 20R22R 27R 33R 47R 56R 62R 91R 120R 180R 390R 430R 470R680R 820R 910R IK15 1K2 1K5 1K8 2K4 2K7 3K3 3K0 5K0 R05(50 milli -ohm) 1% 3w 4 FOR ClW22 or sim 6W 7 OF ONE VALUE £1R47 R62 1R0 1R5 1R8 3R3 6R8 9R1 12R 20R 24R 27R 33R 51R56R 62R 68R 100R 120R 180R 220R 390R 560R 620R 91OR 1K0IK2 1K5 1K8 2K2 2K7 3K3 3K9 4K7 8K2 10k 15K 16K 20KW23 or sim 9W 6 of one value C1

R22 R47 1R0 1R1 56R 62R 100R 120R 180R 220R 300R 390R680R 1K0 1K5 5K1 10KW24 or sim 12W 4 OF ONE VALUE ClR50 2R0 9RI 18R 22R 27R 56R 68R 75R 82R 100R 15OR 180R200R 220R 270R 400R 620R 1 KO 6K8 8K2 10K 15K

PHOTO DEVICESSLOTTED OPTO-SWITCH OPCOA OPB815 £1.302N5777 50pTIL81 PHOTO TRANSISTOR £1TIL38 INFRA RED LED 5/£14N25, OP12252 OPTO ISOLATOR 50pPHOTO DIODE 50p 6/£2MEL12 (PHOTO DARLINGTON BASE n/c) 50pRPY58A LDR 50p ORP12 LDR 70pLEDs RED 3 or 5mm 12/£1 100/E6LEDs GREEN OR YELLOW 10/£1 100/£6.50LEDS ASSORTED RD/GN/YW + INFRA/RED 200/E5FLASHING RED OR GREEN LED 5mm 50p 100/£35

STC NTC BEAD THERMISTORSG22 220R, G13 1K, G23 2K, G24 20K, G54 50K, G25 200K,G16 1M, RES ai 20°C DIRECTLY HEATED TYPE £1 eaFS22BW NTC BEAD INSIDE END OF 1"GLASS PROBERES ai 20°C 200R Cl eaAl 3 DIRECTLY HEATED BEAD THERMISTOR 1kres. Ideal for audio Wien £2 ea

CERMET MULTI TURN PRESETS 3/4"10R 20R 100R 200R 250R 500R 2K 2K2 2K5 5K 10K 47K50K 100K 200K 500K 2M2 50p each

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COMMUNICATIONS FEATURE

n the final parts, we shall examinespecial refinements to receivers andtransmitters, like agc and squelch, as

well as expand on previous sections coveringdetection and amplification.

THE AM RECEIVER

It would be as well to begin with a fewdefinitions.Sensitivity

This is the weakest signal that can bedetected without distortion and is limited bynoise produced within the receiver itself. Theformal definition of sensitivity is the minimuninput carrier voltage required to provide aspecified signal to noise ratio at the output ofthe IF stage.

The signal to noise ratio specified at IFbecause fm detectors provide an improvementin signal to noise ratio. Another definitionapplied to am receivers is the minimum inputcarrier voltage modulated 50% at I kHz andproducing the specified signal to noise ratio atthe output of the detector.Selectivity

The ability of the receiver to tune in therequired signal and reject other signals is theselectivity. This depends on the circuitsbefore the detector, mainly filters in the IFstage.

IF rejectionThis is the ratio in dB of inputs at IF and

the desired carrier to produce equal outputsfrom the mixer and varies with tuning. Forinstance an am receiver tuned to 1605kHzwill be in a better position to reject aninterfering signal at 455kHz than if it weretuned to 555kHz.Image rejection

The image frequency is given by f = f0 +fif where f0 is the oscillator frequency and fifis the IF. When the image frequency ispicked up by the aerial and reaches themixer, it beats with the oscillator. The imagerejection in dB is the ratio off to the requiredinput to produce equal outputs from themixer. Once again the value varies withtuning and 50dB is typical for a goodcommunications receiver.

RF AND IF STAGES

An if amplifying stage has manyrequirements some of which are high powergain and low noise figure, stability andselectivity to prevent spurious, image and IFfrequencies reaching the input of the mixer. Itshould have a linear amplification in order toamplify strong signals without crossmodulation distortion (cmd) orintermodulation distortion (imd), and also

isolate the aerial from the mixer and localoscillator.

In active devices like transistors whichproduce third order distortion when thedesired signal and interfering signal are fed tothe input, amplitude modulation of theinterfering sign can be transferred to thedesired signal. This is called crossmodulation and occurs when the desiredsignal is weak and in the adjacent channel tothe interfering signal from a strongertransmitter. This can occur in the mixer aswell as if amplifier and fets are often used inboth stages.

The local oscillator must be stable so thatthe receiver's tuning does not drift with voltageor temperature, since the local oscillatorfrequency decides the sum and differencefrequencies at the output of the mixer.

An IF of 455kHz for am and 10.7MHz forfm broadcast were chosen in the days of thevacuum tube since a low IF meant less leadinductance and stray capacitance and thereforehigher gain and better stability.

However, this does not help the imagerejection since the IF is separated by only 910kHz from the am broadcast band. For fm(88MHz to 108MHz), the separation betweenif and an image frequency is 21.4MHz whichis no better than for am, percentage wise.

Although valves are still around,particularly in high power transmitters, the

HF RADIONoise figure

This is defined as:

input signal to noise ratiooutput signal to noise ratio

For multistages, 5dB to 10dB is typical fora good receiver from the aerial input to the IFoutput.

FERRITE LOOPAER AL

AVE

+RE

FREOUENCY CONVERTER

455kHz

+VE

4VOLUME

CONTROLFROM AF AMP

1st I F

Part Fiveby Mike Saunders

In this last partwe look at RFand IF stagedesign, filtersand detectors.

2nd IF

DETECTOR

transistor has distinct advantages whenever itis possible to use them. They are smaller,cheaper and give good gain to higherfrequencies. They have a lower impedancethan vacuum tubes and the stray capacitance isless.

Since they need less power, more stagescan be used without increasing the costsignificantly. A frequency up conversion inthe mixer is also possible for producing ahigher IF giving a simpler front end designand better image rejection.

AM receivers are popular right through thefrequency range from if to vhf and the mainapplication is for entertainment broadcast.Since there is a broadcast station in almostevery city, the signal is good and an if stagemay not be required.

Fig. 1 shows a simple am tuner without anif stage and the mixer and local oscillator

Fig 1.Simple AM tuner

PRACTICAL ELECTRONICS DECEMBER 1989 2!

combined in a stage called a frequencyconverter. In addition if a diode detector isused; the result is a cheap, lightweightreceiver which is portable. The IF filters aresingle tuned transformers.

If a 10 µV signal is received at the aerialand a 2 volt signal is required at thedetector, the gain over all those stages is106dB. If the IF stages can provide 90dB ofgain, then the mixer stage need provide only16dB. Two to four transistor stages will dothe job and if ics are used, two are usuallysufficient, one for the rf-IF stages andanother for the audio.

FILTERS

If ics are used then transformer coupling isunnecessarily bulky. This led to research intoceramic and saw (surface acoustic wave)filters. These are small, provide a betterresponse than transformer coupling, andrequire no tuning.

However tuned transformers are still usedas IF filters because they are cheap, providegood impedance matching by the use oftapped windings and high attenuation outsidethe passband. The alternatives arepiezoelectric or magnetostrictive filters.

Mechanical filters are made frompiezoelectric ceramic or magnetostrictiveferrite. These convert electrical signals intomechanical vibrations which are sent througha mechanical resonator of high Q and atransducer at the output. Such filters areuseful at frequencies up to 600kHz withbandwidths of 0.1% to 10% of the centrefrequency.

Ceramic filters are made frompiezoelectric ceramics with centre frequenciesof a ,few kilohertz to 10.7MHz andbandwidths of 0.05% to 20% of the centrefrequency. Fig. 2 shows a ceramic filter, itsequivalent circuit and frequency response.

and so the bandwidth is restricted to a fewtenths of 1% of the centre frequency. Thebandwidth is sufficient for ssb, fm andtelephone (4kHz) circuits in the frequencyrange 5MHz to 350 MHz.

Surface acoustic wave (saw) filters are.another means of cutting down on space andweight and are popular in television IFamplifiers. The insertion loss is 10dB to 30dBwhich is higher than conventional filters butthe compensation is a wider bandwidth andbetter shaping of the band.

Fig. 4 shows the construction of a sawfilter. The spacing of the fingers determinesthe wavelength and the overlap determines thestrength of the signal. The number of sectionsdefines the bandwidth approximately as

Fig 4. SAW filter.

110J GIGS el

FINGEROVERLAP

/SPACINGSECTION SECTION

2

CERAMICDISC

LOU IVALENTCIRCUIT

Fig 2.Ceramic filter.

IMPEDANCE

SERIES PARALLELRESONANCE RESONANCE

FREQUENCY

10.16561

Crystal filters can serve the same purposeas bulky transformers by coupling betweenpairs of electrodes. Fig. 3 shows a monolithic(simple) quartz crystal filter and equivalentcircuit where C is the input and outputcapacitance, C2 the capacitance of the gapand k is the coupling coefficient between theresonant regions where k = bandwidth/resonant frequency.

In the fundamental mode the thickness ofthe wafer is half the wavelength and harmonicnodes occur at odd multiples of halfwavelength. Monolithic fillers have a high Q

follows: resonant frequency = sectionsbandwidth

Therefore delay lines, bandpass filters andpulse compression filters can be created byaltering the length, spacing, and overlap of thefingers. The method of operation is anacoustic wave through the electric fieldbetween the interleaved fingers which isconverted back to an electrical wave by asimilar set of fingers at the output. Filterswith centre frequencies of 10MHz to 800MHzand bandwidths of 0.3% to 20% have beenconstructed.

THE DETECTOR

The most common detector for am receiverapplication is the diode envelope detector,Fig. 5. It is also used for ac -dc conversion involtmeters and as video detectors in tvreceivers since it is simple and the output isfairly linear.

In Fig. 5,. the diode conducts near the peakof each cycle charging capacitor C1 to thepeak value of each cycle. The diode isreverse biased on the downward slope of eachcycle and C1 discharges through R1, Fig. 5b.If Cl is of the wrong value it will be too slowto follow the modulation and clip the waves(Fig. 50) or if it is too fast it will follow the rfvariations more closely, Fig. 5d, producing anexcessive ripple in the output.

Since the detector output has a dccomponent proportional to the modulatingaudio, it can be used for automatic gainFig 5. (a) Diode detector, (b) input andoutput wave forms, (c) clippedwaveform, (d) charge/discharge toofast.

C2

lal

/OUTPUT WAVE

I 1111111INPUT WAVE

(b)

ir\evl.- OUTPUT WAVE

INPUT WAVE

((ADJ616521


COMMUNICATIONS FEATURE.

control (agc). In any case it must not beallowed to reach the audio amplifier or it willcause instability by altering the operatingpoint: Capacitor C2 blocks this dc. Anotherform of detector is the product detector. Thisis not strictly a detector in that it does notdemodulate the signal to its audio componentas in the diode detector. It is more in thenature of a mixer in that it accepts dsb or ssband a local carrier and produces a differencefrequency.

With dsb the phase and frequency of thecarrier are more critical than for ssb and apilot some 20db down on the signal istransmitted to synchronise the voltagecontrolled oscillator (vco) in the receiver'sphase locked loop, Fig. 6.

/, PHASESHIFT

AM IN

PLL

(0 ic,166o]

PRODUCT

DETECTOR our

AUDIO

Fig 6. Product detector with PLL.

In dsb operation if the frequency of thecarrier is correct but the phase is not, then theamplitude of the audio output is reduced. Ifthe frequency is incorrect or drifting then theupper sideband (usb) and lower sideband (lsb)are different.

Fig. 6 shows a 900 phase shift networksince many phase detectors in phase lockedloops (plls) require the vco to be 900 out ofphase with the incoming signal. The pllprovides the required frequency but the audioamplitude can be adjusted for maximumoutput by adjusting the phase network.

Fig 7. Squelch circuit.

RE

0IN

FROMAGC

RF AMP01

FULL ON

SQUELCHCONTROL

OFF

01516611

SQUELCHRANGE

+VE

0-RF IN

IDJGi 66 21

FROMMIXER

DIG16631

RFAMP

IFAMP

FETMIXER

FILTER(BANDPASS)

IFAMP

1-41.IF

AMP

FILTER =Ow, DETECTOR

DETECTOR

Fig 8 (top). FM receiver block diagram.Fig 9. IF stages of FM receiver.

AGC AND SQUELCH

Automatic gain control (agc) is requiredbecause the signal at the aerial of a radio or tvreceiver varies greatly as the receiver is tunedfrom channel to channel. A strong signalwould overload the mixer and causedistortion, therefore the gain of the rf stagemust be reduced by negative feedback.

Agc is also applied to the IF stage in orderthat the input to the detector is constant.However, agc is not applied to the mixer sincegood mixer operation depends on a stablemixer. But agc is used for other purposessuch as squelch circuits, tuning meters, Smeters.

Squelch circuits are required to bias thefirst audio stage to cut off until the receivedsignal is strong enough to switch it on withthe help of agc. This helps removebackground noise which is tiring in channelsshared by multi -users such as citizens band(cb) radio.

Fig. 7 shows a simple squelch circuit. Theoperation of the circuit is a combination of thesetting of the squelch control and the strengthof the agc voltage. With the squelch controloff (base of Q2 earthed), Q3 is switched on,but with the squelch on, Q3 is cut off and theagc needs to go more negative in order toswitch Q2 off and hence allow Q3 to beswitched on again.

FM RECEIVERS

Frequency modulated receivers have thesame front end design as am receivers exceptthat the the bandwidth is wider, up to 200kHzfor broadcast. They differ from am receivers

AMPSQUELCH AF AMP

in that they require frequency detectors,amplitude limiters and a larger IF bandwidth.

Although fm receivers are not affected byamplitude induced noise, they are affected byphase distortion. For instance if the phase ofthe sideband relative to the carrier is notaccurate, the modulating signal cannot berecovered properly.

Fig. 8 shows a block diagram of an fmreceiver. Most commercial ic packagesprovide a gain of about 40db and if say 15dbis allocated to the rf amplifier and mixer, then25db is supplied by the IF stage. The icpackage incorporating the IF also incorporatesthe fm detector, and the manufacturer'sspecification shows the input voltages for achoice of s/n ratio, limiting, audio output, etc.

In order to amplify fm signals withoutdistortion, the transfer function of the IF stagemust be linear, ie, the filters and amplifiersmust have a characteristic whose phasedecreases linearly with frequency. In practiceno device is ideal and the next best solution isto use components such that their non-linearities cancel each other.

Fig. 9 shows a block diagram of the IFstages of an fm receiver. Phase distortionwould be of less interest to a designer of lowquality communications receivers than to adesigner of a high fidelity entertainmentbroadcast receiver.

FM DETECTORS

The basic aim of an fm detector is toproduce an output voltage when the inputvoltage varies in frequency from a reference.The ideal fm detector has a transfer functionas shown in Fig. 10. For input frequencieswith equal spacing above and below thereference the output voltage has the same

Fig 10. Transfer function of ideal FMdetector.

OUTPUTVOLTS

INPUTFREQUENCY

DESIGNCENTREFREQUENCY

10J01664.1


OUTPUTSNR

THRESHOLD

INPUT SNR

ICJG 6651

Fig 11. Input/output SNR of FMdetector.

amplitude but opposite polarity. Practicaldetectors are not entirely linear and havevoltage and frequency limits.

There is a threshold signal to noise ratio(snr) for each type of detector and modulationindex. If the input snr is better than thisthreshold then the output snr is even better,Fig. 11. This improvement results becauseone is trading bandwidth for snr ie, thebandwidth of the audio at the detector outputis less than the IF at the detector input.

Another way of looking at it is that thesignal power in the whole bandwidthcontributes towards the output signal power,whereas the noise contribution is from onlythe limits of the modulating frequency.Above the threshold snr if the modulatingindex is increased while keeping the highestmodulating frequency fixed, the output signalpower is increased but the output noise powerdoes not increase.

Raising the modulation index means thedetector threshold must be raised or the inputsnr will fall below this threshold. Below thethreshold, the output snr gets better is the IFbandwidth or modulation index is decreased.At present a phase locked loop (p11) is the bestavailable fm detector although low thresholddetectors are being developed.

The improvement in snr above thethreshold results in a bonus called 'capture inwhich the strongest signal is heard and anyannoying interference disappears. This is asharp contrast from am and makes fm theobvious choice for mobile radio where thereis not only electrical interference but the radiospectrum is crowded.

The capture ratio is the ratio in db ofdesired to undesired signal to provide a 30dbsuppression of the undesired signal. Theprecise definition is a complex measurementof suppression of one signal generator byanother at a number of standard inputs. Highfidelity domestic receivers have capture ratiosbetween ldb and 2db.

When a strong signal is received, the noisedisappears and this is called quieting which isa direct consequence of capture. The quietingsensitivity of a reciever is the unmodulatedcarfier required to provide a specific reductionin output noise over the noise present in theabsence of a signal. A domestic receiverwould produce 30db of quieting for a 1.7ttVsignal whereas a narrow band fm receiverwould produce 20db for a 0.5ttV signal.

In the days of the valve, the device wasbulky and expensive and because it ran so hot,

Fig 12 (above).Foster Seeleydiscriminator.Fig 13 (right).Ratio detector.Fig 14 (top right).Coincidencedetector.

1.0

IFIN

101616571

SQUAREWAVE

SINEWAVE

ANDGATE

1010,665j

INTEGRATOR

AUDIOOUT

its life was limited. With the invention of thetransistor, the Foster -Seeley detector (Fig. 12)was replaced by the ratio detector, Fig. 13.The ratio detector is less sensitive toamplitude variations of the incoming signal.Am rejection is defined as the ratio in decibelsof fm detector audio power output for aspecific fm input, to the audio power outputfor a specific am input.

In a coincidence detector, the IF is turnedinto a square wave by hard limiting to asquare wave. The square wave is split and fedto an AND gate Fig. 14. One half is feddirectly to the AND gate and the other halfthrough a phase shift network. The phaseshift network includes a resonant circuit ofhigh Q and the input to the AND gate is a sinewave whose frequency is the same as the IFmid -frequency.

The AND gate produces an output onlywhen both inputs are positive going. If eitherinput is negative going there is no output.The length of the output pulse will depend onthe degree of overlap of the positive goingparts of the waveforms, ie, the phasedifference, Fig. 15.

When the waveforms are exactly in phasethe length of the pulse is half the IF periodand when they are in antiphase by 180degrees, there is no output. In between thesetwo extremes, the pulse varies in length.

The output of the AND gate is fed to anRC integrator whose output will beproportional to the average value of the inputpulses and the output of the integrator istherefore a reproduction of the modulatingwaveform.

After hard limiting the IF, one of thedifficulties of fm detection is to decide theinstantaneous frequency of the square wave.One method is to use a zero crossing detectorand measure the intervals at which the carriercrossed the zero line. Doubling this gives theperiod T of the wave and the reciprocal VTgives the frequency.

Fig 15 (right). Coincidence detectorwaveforms.Fig 16 (below). Zero crossing detector.

IF

INTRIGGER

I 0.1Gib701

MONOST AB LE

MULT [VIBRATOR

SQUARE

SINE

I-1 OUTPUT

45° PHASE DIFFERENCE

SQUARE

SINE

n n OUTPUT

T35° PHASE DIFFERENCEIDJ015691

AUDIODIFFERENTIALINTEGRATOR OPERATIONAL-1111.

OUTAM PL In ER


COMMUNICATIONS FEATURE

This method is expensive and the output isbetter than normally required forcommunications. A simpler method is to usea monostable. The Q and Q outputs, Fig. 16,are applied to RC integrators and then to adifferential operational amplifier.

The output of the operational amplifier isthe difference between the average values ofQ and Q multiplied by a constant. If the inputsignal frequency drops, the average voltage atQ increases and that at Q decreases makingthe output of the operational amplifiernegative. Similarly the output of theoperational amplifier will become positive ifthe input signal increases in frequency. Theresult is a linear relationship between the IFdeviation and the amplitude output of theoperational amplifier.

STEREO RECEPTION

A stereo reception relies on the broadcastof a right (R) and left (L) channel. Whenstereo records were produced in the 1950s, anattempt was made to also broadcast stereomusic. The early attempt was by transmittingone channel on am and the other on fm bythose stations which held both licences.

This was not satisfactory and the presentmethod of transmitting both channels on thesame fm carrier replaced it. Instead oftransmitting the left and right channels, thesum (L + R) and difference (L - R) aretransmitted. This is because mono receiversmust be able to receive stereo broadcastswithout the necessity for modification.Therefore a mono receiver detects the (L + R)signal which is also what a mono transmitterwould send out.

The modulating wave of an fm stereosignal is shown in Fig. 17 where thedifference signal (L - R) modulated a 38kHzsubcarrier to produce a DSB/SC. The sum (L+ R) is transmitted in phase with thesubcarrier so that the subcarrier oscillator atthe receiver can be synchronised with thesubcarrier oscillator in the transmitter.

AMPLITUDE

(L+R) PILOT

0.03 15

101016711

(L-R)

nn23 38 53 IcHz

FREQUENCY

Fig 17. Modulating wave of FM stereo.

A pilot is transmitted instead of thesubcarrier because the pilot fits in a slot farremoved from the channels whereas thesubcarrier is only 30Hz away from thesidebands and therefore requires expensivefilters.

An fm stereo receiver adds and subtractsthe (L + R) and (L - R) signals in order toobtain the original L and R channels. A monoreceiver on the other hand does not expect tosee anything above 15kHz therefore the (L +R) signal is accepted and everything elsediscarded by filtering.

VE

VL OUT VR OUT

COMPOSITESIGNAL

v,

PILOT196 Hz

19kHzREFERENCE

PHASEDETECTOR

TO STEREO

DECODER38kHz

FILTER

101616731

VCO

175 Hz)

COMPOSITESIGNAL

10/616721

AUDIO

IN

dB

0 75 H

10k

AUDIO

1p' OUT

01016741

LOG

2122 Hz t FREQUENCY

7k5FROM

DETECTOR

dB

2122Hz

0.01p

101916751

TO

- AUDIOAMP

LOG

FREQUENCY

Fig 18 (top left). FM stereo decoder. Fig 19 (bottom left). Generation ofsubcarrier. Fig 20. (top right). Pre -emphasis. Fig 21 (bottom right). De -emphasis.

A popular form of fm stereo decodingcircuit is shown in Fig. 18. The 38kHzsubcarrier is usually generated on the samechip by means of a pll, Fig. 19. The 19kHzpilot is applied to a phase detector whoseoutput controls a 76kHz vco which is an RCrelaxation oscillator.

A divide by two circuits provides the38kHz subcarrier and a further divide by twoprovides a 19kHz for comparison into thephase detector.

During the propagation the higherfrequencies get attenuated more than the

extra amplification (pre -emphasis). At thereceiver the higher frequencies, above2122Hz, are de-emphasised.

Pre -emphasis is carried out by a

differentiator, as in Fig. 20, consistingessentially of an RL network of time constant75 ps, and de -emphasis by an integrator (Fig.21) consisting of an RC network of the sametime constant.

Next month we conclude the series bylower frequencies and therefore they need discussing transmitters and converters. 111

A practical method of improving radio reception amplification using avacuum tube.


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Tin part one, Oct 89, I

described the delaycircuit options and

modulation control. On nowto the signal routing options.

INPUT AND OUTPUT

Since, in nature, echo and reverb signalsare not necessarily direction conscious,especially in enclosed spaces, only onedelay circuit is used, even though the unit isintended for stereo operation. There is, ofcourse, no reason, other than financial, whyseparate delay stages should not beprovided for each of the stereo channels.But, being cost conscious, I have used justthe one delay path, routing both channelsthrough it, and feeding the processed signalequally to both stereo output channels.Fig.9 shows the input and output signalcircuit.

BAND PASS

Both input signals are also taken to themixer -filter stage around IC I a. Here thecombined signal level amplitude can beincreased by VR1, from xl one up to aboutx10. Some of the upper frequencies areremoved by the action of C3 and C4, toreduce distortion through the delay stage.The band pass range for IC I a is about100Hz to 5kHz.

Bass frequencies are those most prone tocause feedback howl in reverb modes andso I've included a bass -cut control, VR2.The cut can be set from nil up to around1kHz. From the junction of C6 and R7, thecomposite signal is taken into C22 of thedelay stage.

Following the delay stage, the signal isbrought into IC 1 b. Here, the steps inducedby the clocked sampling referred to earlierare partially filtered out. C8 and C9 set the

upper frequency filtering limit. The secondfilter around IC2 further smooths out thesampling steps, with C7 and C12 setting thefrequency cut off. The upper limit is set ataround 3.5kHz which allows adequatefiltering out of clocking frequencies as lowas 10kHz. Since the dominant frequenciesin music lie at less than about 3.5kHz, it isbasically the upper harmonics which arelost in the filtering, not the main signalcontent.

SPLIT AND MIX

The delayed and filtered signal is takenvia the level control VR4 and split equallybetween 'Cie and IC1d, where it is mixedwith the original stereo signals. Theprocessed signal can also be fed back intothe delay chain at a level set by VR3.

Don't forget that when using more thanone output from the MN3011, the signal

MONO -STEREOECHO STATION

The stereo signal is input to the identicalmixer stages ICIc and IC1d. The twochannels retain their separate identities andamplitudes at the output of these ics,although there is an inversion of phase. Thedirect through frequency range is around20Hz to better than 30kHz.

Part two - in whichJohn Becker finds aconstructive route

through the operationaldelays.

levels will be added together, which couldresult in distortion if the levels are too high.

For effects where the processed signal isrequired on its own without being mixedwith the original, the output may be takendirectly from the wiper of VR4 to aseparate output socket. (I have not included

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Fig.9. Input - outputinterface circuit.

the option on my own model and have notshown it on the wiring diagram - but I'msure you know how to wire up a jacksocket, don't you?) As an alternative, youcould cut the pcb tracks at the junctions ofC15/RI 8 and C18/R22, and insert a dpstswitch so that the original signal can beswitched out from the mixers ICIc andIC1d.


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Fig.10. Component layout for the interface pcb.

Fig.11. Track layoutsfor the interface anddelay pcbs.

SUPPLY VOLTAGES

The circuit has been designed to workwith a single 9V battery as the powersupply, though voltages up to around 15Vdc are acceptable. At 9V the currentconsumption is around 11mA.

The maximum signal input strength tothe delay chips is 1V rms for the MN3011,and 1.8V rms for the MN3004. Input levelsabove these figures will cause distortion,but will not damage the chips, providing thelevels don't exceed the psu voltage.

The unit is designed for use withpreamplified signals, and immediately priorto a power amplifier.

RATES ANDMEASURES

Now for a list of the various clock ratesand delay times to be expected. Individualunits may vary slightly in theirmeasurements due to normal componenttolerance factors, but not too significantly.

Signal delays with the clock rangecontrolled by VR9 set for 10kHz to 100kHzare:

MN3004MN3011 output 1MN3011 output 2MN3011 output 3MN3011 output 4MN3011 output 5MN3011 output 6

2.56ms to 25.6ms1.98ms to 19.8ms3.31ms to 33.1ms5.97ms to 59.7ms8.63ms to 86.3ms13.95ms to 139.5ms16.64ms to 166.4ms

Delay times may be extended beyondthese figures by decreasing the clockfrequency below 10kHz, though with aconsequent reduction in signal quality andincreased possibility of clock signalbreakthrough. The latter may becompensated for by equivalent increasingof relevant capacitor values in filters ICIband IC2.

Reverb lengths are controllable from nilto infinite (total feedback) irrespective ofdelay clock frequency.

Modulation rate : 8Hz to 13 secs. Afaster rate range can be achieved bydecreasing C27.

Modulation control example : VR9 setfor basic clocking frequency of 16kHz,VR7 set midway, VR8 set to maximumoutput - clock sweep range becomes 10kHzto 23kHz. The sweep range can be extendedby reducing the resistance of VR7. Notethat the basic clock frequency should be setso that the sweep control does not drop theclocking rate into the audible range.

SETTING UP

Turn VR4 to minimum level, apply asignal to either or both of the inputs, plugthe outputs into your amplifier and checkthat the original signal passes through.


AUDIO PROJECT

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IF USING 105 INSTEAD OF 1C3 DELETE S2 -57 AND LINK 30 TO 16

5

LINK 8 TO 21LINK 10 TO 22LINK 12 TO 20

Fig.12. Control wiring diagram.

Set VR1 and VR3 to minimum, switchoff S8 and turn up VR4 until a noticeablydifferent signal quality is experienced. It isbest if the signal has a significant beat to itso that echo repeats will be clearly heard.Play around with the delay switchingoptions and the setting of the delay rate potVR9, determining that the delay stage isfunctioning. Then check that feedbackoccurs when the reverb control VR3 isused. Now switch on S8 and test out the

modulation rate and control functions ofVR6 and VR8.

Any malfunction is likely to be due toincorrect assembly or wiring, in which casethoroughly recheck your work.

Finally, set the preset pots for optimumconditions. If you have a scope, the settingof VR5 should be made to give symmetryto the delayed signal at VR4. For this, thesignal strength should be close tomaximum. Without a scope, adjust VR5

until minimum distortion is heard withstrong signals. If you can hear nodifference, set it about midway.

Adjust VR7 for the smoothestmodulation when VR8 is at maximumlevel. VR 10 should be adjusted until theclocking oscillator is not heard when VR9is at minimum level (lowest clock speed).

So, there you are, a simple multipurposedelay unit using bbd chips other than thesadly missed TDA 1022 and TDA1097.

puBus" r4-7,1°

FEATURES

Psi

x--

* LEARN

PROBY BUILDING

JECT* ENJOY

KITSBY USING *

I10

isr 4 44/AvPE ECHO STATION* BE CREATIVE * GET KITTED! * HAPPY 25TH BIRTHDAY

TO PRACTICAL ELECTRONICS! DETAILS IN CATALOGUEPHONOSONICS TOO ARE

0 0 0 VETERANS - WE'VE SUPPORTEDPE FOR 17 YEARS! I al -4. 90'. .

PE

b <,e 90 * 40 omse,,,

404 4 41010Q4 *COMPUTER KITS

The software listings published with the computer kit

1PE DUAL BEAM OSCILLOSCOPEDetails in catalogue.

projects are for use with C64, PET and BBC corrputers.CHIP TESTER SET258F £41.50Computer controlled logic and chip analyser.

EPROM PROGRAMMER SET277 £26.20

PE FREQUENCY COUNTERAND GENERATOR

DETAILS IN CATALOGUEComputer controlled unit for 4K Eproms.

MORE KITS IN CATALOGUESEND MEDIUM S.A.E. FOR CATALOGUE AND

WITH ALL ENQUIRIES(OVERSEAS SEND 01.00 TO COVER POSTAGE)

MICRO -CHAT SET276 £69.50Computer controlled speech synthesiser.

MICRO -SCOPE SET247 £49.50ENVIRONMENT

ELECTRONIC BAROMETERTurns a computer into an oscilloscope. SET285 £41.20

BURGLAR ALARMCONTROLLERS

MICRO -TUNER SET257Computer controlled, tuning aid and freq counter,

£57.40

MORSE DECODER SET269 £26.70Computer controlled morse code -decoder.

Computer controlled unit for monitoring atmosphericpressure.

GEIGER COUNTER SET264 £65.50A nuclear radiation detector for environmental and

MULTIZONE CONTROL geological monitoring. With built in speaker, meter and

SET280 £23.90 VARIOUSdigital output. This project was demonstrated on BBC TV.

Two entry -zones, anti -tamper loop, personal attack, entry -exit timing, timed duration, automatic resetting, latchingLED monitors.

SINGLE ZONE CONTROLSET279 £10.50With timed duration control and latching LED monitor.Both units can be used with any standard detectiondevices, such as contact or magnetic switches, pressurepads, tremblers, ultrasonics, infrared etc., and willactivate standard bells, strobes or sirens.

VOICE SCRAMBLER SET287 £49.5032 switchable channels to keep your communicationsconfidential.

STORMS! £35.50 each unitRaw nature under panel control! Wind & Rain SET250W.Thunder & Lightning SET2507.

DISCO -LIGHTS SET245F £69.503 than sound to light, chasers, auto level.EVENT COUNTER SET278 £36.604 -digit display counting for any logic source.

ORDERINGAdd 15% VAT. Add P&P - Sets over £50 add £3.00.Others add £2.00. Overseas P&P in catalogue. Textphotocopies - Oscilloscope £3.00, Geiger £3.00,Weather £2.00, others £1.00, plus 50p post or largeSAE. Insurance 50p per £50. MAIL ORDER. CWO, CHO,PO, ACCESS VISA. Telephone orders: Mon -Fri, 9am -6pm. 0689 37821. (Usually answering machine).

MORE KITS IN CATALOGUE

PHONOSONICS, DEPT PE99, 8 FINUCANE DRIVE, ORPINGTON, KENT, BR5 4ED. MAIL ORDER


nn the words of the old song, What anight it was, it really was such a night!Such a night was September 27th 1989

when. PE and friends gathered together tocelebrate PE's twenty-five years of history.The venue was the Kew Bridge SteamMuseum, London.

PREPARATORIES

The preparations for the celebrations hadbeen on our minds for months, the partydominating the thoughts and actions ofeveryone in the office, especially Mary -AnnHubers, Louise Hewett and Publisher AngeloZgorelec. Questions like: Who should come?What shall we eat and drink? Whatdecorations do we have? What gift shall wepresent to Sir Clive Sinclair as his award? Allhad to be thoroughly discussed and solutionsconcluded.

The Birthday issue itself, its contents andappearance, had occupied me far longer thanany other issue, until a week before the partywhen the pages at last were sent to theprinters. Somewhat anticlimactically, I nowhad to prepare the December issue, trying to

Some of PE's well-wishers enjoy nostalgic reunions and spirited chats at theKew Bridge Steam Museum. The rest of the crowd are off-stage, touring theancient pumping machinery.

CELEBRATING A PARTYTO REMEMBER!

ignore the November one which was out ofmy hands until we received the printedcopies. These arrived at the office with onlyhours to spare before the party, at which wewere to present them to everyone there!

The party preparations were going on rightup to the last minute before our guestsarrived. Balloons were being blown, bannersgoing up, food laid out, microphones placed,

The glittering cut -glass PE TechnologyAward

Presemcd4c,

SIRct111,

.'n recogkiA,),.

b,r,

Ignoring hisBacchanalian

hangover, JohnBecker recalls thefestivities of PE's25th Anniversary

Party!

and the steam engines brought up to steam - itwasn't just a party to celebrate the hi -tech, buta grand tour of Ancient Technology as well.

GREETING GUESTS

Angelo and myself greeted our guests asthey arrived at the main entrance, invitingthem to sign our commemorative visitorsbook, and directing them through the maze ofcorridors between the historical machinery,and along to the main hall. It was a realpleasure to greet so many people whom I hadcome to know over the years, and to welcomethose I only knew by reputation but had nevermet. When Sir Clive arrived, he too wasobviously pleased to see people he had knownsince his early days in electronics.

In fact, it was obvious that many peopleknew each other, and the hubbub of

reminiscing chatter and the gentle murmur ofworking steam engines was an exhilaratingand heart-warming sound.

HEART -FELTWELCOMES

We'd agreed, Angelo and I, that we wouldkeep speeches to a mere minimum - wewanted people to enjoy themselves in trueparty fashion, not to have their ears bent fortoo long by formal speeches. So, it was almostwith reluctance that I intruded upon theconversations by asking for people's attentionto a few formalities!

Angelo then warmly welcomed everyone,and said a special word of thanks to Sir Clivefor joining us all, and how pleased we werethat the readers had shown such discernment innominating him for the PE Technology Award.

On accepting the mic back from Angelo, Ioffered my own greetings to all present, mostof whom were part of PE's story, going rightback to the early days. There were PE staffmembers, advertisers, authors, printers,distributors, manufacturers, retailers andreaders. It was certainly a proud day for uswho now work on PE to have them all join inour celebrations. We were especially pleasedto have with us PE's founding editor, FredBennett, and his 'successor Mike Kenward.Sadly, one expected special guest, Dr PatrickMoore, was unable to join us as he'd beendelayed in South Africa.


Sir Clive Sinclair, awarded inrecognition of his technologyachievements over the last twenty-fiveyears.

AWARDING SIR CLIVE

We were proud too that our readers hadoverwhelmingly voted Sir Clive as thepersonality who has done most to promotetechnology over the last twenty-five years.Loud applause greeted him as I invited him tocome forward to receive the award, anabsolutely superb glittering glass pyramid,standing a good ten inches high. It wasinscribed with the words, "The Practical

CELEBRATORY FEATURE

Electronics 25th Anniversary Award 1989Presented to Sir Clive Sinclair in Recognitionof his Achievements in Technology."

Beaming broadly, Angelo presented theaward to Sir Clive, who thanked us allsmilingly, accompanied by the brilliance ofnumerous camera flash guns, followed bymore loud applause.

TECHN-ODE-OLOGY

Then I broke my oath of brevity! I

couldn't let an occasion like a PE SilverAnniversary Party pass without acommemorative Odd -Ode, which I'd justhappened to scribble down on the back of anold pcb while waiting in a traffic jam on theway to the party!

In days of oldYoung Fred, so bold,This PE mag invented -

He'd seen the signsThat hi -tech timesWere for the fans intended,And his battle cryWas heard on high:"This market's undefended!"

So from Sixty -FourWith PE's galoreThe hobbyists he contented,Bringing folks to gripsWith boards and chips,And features techno-splendid!

Then Mike from FredBecame the Ed,And though now relocated,Through the Micro yearsHe brought great cheers,And PE much commended!

Devotees to technology: PE founding editor Fred Bennett, his successor MikeKenward, Sir Clive Sinclair, PE publisher Angelo Zgorelec, current PE editorJohn Becker.

But recession's holdBrought plights untoldAnd fortunes were upended,Till from the wings,"Allelujah!" rings -Guardian Angelo has descended!

Now we're legend boundAnd techno-crownedAnd our pages are well tended,Thus to such acclaimWe admit our fame,With glowing pride subtended.

So raise your beersTo hi -tech yearsAnd make us all beholdenTo turn PEFrom Silvered gleeInto the bloomin' Golden!

GRAND TOUR

After which, a libation to PE andTechnology; then on to the Grand Tour!

You probably don't know, but DerekGooding, as well as being our illustrioustechnical illustrator, also helps to run the SteamMuseum. It was only natural then, that he shouldguide us round the highly impressive pumpingmachinery, gleaming with the typical well-oiledbrightness of well -cared -for precisionengineering. Derek's knowledgeable and wittycommentary had us enthralled while the massivemachinery sleekly drove the beams and pistonsthrough their motions.

PE's technical illustrator DerekGooding describes the fascination ofthe Steam Museum machinery, flankedby Helen and Andrew Armstrong, PEsub editor and author respectively.

If you're ever near the Kew Bridge SteamMuseum, do call in for a look round, it's reallyinteresting. You'll find it in Green Dragon Lane,Brentford, Middx, phone 01-568 4757. It's opendaily, and the engines are 'In -steam' atweekends.

EAT, DRINK AND BEMERRY!

Many of us then stayed on for more chats,while attempting to consume the remainingconsumables, animal, vegetable, but not mineral- champagne and other similar beverages are farmore appropriate than mineral waters!

Then off home, exhilarated!


BMIMEHU MEM HOCOPROJECT KITSMagenta supply Full Kits: Including PCB's (or Stripboard), Hardware, Components, and Cases(unless stated). Please state Kit Reference Number, Kit Title and Price, when ordering.REPRINTS: If you do not have the issue of P.E. which includes the project, you will need toorder the instruction reprint as an extra: 80p each. Reprints are also available separately -Send£1 in stamps. Magazine dates refer to Everyday Electronics.

816 STEPPING MOTOR DRIVER £7.94As featured in Aug P.E. kit onlySuperb controller with half-step ability

581

584578569563

VIDEO GUARD Feb.87SPECTRUM SPEECH SYNTH. (no case) Feb. 87SPECTRUM I/O PORT less case Feb. 87CAR ALARM Dec. 86200MHz DIG. FREQUENCY METER Nov. 86

£8.94£22.28£10.05£13.28f67.98

Offer Price with MD35 Motor £14.99 561 LIGHT RIDER LAPEL BADGE Oct. 86 £10.86560 LIGHT RIDER DISCO VERSION £20.89

812 ULTRASONIC PET SCARER May 89 £13.80 559 LIGHT RIDER 16 LED VERSION £14.52

807 MINI PSU Feb 89 £22.71 556 INFRA RED BEAM ALARM Sept 86 £30.19

806 CONTINUITY TESTER f 10.28 544 TILT ALARM July 86 £8.33

1303 REACTION TIMER Dec. 88 £31.93 542 PERSONAL RADIO June 86 £12.28

801 DOWNBEAT METRONOME Dec. 88 £18.71 528 PA AMPLIFIER May 86 £28.70

800 SPECTRUM EPROM PROGRAMMER Dec. 88 f28.72 523 STEREO REVERB Apr. 86 £28.16

796 SEASHELL SYNTHESISER Nov. 88 f26.61 513 BBC MIDI INTERFACE Mar. 86 £29.76

795 I.R. OBJECT COUNTER Nov. 88 £31.56 512 MAINS TESTER & FUSE FINDER Mar. 86 £9.39

790 EPROM ERASER Oct. 88 £26.57 497 MUSICAL DOORBELL Jan. 86 09.95786 UNIVERSAL NICAD CHARGER July 88 £7.44 493 DIGITAL CAPACITANCE METER Dec. 85 £44.25

780 CABLE & PIPE LOCATOR April 88 (1 6.35 481 SOLDERING IRON CONTROLLER Oct. 85 £5.83

775 ENVELOPE SHAPER Mar 88 £15.96 464 STEPPER MOTOR INTERFACE FOR THE BBC

769 VARIABLE 25V.2A BENCH POWER SUPPLY Feb 88 £52.96 COMPUTER less case Aug. 85 E8.95

763 AUDIO SIGNAL GENERATOR Dec. 87 £14.53 1D35 STEPPER MOTOR EXTRA (16.50

739 ACCENTED BEAT METRONOME Nov 87 £22.31 OPTIONAL POWER SUPPLY PARTS £5.47

740 ACCOUSTIC PROBE Nov 87 (less bolt & probe) £18.46 461 CONTINUITY TESTER July 85 E5.60

744 VIDEO CONTROLLER Oct. 87 £31.03 455 ELECTRONIC DOORBELL June 85 HBO

745 TRANSTEST Oct 87 £10.33 453 GRAPHIC EQUALISER June 85 £28.69

734 AUTOMATIC PORCH LIGHT Oct 87 E1829 444 INSULATOR TESTER Apr. 85 £20.85

736 STATIC MONITOR Oct 87 £9.22 430 SPECTRUM AMPLIFIER Jan 85 E7.36

723 ELECTRONIC MULTIMETER Sept 87 £50.01 392 BBC MICRO AUDIO STORAGE SCOPE

728 PERSONAL STEREO AMP Sept 87 £1524 INTERFACE Nov 84 £38.61

730 BURST -FIRE MAINS CONTROLLER Sept 87 £14.45 387 MAINS CABLE DETECTOR Oct. 84 £5.89

724 SUPER SOUND ADAPTOR Aug 87 £40.89 386 DRILL SPEED CONTROLLER Oct 84 E9.24

718 3 BAND 1.6-300MHz RADIO Aug. 87 £2815 362 VARICAP AM RADIO May 84 £14.00

719 BUCCANEER I.B. METAL DETECTOR inc. coils and 337 BIOLOGICAL AMPLIFIER Jan 84 £25.71

case, less handle and hardware July 87 £28.17 263 BUZZ OFF Mar. 83 £6.05

720 DIGITAL COUNTER/FREQ METER 110MHz) 242 2 -WAY INTERCOM no case July 82 £6.06

inc. case July 87 £71.43 240 EGG TIMER June 82 £7.31

722 FERMOSTAT July 87 £12.93 205 SUSTAIN UNIT Oct 81 £18.78

711 VISUAL GUITAR TUNER Jun 87 £24.48 108 IN SITU TRANSISTOR TESTER June 78 £10.03

715 MINI DISCO LIGHT Jun 87 £13.41 106 WEIRD SOUND EFFECTS GEN. Mar. 78 £8.33

707 ACTIVE I/11 BURGLAR ALARM Mar. 87 £37.97 101 ELECTONIC DICE Mar. 77 £6.57

EE TREASUREHUNTER

£39.95OUR KITREF 815Suitablehead phone£1.99

P.I. METAL DETECTORA sensitive pulse induction MetalDetector. Picks up coins and ringsetc. up to 20cms deep. Low"ground effect". Can be used withsearch -head underwater. Easy touse and build, kit includes search -head, handle, case, P.C.B. and allparts as shown.

DIGITAL FI-ItLIUENCY200 MHz METERKIT REF 563An R digit meterreading front 4.Fup rct 200 MHz intwo ranges. Large

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Crystal controlled, witha super rotating LEDdisplay. indicates high,low, and exact degreeof mistuning Ilse with,pick-up or roic Also has'audible output.

KIT REF E711 £24.98

3 BAND SHORTWAVERADIO

IOUR KITREF 718 £28.25

Covers 1.6-30 MHz in 3 bands usingmodern miniature coils. Audiooutput is via a built in loudspeaker.Advanced design gives excellentstability, sensitivity and selectivity.Simple to build.

TOP KITS FMOSFET VARIABLE BENCH25V 2.5A POWER SUPPLY

A superb design giving 0-25V and 0-2.5A. Twin panelmeters indicate Voltage and Current. Voltage is vari-able from zero to 25V.Current -Limit control allows Constant Current charg-ing of NICAD batteries, and protects circuits from over-load. A Toroidal transformer MOSFET power outputdevice, and Quad op -amp IC design give excellent per-formance.

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SMALL (WIGS) £3.99 LARGE (MGL) £4.55

ADVENTURESWITH ELECTRONICS

BOOK £4.75 PACK £22.35An easy to follow book suitable for allages. No soldering, uses an S DECbreadboard. Lots of clear diagramsand instructions to build 16 projects.Component pack includes S DEC andall components for the projects.

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STEPPING MOTORS 12VOLT

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INSULATION TESTERAn electronic High Voltage tester for mainsappliances and wiring. An inverter circuitproduces 500 volts from a PP3 battery andapplies it to the circuit under test. Readsinsulation up to 100 Megohms. Completelysafe in use.

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Full 8MHz 68000 Processor with 16 bit data bus. Top -class monitor program Included on two Eproms. Communicates via RS232 interface to terminal, BBC computer,

P.C. computer etc. Use as a Trainer, or as a stand-alone computer system. Fast, powerful processing - Ideal for real time control systems.

MAGENTA ELECTRONICS LTDPE16 HUNTER STREET,BURTON -ON -TRENT,STAFFS. DE14 2ST.0283 65435, Mon -Fri 9-5 Sat 9-2Access/Barclaycard (Visa) byphone or post.24 hr Answerphone for creditcard orders.Our prices include VAT

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DIGITAL CAPACITANCE METERSimple and accurate (1%) measurementsof capacitors from a few pF up to 1,000uF. Clear 5 digit LED display indicatesexact value Three ranges pF, nF.and uF. Just connect thecapacitor, press thebutton and read thevalue.

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PLEASE ASK FOR DETAILS

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OTHER OPTIONS, DATA PACKS,AND LITERATURE AVAILABLE- SEND FOR DETAILS.

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Add-on options Include: G64 Bus, 68230 P.I.T. 8k and 32K Ramupgrades, and line by line assesmbler.

Available from stock now - with support literature and fulltechnical back up.

STANDARD SYSTEM IN KIT FORM (Ref 600) £110.00 68230 P.I.T. ADD ON OPTION (Ref 603) £11.97

STANDARD SYSTEM BUILT & TESTED £190.00 G64 BUS ADD-ON (Ret 604) £ 5.49LINE BY LINE ASSEMBLER PROGRAM (IN EPROM) £ 27.G0 8K x 16 RAM UPGRADE (Ref 605) £24.00POWER SUPPLY (Ref 609) £ 13.99 32K 16 RAM UPGRADE Ref 606 £52.00


4

VOICE RECORD/PLAYBACK KITRECORD LED

MICROPHONE

AUDIO AMP

HIGH DUALITY PCB

This simple to construct and even simpler to operate kit will record and playbackshort messages or tunes. It has many uses - seatbelt or lights reminder in thecar, welcome messages to visitors at home or at work, warning messages infactories and public places, in fact anywhere where a spoken message isannounced and which needs to be changed from time to time. Also suitable fortoys - why not convert your daughters £8 doll to an £80 talking doll!!Size 78x60x15mmMessage time 1 -5 secs normal speed, 2 -10 secs slow speed

PROCESSOR

121K MEMORY

SPEAKER

XK129 £22.50

DISCO LIGHTING KITS

DL8000K 8 -way sequencer kit with built-in opto-isolated sound to light input. Onlyrequires a box and control knob to com-plete £34.60DL1000K 4 -way chaser features bi-directional sequence and dimming 1kWper channel £21.00DLZ1000K Uni-directional version of theabove. Zero switching to reduce in-

terference £11.80DLA/1 (for DL & DLZ1000K) Optional op-ts input allowing audio 'bear/lightresponse 80p013000K 3 -channel sound to light kit.zero voltage switching, automatic levelcontrol and built-in mic. 1kW per

channel £17.00

POWER STROBE KITProduces an intenselight pulse at a

variable frequency ofI lo.15Hz. Includes ',4t.,high quality PCB.components. connec-tors, 5Ws strobe tube and assembly in-structions. Supply: 240V ac. Size:

80)150 x45.06124 STROBOSCOPE KIT £15.00

VERSATILE REMOTECONTROL KIT

QI

Includes all components ( + trans-former) for a sensitive IR receiver with 16logic outputs 10 -15V) which with

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TEST GEAR PROJECT

T he true rms voltmeter of this designuses exactly the same electroniccircuit as the ammeter already

described in PE June 89. The only differenceis that the heater coil on diode sensor D1,which is a low -resistance heater consisting of afew turns of thick Nichrome wire in theammeter version, is now a high -resistanceheater consisting of a large number of turns offine, insulated Nichrome wire. There are alsosome physical differences which will bedescribed later.

HEATER COIL

The wire used for the voltmeter heater coilis 47 or 48 swg enamelled Nichrome (availablefrom Magenta Electronics) which is doubledback on itself and then twisted together beforebeing wound onto the coil former. Thisreduces any inductive effects which wouldmake the effective input resistance dependenton frequency, and thus affect the reading. Fig.1 shows how to wind the twist into the wire.

Do not worry about handling such finewire; Nichrome is much tougher than copperwire of the same diameter. It is howeverdifficult to see the wire in poor light, so thisjob is best done in strong light. I did it out-of-doors on a sunny day and had no problem.

Fig 2. Diode and card cheeks before and after assembly.

with Araldite. Fig. 3 shows how. The cheeksare made oversize to begin with, and are cutdown to size after coil -wind is complete.Make sure there is a generous fillet of Aralditeas shown, since this ensures good electricalisolation between input and the instrument.

The best way to wind the coil is by hand -there are not so many turns that a mechancalmethod is needed. There is however a knackin starting the turns, because Nichrome wire isslightly springy. Use the cotton thread at the

Fig 3. How the coil former should beglued for maximum insulation.

THERMAL -CONVERSIONVOLTMETER

First, wind off a length of wire which givesa resistance of just over 5k. With 47 swg thislength is about 10 metres and with 48 swgabout 7 metres, but these figures can vary dueto slight differences in the alloy andmanufacturing tolerances when drawing thewire. Err on the generous side.

Now, find the mid -point and attach a pieceof cotton thread. Combine the two loose endsand tie on a thin rubber band. Fix the rubberband to something solid at waist height (a doorhandle is very convenient!) and put the cottonthread in the chuck of a hand -drill. Do not tryto use a power drill or the results will bespectacularly messy. Now, keeping sometension on the wire via the rubber band, turnthe drill and twist the wire, stopping ifnecessary to even out the twists along thelength. There is no well-defined number oftwists to recommend - just keep going until itgets difficult to produce even twisting.

COIL FORMING

The coil former is made by taking diode D1and cementing two thin card cheeks onto it

Rod Cooper followshis previous

ammeter withanother meter for

use in powerelectronics.

mid -point to hold the wire in place while youwind on a few turns, but do not put any turnsover the thread. After about ten turns, thewire will hold itself in place and you can slipthe cotton thread off, and you can thencontinue winding. Without this technique,you will find it difficult to commence theturns.

After a number of turns, you can double -back on the winding to help minimiseinductance, although this is not strictlynecessary for general use on the types ofapplications outlined in the June issue. Whenwinding is finished, test the resistance with anohmmeter, and then wind off a few turns at a

Fig 1. How to wind the twist into the wire.

DRILL THREAD

,FINHERE

WIRE 1010,6501 RUBBERNBAND

time testing as you go until you have exactly5k ohms. The heater coil can then becompleted by winding on a few turns of finecotton thread to hold the Nichrome wire in,place and to provide some mechanicalprotection during assembly. Leave about 2 or3 cm of wire as lead -outs.

HEAT SINKS

Unlike the ammeter, which used fourcopper heat -sinks, the volmeter has onlytwo heat -sinks. The amount of heat whichwould be conducted down fine Nichromelead -out wires is negligible compared withthe much thicker ammeter lead -out wires, soit is not worth providing a heat -sink. Theheat exit path is confined to the diode leads,and the heat -sinks for these are exactly thesame as those used in the ammeter, exceptthat the spacing dimensions are different.The recommended distances are shown inFig. 4.

Because there are only two heat -sinks,the heat exit path length must be shortenedto keep the right balance between thethermal time -constant and sensitivity. Ifyou reduce the distances shown in Fig. 4,the sensitivity goes down, but the timeneeded to take a reading also goes down,and vice versa. Like the ammeter, thepractical result is a compromise, which canbe altered at will to suit the constructor.


tOlG1652L

DIODEASSEMBLY

n.

4mm 4mm

HEATSINK

Fig 4. Diode to heatsink dimensions.

SENSOR ASSEMBLY

As with the ammeter, the diode and heaterassembly is covered in a small piece ofpolystyrene top and bottom. I used 7mmpolystyrene, with a piece of bright capacitor foilsandwiched between two layers for both top andbottom sections to reflect heat and thus preventradiation losses. The pieces of polystyrene weresimply pushed into place between the heat -sinksand held in place by friction. To enclose theheater coil completely, a little pressure wasapplied to squash the polystyrene down over theassembly. There is sufficient "give" inpolystyrene to do this, but be careful.

Don't forget to pull out the two lead -out wiresbefore finally encapsulating the diode/heaterassembly. These two wires need to be anchored,otherwise they are likely to be damaged duringassembly. The easiest way is to mount a piece ofterminal strip as close as possible to the heatsinks. Although it is quicker to make flying leadsout of these wires by simply soldering onextension wires, if you pull out one duringassembly, you will have to start all over again.The photograph shows an alternative to tag -stripanchoring.

The ,compensating diode D2 can be clippedonto the heat -sink or glued on with Araldite. In

the photograph, this has been removed for clarity.

CIRCUIT ASSEMBLY

Assembly of the rest of the circuit isstraightforward and has been described in theJune article. Calibration is similar and is donewith a dvm and a variable voltage power supply.First, zero the meter with VR1, connect thepower supply to the input and set up 25 asmonitored on the dvm. Adjust R8 for fsd on themeter and mark on the scale with Letraset orsimilar. The scale, you will remember, will needto have the existing numerals removed. Thepower supply is now set up for 20V, 15V, 10V,and 5V, and the corresponding points put in onthe scale.

Having set the instrument for a basicsensitivity of 25V f s d , it is now possible toproduce other ranges using the usual,resigor divider chain, and I suggest ranges of 25,50, 100, 250, and 500V. These cover the likelyuse on 110y, 240V, and 440V, supplies, and theranges can be obtained using readily availablepreferred resistors if you copy the network shownin Fig. 5. Bear in mind that the resistor chain willbecome warm with prolonged use, so mount allthe resistors as far away as possible from theheat -sensitive parts of the circuit.

Fig 6. Completed heater coil (lp piece indicates size)

VOLTMETER USE

The instrument is as easy to use as anyvoltmeter, but you must bear in mind that it isintended for use in power electronics. Theinput current needed for fsd is 5mA on anyrange, and when measuring a circuit alreadytaking 10 amps for instance the added load of5mA is neither here nor there. What thisinstrument will not do is measure voltageswhere power levels are small and impedancesare high.

Before use, and especially if you intend to`float' the instrument, it is advisable to givean insulation test to make sure that the input iswell isolated from the rest of the circuit, andthe metal case. Most insulation testers (suchas Megger) can give a 1000V test.

100k 100k

5k

CARBON HI -STABRESISTORS 2W 700V

15k

Ok

10k 10k

DIODEHEATERCOIL 15k)

50mA

INPUT

101G,653.1

0

Fig 5. Range network giving 25, 50,100, 250, 500.

Fig 7. Heater coil assembled with heatsinks on an aluminium plate fordemonstration. Note tags for heaterleadout wires and position of lowerpolystyrene insulator (top layer not yetin place).

INPUT PROTECTION

As with most test gear, overloads willoccur from time to time even with the mostcareful user. With the ammeter, a fuse wasused to prevent burn -out of the heater coil, butthis method is not such a good solution to theproblem with this voltmeter because 5mAfuses are just not available. The lowestcommercial fuse I have seen has been 50mA,and this does not give sufficient protection tothe input heater coil.

The circuit of Fig. 8 gives reasonableprotection against most of the overloads likelyto be encountered. It works like this; thenormal maximum output from the is isapproximately 2 volts, (and if it is not, itshould be adjusted to be 2V by altering thefeedback resistor R5. If it rises to 3V thenzener diode DI conducts and the small -signalscr TH1 fires, energising the relay. The heatercoil can easily withstand 3V, so no damage isdone by this level of overload. The change-over contacts on the relay switch the inputaway from the heater coil and into a dumpresistor R2. This resistor is required to lessenthe possibility of a voltage higher than the


TEST GEAR PROJECT

RELAY C/0CONTACTS

DUMPRESISTOR

R2tqc

2W

TO REST OF RESISTOR CHAIN

HEATERCOIL INPUT

{0161654o I

2V 7400mW

10.IGI654b1

UNSMOOTHED 9V LINE

RS*SEE TEXT

R4 ik 1/4W

+VE

OV

Fig 8. Input protection relay

blocking voltage of the contacts appearingacross contacts a and b. The scr latches on,since the relay current is chosen to be higherthan the holding current of the scr, and thismeans that there is no input to the heater coileven when the overload is removed.

Without latching, an overload could comeand go without the user noticing, and this isnever a good practice because damage could bedone to the circuit under test with no apparentcause. Also, if the instrument is left unattended(feeding a chart recorder for example) a standingoverload could cycle the relay indefinitely, andlatching prevents this situation arising.

Indication of overload is given by led D2.

OPERATIONAL NOTES

There are two facets of this system to benoted. Firstly, it will not operate if theinstrument is turned off and an overload appliedto the input. Secondly, it will not cope with acataclysmic overload. A sudden enormousoverload will damage the heater coil before therelay has time to operate, so a 50mA fuse in theinput circuit is definitely a good idea.

Other forms of input protection used in moreconventional instruments are not applicable.Back-to-back diodes for example would clip anyspikes that might normally occur in properoperation. These should of course be allowed

through to contribute to the heating effect in theinput heater coil. A high crest factor cannot beclaimed when back-to-back diodes are used.

COMPONENTSD1D2D3R1R2R3R4R5

ThlRelay

2.7V 400mW zenerledI N4001100R 1/4 watt10k 2 wattlk 1/4 wattlk 1/4 wattchoose to give approx 6V acrossrelay coil when Thl firessmall scr BR103submin 6V c/o type

ARISING POINTS

Finally, there were a couple of errors in theJune issue. Firstly, on page 50, the division lineon one of the equations was missing, whichmust have been very puzzling for some readers:this was Eav = V2 T

2RSecondly, the term T was missing from the

right hand side of the equationv2 T = V22 T2R

THE DREADED VIRUS

T he greatest current nightmare for anyoneusing a computer must surely be the fear ofthe system becoming infected by a virus.

That nightmare recently became reality for us at IntraPress, and the production of all three magazines, PE,Program Now and Astronomy Now was severelythreatened.

Eagle-eyed David Hewett, who is responsible foroperating our sophisticated Macintosh DTP system,began to suspect that the software routines were notresponding as fast as they should.

Prudently, he ran a virus -detecting software routine.To our horror it indicated a positive response - we wereinfected! A grand pow -wow ensued between David,PN editor Nigel Stuckey and PN technical editor JonLansdell. Thank heaven for the expertise of our sistermagazine's analysts! They discovered the infection wasby a less-virulant virus known as `nVIR'. With theamount of material that comes through our offices, thevirus might have originated from many sources. A bitmore detective work, though, strongly suggested it hadcome in from Yugoslavia.

With diagnosis complete, a vaccine was providedby the Apple Centre at Chiswick, and administered tothe sick system. "Apple today keeps the virus at bay",says the old proverb - and with due justification. OurDTP is fully restored to natural health, and not a sign oflethargy, even among the operators!

Newsagent Order FormDear Newsagent, I like reading Practical Electronics magazine. Please deliver/save

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,47, ASK PE

T hough the circuit shown last monthwill work better than a design usingonly 555 type timers, it is possible to

'reduce' cost and complexity while improvingperformance. Here is the reasoning leading tothis conclusion:

It is intuitively obvious that one could gatetogether the outputs of the counter stages togenerate waveforms with differing time periods.The basic requirement to achieve this is that thecomplete cycle time of the counter (from allzeros to all ones) must be the required timingcycle. In this case the figure is half an hour.

Andrew Armstrongcontinues the topic oftiming circuits startedlast month, examiningthe logic of digitally

controlledintermittentswitching.

transistors, so that the logic output must be atlogic 1 to switch on. The output is to be offfor more of the time it is on, so choose aNOR gate because its output is at 0 if eitheror both inputs is at 1. This is the best functionfor the output gate.

Gating together Q9 and Q8 immediatelygives a duty cycle of 4:12. It is thennecessary to add logic to change this to 3:13.The NOR of Q9 and Q8 is at logic 0 most ofthe time, and logic 1 is required to inhibit theoutput, so invert this signal.

That uses three gates, and leaves one input

Fig 1. Gated extended timing method in which:Q6 = 1.875 mins/half cycleOn time = approx 5 min 40 secsCycle time = approx 30 min

ov

MR 4060

RTC CTC 06 07 08 09

C1

100p

R2

3M3

R1

10m

R3

270k

f .0.57Hz

220nmim

4001

Do

+12V

POWER

OUT

RELAY

BC182

OV

GATED TIMING2onsidering only the ON times, the lowest

fi juency output is on for 15 minutes, the nextone for 7.5 minutes, and so on. A very simplesolution would be to gate together the final twocounter Outputs from last month's circuit to givea time period of 7.5 minutes. Such a simplerequirement could be carried out by two diodes,but I would not say that 7.5 minutes is closeenough to 5 minutes.

0100 AND COUNTING

To choose a time period to better than ±minute, four adjacent counter outputs areneeded, with the lowest frequency outputrunning at one cycle per half hour. The databook shows that the 4060 lacks a Q10 output, sothat the lowest frequency outputs in a group offour are Q6 to Q9 inclusive. To produce therequired frequency on Q9 requires a frequencyof 0.57Hz on the oscillator, much lower than lastmonth's design.

The oscillator design itself deserves mentionat this point. The basic time constant is

(R2+R3)xCl, but this is affected by the loadingof the diode protected cmos inputs.Conkquently RI is added to minimise thespurious capacitor charging current which flowswhen the voltage on the bottom end of Cl isoutside the power supply rails. Unfortunatelythe addition of this resistor leaves the inputvulnerable to noise, so to make sure that thecounter only receives one clock pulse per clockcycle rather than a double bounce, Cl is added.

Q9 08 Q7 Q6 A B C

0 0 0 0 0 1 0

0 0 0 1 0 0 1

0 0 1 0 0 0 1

0 0 1 1 0 0 1

0 1 0 0 1 1 0

0 1 0 1 1 0 0

0 1 1 0 1 0 0

0 1 1 1 1 0 0

1 0 0 0 1 1 0

1 0 0 1 1 0 0

1 0 1 0 1 0 0

1 0 1 1 1 0 0

1 1 0 0 1 1 0

1 1 0 1 1 0 0

1 1 1 0 1 0 0

1 1 1 1 1 0 0

Truth table for Fig. 1.

DECODING

The method of designing the decodingcircuit is not totally obvious. Inspection ofthe counter truth table suggests that thefunction could be performed with a singlecmos ic, probably some sort of quad 2 -inputgate. The reasoning was as follows:

"The relay is to be driven by a pair of npn

of the output gate spare. What happens if Isimply gate together Q7 and Q6 and feed theresulting signal to the output gate? (Addsthree columns to truth table.) That works!"

The method of design is thus acombination of straight analysis, and drawingsomething out to see if its logic is correct.The same style of reasoning would apply todifferent timing requirements (and othertypes of problem) though in somecomplicated cases it might be easier to usetwo binary comparators to build a digitalwindow comparator to determine the requiredrange. It is more satisfying, however, todesign a simple circuit which does the jobwell.

ABOUT ASK PE

Ask PE is a most -monthly column inwhich the most interesting readers' technicalenquiry (in the opinion of the Editor) will beanswered to the best of the columnist'sability. Individual queries will not beanswered, even if stamped addressedenvelopes are sent. The sole exception to thisis that, when a column has been written, aphotocopy of the material may, at the Editor'sdiscretion, be sent to the questioner ahead ofpublication if an sae is enclosed. Please markenvelopes clearly "ASK PE", and enclose noother correspondence because theseenvelopes will be forwarded straight to thecolumnist.


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40 PRACTICAL ELEC l'RONICS DECEMBER 1989

TIME AND MEASUREMENT t

Last month we looked at "faithfulreadings" and errors ofmisapplication and frequency.

PERIODMEASUREMENT

Period error = ±1 count± time base error± trigger error

The measurement of signal period usesthe same circuitry as that used for frequencymeasurement; however, when measuringperiod, the input signal itself produces thegating signal, such that the gate time equalsthe input period tp, and the pulses countedduring this time are generated by the timebase (see Fig. 9 and compare with Fig. 7).

Just as with frequency measurement, thelack of phase coherence between the gatingsignal and the counted clock pulses meansthat we again have a ±1 count uncertainly inthe reading. This time, however, the ±1count corresponds to ±1 clock period, and so

TIMEBASE

CONDITIONED INPUT SIGNAL1

IfERIOD.t p

CONDITIONEDINPUT SIGNAL

GATETIME.tp

GATING LSIGNAL

MAINGATE

_DCOUNTED PULSES

0.131181

Tr.

Fig.9 Basic period measurement scheme.

In the example of Fig. 10, the gate shouldnormally be opened at point A, but the noisespike causes it to open too early (at point B)by the amount te seconds.

If we denote the noise amplitude by Vn, itcan be seen that:

Vn = dV, and rearranging: te = Vnte dt dV/dt

Consequently, the trigger error is givenby:

Trigger Error, TE = ±2te =±2Vn seconds,dV/dt

zero). Consequently, when making timemeasurements, it is always best, whereverpossible, to choose signals with very fast riseand fall times.

For sinewaves, the trigger points shouldbe located at zero -crossings, since thesinusoid's slew rate is largest here. We cancalculate the magnitude of the trigger errorfor a sinewave with, say, a 40dB signal-to-noise ratio, as follows:

Input signal, V = V sin 2aft;differenting: dV = 27tiV cos27tft.

dt

UCT MEASUREMENTSthe quantisation error contribution is simply±te, where te = one clock period.

Furthermore, since the counted clockpulses are derived from the time baseosciallator, we must remember to include the± time base error in the assessment of thetotal period error.

So far, the error sources are similar tothose for frequency measurement. However,we must also consider one more, namely'trigger error.

Trigger error is a random error caused bynoise on the input signal and noise inherentin the counter's input circuitry. For ourpurposes, we will ignore circuit noise sinceit is usually much smaller than signal noise.

In both period and time intervalmeasurements, the input signal(s) controlsthe opening and closing of the main gate;thus, any noise on the signal will cause thehysteresis threshold to be crossed either toosoon or too late, leading to a distortion of thegate time.

Fig.10 Noise causes trigger error.

NOISE SPIKEAmPLiTUCE.

dt

OPENS TOO rEARLY

dV

NOISYINPLfTSIGNAL

VTU

SCHMITTTRIGGERHYSTERESISWINDOW

VTL

GATE SIGNAL

10,61380

PART TWOBY ANTHONY H. SMITH BSc

For anyone with anexpensive counter -

timer it's vital tounderstand it

correctly and avoidthe pitfalls.

where dV/dt is simply the slew rate of theinput signal at the trigger point. We mustinclude the factor ±2 because the noise maycause the gate to:

open early and close late: TE = +2te,or: open late and close early: TE = -2te.

It is quite possible, of course, for the gateto open early and close early, or open lateand close late, such that the error 'cancelsitself out'. However, when specifying errorswe must deal with worst -case possibilities,such as those above.

We see immediately from the expressionfor TE that this error can be minimised byreducing the input signal noise, and/orincreasing the signal slew rate. (Indeed, youmay be wondering why trigger error is notincluded in the error specifications forfrequency measurement. This is because themain gate is controlled by digital signalsderived from the time base, which -theoretically at least - are noiseless, andhave infinite slew rate, thus reducing TE to

This is a maximum at the zero -crossing ofthe input sinewave, where cos27tft = 1, ie,

dV = 2/rIV. Thus, TE = ± _n_2Vdt MAX

and since the SNR = 40dB, we know thatVn = 0.01V. Hence:

TE = ± 0.01 = ± 0.003 seconds.

We can specify the trigger error time as apercentage of the period itself, as follows:PercentageError =TE x 100 TE x 100 = TE x f x 100

PERIOD 1/f

=±0.003 xfx 100=±0.3%pf

Thus, the trigger error can be as large as0.3% of the period itself. This is quite asubstantial error, and we can see how itaffects the overall period error in thefollowing example:

EXAMPLE:Input Signal = 100kHz sinewave,40dB SNR.

Time Base Oscillator NominalFrequency = 10MHz.

Time Base Osciallator FrequencyError = ±8.8 x 10-6.

Quantisation error component =±te = ±10Ons.

Time Base error component= ±8.8 x 10-6 x PERIOD = ±8.8 x10-6 x 10µs= ±88 picoseconds (which is negligible!)

Trigger error component = ±0.3%of PERIOD = ±0.03µs = ±3Ons.


Thus: total error = ± 13Ons.

Forsthis example, quantisation error is asmuch as 1% of the input period. However,had the period been, say, 10milliseconds,this error would have been a mere 0.001%.

RECIPROCALCOUNTERS

We see from the above example thatquantisation error is the dominant factoronly when measuring short periods (ie, forhigh frequencies), whereas for frequencymeasurements, this error dominates only atlow frequencies.

For example, assume we wish todetermine the exact mains supply frequency.In frequency mode, using a 1 second gatetime, the reading will simply be 50Hz, ie,we can measure the frequency with aresolution of 1Hz. The quantisation error asa percentage of the input frequency will be:

Percentage QuantisationError =± 1 x 100=±1 x 100 = ±2%

FREQ. 50

If we now switch to period mode and findthat the display reads, say, 19.9970milliseconds, we can determine thefrequency simply by taking the reciprocal:

InputFrequency = 1 = I = 50.0075Hz.

PERIOD 19.9970 x 10-3

(Note the improved resolution).The percentage quantisation error

(assuming a clock period of tc = 100ns) issimply:

Percentage Quantisation Error=± tc x too=± too x 10-9x 100 = 0.0005%.

PERIOD 19.9970 x 10-3

Obviously, we can dramatically improveboth resolution and accuracy by making aperiod measurement and then taking thereciprocal. A reciprocal counter works in thesame way: first, the input period ismeasured, then arithmetic circuits computethe reciprocal which forms the requiredfrequency display.

To ensure that quantisation error isminimised even when measuring highfrequencies, most reciprocal counters use amethod known as 'mutiple period averaging'which maintains high accuracy and optimumresolution at all frequencies (we willexamine this powerful technique in the nextpart).

The reciprocal measurement technique isalso ideally suited to making burst frequencymeasurements. Fig. 11 shows a typical burstfrequency signal.

POSITIVE SLOPE TRIGGERPOINT, VT + VH/2

TRIGGERLEVEL.VT

INPUT SIGNAL

VH/2 HYSTERESISVOLTAGE.VH

TRIGGER LEVELTIMING ERROR lote1394.1

Fig 12. Time intervalmeasurement.

NEGATIVE SLOPETRIGGER POINT= VT-VH/2

Using the conventional frequency mode,the cycles occurring during the frequencyburst are accumulated as normal, but thedisplayed reading will be totally incorrectdue to the absence of cycles during the'silent' period.

The reciprocal counter, however, starts themeasurement with the first cycle of thefrequency burst, measures its period, andthen displays the correct frequency: thesilent periods thus have no effect onmeasurement accuracy.

TIME INTERVALERRORS

Time interval error = ± 1 Count± Time base error± Trigger error± Systematic error

Like period measurement, a time intervalreading suffers from quantisation error, timebase error, and trigger error, although thelatter must be specified slightly differently toaccount for possible differences in slew rateat the 'start' and 'stop' inputs:

Trigger Error= ± (Peak Noise Voltage on Start Signal

Start Signal Slew Rate

+ Peak Noise Voltage on Stop Signal)Stop Signal Slew Rate

The additional 'systematic' error actuallycovers two new sources or error, namely'differential channel delay' and 'trigger leveltiming error'.

The time interval measurement scheme isrepresented in Fig. 12, where the start signalsets the flip-flop, thus opening the maingate, and the stop signal resets the flip-flopwhich closes the main gate.

It takes time for each input signal topropagate through the probe cable andconditioning circuitry; consequently, thestart and stop channels have small, but finite,propagation delays. Any difference in thesedelays gives rise to the differential channel

FREQUENCYBURST

VYYYYYI

SILENCE FREQUENCYBURST

101013901

Fig 11. Pulsed frequencysignal.

delay error. For example, if the start channeldelay is 8ns, and that of the stop channel islOns, the difference will set the flip-flop 2nstoo early, causing a corresponding erroneousincrease in the gating signal.

When measuring time intervals of 10µs ormore, such errors are usually small enoughto be negligible. However, several highperformance counters such as the HewlettPackard HP5345A can measure single timeintervals as short as 2ns, such that thedifferential channel delay can no longer beignored and must be removed.

This is no easy matter, however, andrequires that the conditioning circuits befitted with special variable delay elementswhich can be trimmed so as to equalise thepropagation delays. Furthermore, the probecables must be exactly the same length: adifference of just one inch is enough tointroduce a differential delay of around9.1ns.

Unfortunately, even if we manage toremove the differential channel delay, wemust still contend with the trigger leveltiming error.

If all input signals were clean, sharppulses, trigger level timing error would notexist. In practice, however, all signals have aparticular slew rate; for pulses, this meansfinite rise and fall times, and indeed, thetime interval mode is often used to measurethese parameters.Fig 13. Time interval mode measuresrise time.

900CAMPLITUDE

INPUTPULSE -N,

10%0FAMPLITU.

[mina 1

STOP' CHANNELTRIGGERS HERE

'START' CHANNELTRIGGERS HERE

MEASURED TIMEINTERVAL EQUALSPULSE RISE TIME

GATING SIGNAL

Fig. 13 shows how rise time is determinedby measuring the time interval between the10% and 90% amplitude points. Since boththe start and stop channels are required totrigger on the same waveform but atdifferent points, the accuracy with which wecan set the trigger points determines theultimate accuracy of the measurement itself.

The trigger points are set by adjusting theuct's trigger level potentiometer. If thecontrol knob is the only guide as to the value


TIME AND MEASUREMENT 1,W

of the trigger level, it is almost impossible toset. the trigger points accurately, even if thecontrol has a calibrated scale.

Those counters which feature signalmonitor and trigger monitor outputs allowthe conditioned input signal and Schmitttrigger output to be superimposed on anoscilloscope screen; the point where thewaveforms cross is the apparent triggerpoint (Fig. 14).

ACTUALTRIGGER --__OUTPUT

TRUETRIGGER

POINT

1

-SIGNAL

..._ APPARENT- TRIGGER

POINT

DELAYEDTRIGGEROUTPUT

10161]9)1

Fig 14. Problems in setting triggerpoint.

For measurements above a fewmicroseconds, this technique provides areasonably accurate means of setting thetrigger points.

Unfortunately, when dealing with veryshort time intervals, delays within thecounter which result in the trigger outputbeing delayed relative to the signal outputby the amount tD, cause the apparent triggerpoint to be substantially different from thetrue trigger point.

In fact, correct setting of the trigger pointscan only be accomplished if the trigger levelvoltage itself can be read from theinstrument panel. However, even this has itslimitations. Fig. 15 shows that if we assume(incorrectly) that triggering occurs at thetriggerlevel, we will incur a trigger leveltiming error given by:

Trigger Level timing Error =Half Hysteresis Voltage (=VH/2) secondsSignal Slew Rate at Trigger Point

Since triggering occurs at the triggerpoints, rather than at the trigger level, wemust bear in mind that:

Positive Slope Trigger PointVoltage = VT + VH/2 ,

and: Negative SlopeTrigger Point Voltage = VT - VH/2 .

STARTSIGNAL

PROBE CABLE

CLOCK PULSES

TIMEBASE

11.=1CONDITIONING

SET1--FFLIP-FLOP

START CHANNEL PROPAGATION DELAY

STOPSIGNAL

PROBE CABLE CONDITIONING

CIRCUITRESET

11=5.

STOP CHANNEL PROPAGATION DELAY

MAINGATE

_DCOUNTEDPULSES

_111

GATINGSIGNAL

TI MEINTERVAL

D1513911

Fig 15. Trigger level timing error (shown for positive slope).

Take care, however, if the input signal hasbeen attenuated: for example, if we attenuatethe input by a factor of x10, a trigger levelreading of, say, 0.5 volts corresponds to atrigger level of 5 volts relative to the signalitself.

SUMMARY

Fig. 16 summarises the errors associatedwith each type of measurement.

Remember that any additional modes(such as Pulse Width, Rise/Fall Time, andPhase Measurement) which rely on a timeinterval measurement are likely to sufferfrom all sources of error.

Incidentally, provided the input pulsesare large enough to trigger the counter'sinput correctly, the 'totalise' mode ofmeasurement (sometimes called 'eventscounting') will suffer no error at all, andwill simply continue to accumulate allinput events as they occur. However, it isarguable that this mode is not a true formof measurement, since the input signal isnot being compared with any referencestandard.

In the next article, we shall look at aspecial averaging technique used tominimise random errors, and at severalunusual techniques employed to extendthe range of the basic uct.

Fig 16. Summary of errors associated with each type of measurement.

Measurement Mode

Types of Error

Frequency Period Time Interval

±1 Count (A Random Quantisation Error) / i i± Time Base Error / i i± Trigger Error (A Random Error) ./ i± Systematic Error /

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PE COMPETITION RESULTS

MAPLINOSCOPY!WE'VE TRACED THE TRIPLE WINNERS!

12)

AND

COMBINED FORCES IN THESEPT 89 ISSUE TO LETTHREE OF YOU WIN A

SUPERB MAPLIN TRIPLE -

TRACE 20MHZOSCILLOSCOPE!

THESE ARE THE LUCKY WINNERS :

Dr Colin G. Owen of Luckington, Wilts.J.F. Young of Totnes, Devon.G. Harris of Bromyard, Herefordshire.

CONGRATULATIONSTO THE THREE OF YOU!

THE ANSWERS!

These are the answers I was after:

1. The initial letters CRT stand for Cathode Ray Tube.2. The name Rayleigh is renowned for electronic supplies (Maplinhave their HQ there of course!)3. Signal phase and frequency comparison patterns on a scope screenare called Lissajous Figures.4. In a scope tube the electron beam starts its journey to the screenfrom the cathode.5. In the list including oscilloscope, video camera, tv set, radio,computer, cash dispenser, radar, the odd one out was radio.6. The common link in the list was that all except the radio commonlyhave a crt or other visual display.

ANALYTICALSThe response to this competition was astonishingly high and it's ashame that you all couldn't win. Owning an oscilloscope giveselectronic enthusiasts a tremendous advantage when pursuing theircircuit building or testing. We know the winners will really benefitfrom their good fortune.

It was most unexpected that quite a few of you did not know whatthe letters CRT stood for. Those who were wrong on this mostlythought the answer was Cathode Radar Tube - well fooled, weren'tyou! A handful thought the answer was Caesium Ray Tube. At least Ididn't spot anyone answering Capacitive Resonance Trigger! But whydidn't a lot of you write in the answer as requested? you just circledOther and left it at that.

RAYLEIGHThe answers to Q2 were interesting. Most of you spotted that

because Maplin were so generously involved in the competition thisquestion was related to them. The commonest wrong answer wasBicycles; no, bikes are not manufactured by Rayleigh - the spelling forthat is Raleigh. Sir Walter also used the latter spelling, so the answerof Plymouth Hoe should have been regarded as a trick one - a few ofyou fell for it! The answer of Maplin Sands was also a trick one -certainly Maplin named themselves after Maplin Sands, but their HQis not on the Sands (which are tidal and a Ministry of Defence DangerArea!). I was happy to accept either Electronic Supplies or a written -in answer relating to Maplin's electronic activities.

But I was also interested by the number of you who knew thename Rayleigh had other historical significance. Baron Rayleigh(1842-1919) was one of the most eminent British physicists; anauthority on sound vibrations, and the co -discoverer with Sir WilliamRamsey of the gas Argon. Several physical terms are named afterRayleigh, but so far as I know he had no connection with theinvention of the CRT (as one reader suggested). He was awarded theNobel Prize for physics in 1904.

Q3 ONWARDSNearly all knew that Lissajous Figures (or patterns) were Q3's

answer. Only one person said Lissotrichous, and no one said Listeria.The only wrong answer given to Q4 was Anode, and only a small

percentage of you quoted it.Q5 and Q6 proved what versatile imaginations there are among PE

readers when it comes to spotting relationships. Most of you actuallyspotted that Radio was the one I was looking for, and that the link wasrelated to crts or some other form of visual display. A good case forCash Dispenser was made by several people, and Oscilloscope alsohad a few well -reasoned supporters. There were too many answers toQ6 for quotation, but two I especially liked: Computer was oddbecause electronics is not essential to its operation (examination of thedefinition of a computer reveals the truth of this); Oscilloscope wasodd because only it could be won in this competition!

Well done, the winners; commiserations to the rest of you -better luck next time!


e have still not heard just what hasbeen the result of the supernova inthe large cloud of Magellan which

made headline news in 1987. Reports of apulsar have still not been confirmed, and we dohave to admit that at the moment the situationis unclear. The supernova itself has faded farbelow naked -eye visibility, but it can still beseen with large telescopes - provided you arelucky enough to be observing from thesouthern hemisphere!

Astronomers are still awaiting the launch ofthe Hubble Space Telescope, which shouldhave been in orbit long ago, but was delayed bythe Shuttle disaster. (To refresh your memory,the HST is a 94 -inch reflector which will besent into an orbit 300 miles above the Earth'ssurface). Now, at last, there is a definiteschedule: March 1990. Let us hope there are nofurther delays.

On the debit side, we have the virtual lossof the astrometric satellite Hipparcos, whichwas intended to provide the most accuratecatalogue of stellar parallaxes and propermotions ever compiled. It was launched by theAriane rocket in August, but its own propulsionfailed - this time, one cannot blame Ariane -and Hipparcos ended up in an orbit whichmakes it virtually useless. Unfortunately therewas no tack -up'. No doubt another Hipparcoswill be built, but there is no denying that this isa grave setback, and one's sympathy goes outto the scientists who have given years of theirlives to the project and were looking forward tothe final catalogue to be produced in the mid -1990s. The cause of the failure is still obscure;in the end it may have to be put down to 'justone of those things'.

SPACE

WATCHBY DR PATRICK MOORE CBE

The fate of Hipparcosis distressing to allconcerned, but the

success of the Neptunefly -by is a magnificent

achievement.

NEPTUNE

Now, weeks after the event, we can start toevaluate the results of the Voyager 2 pass ofNeptune on August 25. There is no doubt thatit was a major triumph. Remember, Voyagerwas launched in August 1977, and its designhad been completed several years before that.This means that by modern standards it is anancient vehicle; the technology of the 1970sseems to take us back into a sort of StoneAge. To complicate matters further, Voyager2's main receiver failed as early as 1978.After a brief recovery it went out actionpermanently, and ever since then Voyager hasbeen operating on its back-up system, whichis in itself faulty. This makes it all the moreremarkable that at the Neptune pass, after ajourney of more than 4,000 million miles,everything was working even better than atany other time during the mission.

How could this be done? Simply byupdating the equipment on Earth - so that the1970s Voyager was operating with thetechnology of the late 1980s. It reached itstarget within six minutes of the predictedtime. (British Airways, please copy.)Moreover, it was under full control. As itdrew in towards Neptune, six new innersatellites of the planet were discovered; one ofthese was well over 100 miles in diameter(larger than one of the previously knownsatellites, Nereid), and it was impossible toadjust Voyager so that a close range picturecould be taken of it. The satellite proved to bevery dark, and rather irregular in shape, withone large crater. The second of the new

THE NOVEMBER SKYNeveral of the planets are on view this month, but Mercury isnot among them; it passes through superior conjunction onNovember 10, and will too close to the Sun to be seen.

Venus, however, is a brilliant evening object. It is a long nay southof the celestial equator (its declination is around -26c'1 but by theend of the month it is visible for almost two hours after sunset. Itreaches eastern elongation on November 8. and should thentheoretically he at half -phase: but during evening elongationsdichotomy is always slightly early (Schroters effect). 13y the end ofthe month the phase will he reduced to 36(4, and it will be time tostart looking out for the elusive Ashen Light; observers withadequate telescopes please note.

Mars is drawing slowly away from the Sun, but the apparentdiameter is still below 4 seconds of arc, and at magnitude 1.6 Marsis no brighter than Castor in Gemini. Jupiter, however, is coming opto opposition (which it reaches at the end qt.December) and since itis well north of the equator it is on view for most of the night. Jupiterhas been of unusual interest recently,' the South Equatorial Belt,which at the start of the year was the most conspicuous belt on theplanet's disk, liar 1'11111[111y disappeared. SO that telescenfic observerswill be very much on the alert, Saturn, the other giant, is still anevening object, but is very low, and ,sets not long after the Sun.Uranus and Neptune are both fools near it, but at the momentneither of these can he seen without optical aid.

The Moon is at First Quarter on November 6, full on the 1.311r,Last Quarter on the 20th, and new on the 28th. Perigree is due onNovember 12, and apogee on the 25th. There are no solar or lunareclipses this month, but the Sun is becoming more and more active,

so that we may well have more displays of auwrae - though it isimpossible to be sure, and we may hare to wait a long time for adisplay as good as last March.

Comet Brorsen-Metcalf will still be visible. There are also twometeor showers. ihe Taurids are active throughout the month, butare usually rather sparse. Not so with the Leonids, which reach theirpeak on the 17th. Occasionally, the Leonids are spectacular, as lusthappened in /966; we do not really expect much this vein: but wemay he fairly sure of another 'storm' in 1998 or 1999, and it isworth keeping watch just in case the Leonids take us by ,surprise.

Orion non, rises at a respectable hour, and will dominate theevening sky all through the winter and into spring; with its twoleaders. the red Betelgear and the pure white Rigel, it is certainly themost impressive of all the constellations. Following it round, Sirius,the brightest .star in the sky. comes into view after midnight. Capella,the yellow star in Auriga, is almost Overhead; Ursa Major. the GreatBear is low in the north, though from Britain it never sets. 'The so-called 'Summer Mangle' {Vega, Altair and Deitch) is descending inthe west, though of these bright stars only Altair is not circumpolar.The Square of Pegasus remains prominent in the southwest; below itis the large, dim constellation of Cetus. the Whale, who.se only realclaim to fame so far as the naked -eye observer is concerned is thepresence of the red variable star Mira.

The W of Cassiopeia is very high up. Its three leading stars(Alpha, Beta and Gamma) are all about magnitude 2, but Gamma isdefinitely variable. while Alpha may be. Look at them, and seewhether you can decide which of the three is the In-ightest!


satellites was also imaged satisfactorily.Voyager made a whole series of

discciveries during its pass. Neptune doeshave rings - three of them, though they arevery faint, and parts of them are almostbeyond the threshold of Voyager's visibility;there is no chance of seeing them directlyfrom Earth. Neptune itself is a dynamicworld, very different from the bland Uranus;there is one feature, the Great Dark Spot,which dominates the entire scene. It has beenlikened to the Great Red Spot on Jupiter,though it is not red (it is merely rather lessblue than the rest of the planet). The magneticfield of Neptune is a surprise. It is weakerthan those of the other giant planets, and, aswith Uranus, the magnetic axis is inclinedsharply to the axis of rotation; the angle isaround 50 degrees, as against 58 and a halfdegrees for Uranus. Oddly, then, Neptuneseems to be much more like Uranus thaneither Jupiter or Saturn from a magnetic pointof view.

Triton, Neptune's large satellite, was thehighlight of the whole mission; its pink andwhite surface, coated with nitrogen andmethane ice, was seen in detail (earlier fearsof a cloud -laden atmosphere proved to beunfounded; the Tritonian atmosphere is verytenuous indeed, with a ground pressure of nomore than 0.01 millibar). It may also be thatbeneath the surface, at a depth of no morethan a hundred feet or so, there may be a seaof liquid nitrogen; when this nitrogenpercolates upward, it 'explodes' and produces

Neptune, featuring the Great Dark Spot. Photo by courtesy of NASA-JPL.what may be termed an ice volcano (or, moreappropriately, an ice geyser). Nothing of thesort had been expected, particularly as Triton,with its temperature of -400 degreesFahrenheit, is the chilliest place ever visitedby a spacecraft.

All in all, I feel that the most remarkabletriumph of the mission was the endurance ofVoyager 2 itself. Even now it has notcompleted its task, though Neptune and theother planets have been left far behind. Wehope to maintain contact for the next 25 years,

by which time Voyager will have reached theedge of the heliosphere (that part of theGalaxy over which the influence of the Sun isdominant). Until its power fails, it will go onsending back data about the solar wind,interplanetary magnetic fields, cosmic raysand much else. Then, sadly, it will be gone. Injust under 300,000 years time it will passwithin 4.3 light-years of Sirius; I wonderwhether any `Sirians' will find it and examinethe record that it carries, hoping to find outwhence? I would like to think so. PE

Astronomy NowBritain's leading astronomical magazine

11K Prier £1.50

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SPACE TELESCOPE READIED FOR LAUNCH

JEREMIAH HORROCKS AND THE TRANSIT OF 1639

NEPTUNE IN CLOSE -LIP

METEOR MYTHS

In the November issue:Space Telescope Readied for

Launch * Jeremiah Horrocks andthe Transit of Venus Neptune in

Close-up Meteor Mythsplus: News Uptdate - The Night Sky -

Sky Watch Down Under and more...

WIN A MEADE TELESCOPE!Remember to look out for ourDecember issue competition!

November issue on sale now - price f1.50!Published by Intra Press - Publishers of Practical Electronics


Six months ago, in the June 89 issue,Robert Penfold described a reallysophisticated metronome that would

suit the music workshop of any equally -sophisticated musician. But I know that not allof you music fiends need a deluxe machine toknock out the beats in the back bedroom. So forthis month's Easibuild project, I've a circuitthat's somewhat further down the spectrum indeluxification, but which offers you a broadselection of beat markings and rates.

FUNCTIONS

Not only does the unit have an audio output,it has a red/green led visual display as well. Youcan 'program' it for accenting one beat in two,or one beat in three, and so on up to one beat inten, plus constant running without any accent atall. The basic rate is controllable from the panelbetween about 20 and 200 beats per minute,though you can change the range at the time ofsetting -up if you prefer.

In keeping with the nature of this series, I'vemade the circuit quite simple, using just two ics:a quad opamp, and a decade counter.

CLOCKING

The first of the opamp sections is shown inFig.l as IC la. This forms the master clockoscillator, and is of a type which I havedescribed in other Easibuild projects, for

6

7

9

10

11

12 13 1.

T

5

R6

C.2

Fig 2. PCB layout for theMini Metronome

-f R5 F.

zz

19

Whatever theclosing time you

can count the barswith John Becker'sflashy simple supersonic metre beater.

example in the Wheehy-Jeeby Sirens of June 89.The oscillation rate is set by CI, VR2 and thetotal resistance of VR I plus RI. VR2 set thebasic range, and VR1 then controls it from thepanel.

The 324 opamp used does not reliably swingcleanly between the levels required to drive thedecade counter chip, and so TR1 has beenincluded to ensure the maximum swing. Alsoconnected to TRI is S I a which allows you to

MINI METRONOME

.R1 im R1 470.a C,

R2 2M 2 20n

47.47°046.41R3 10k

2

IC in

1.,VR 2

C2220p

100k

+VE0

RI.

TR1BC 549

0 +VER5 467

R6

467

21

1C1 321.PIN!. - +VEPIN 11 - GNU (0V)

03 D6 R12 1006

1C24017

11

986 7b

5 6-o1 5

104-7 33p

4 2P

10

S2

0

R7 100k

ID1G17,51

15

(sib R10

10k

05

1N4148

1N14.18

DI IC1d

111N1.148

10

R8 100k

1N4148D8 GA)

D213

LR9 4.7011

(SEE TEXT14iN4148

2

IC1d

10011 1N4148R11

100k

106

C4 % D7 614 10

220n 1N /..103

R13

1M

'00k

IC:b

Fig 1. Completecircuit diagram forthe MiniMetronome

stop the clock signals from reaching the counterby holding the base of TR1 at ground level.

DECADE COUNTER

With S I a open TRI amplifies the clocksignal which then triggers the decade counterIC2. It's a cmos device and has ten outputs


EASI-BUILD PROJECT

which normally are consecutively triggered highonce every ten clock pulses. As one output goeshigh, ?D the preceding one goes low. The ic hasa reset input too, of course, and so the cycle often can be shortened by selecting any of theoutputs to trigger the reset on whicheverdivision beat you wish.

CLICK ROUTE

You will see that the signal from TRI also isrouted to the circuit around IC1b. The pulses areshaped by C3 and R11 and their spikiness isdictated by the value of both components. Thespiky pulse is amplified by ICIb and can be fedto any normal amplifier and speaker system,from which they will be heard as clicks. Thesound of the click will depend on how spiky itis. The clicks coming along the C3 route arethose for the 'tick' sound. The lock' soundcomes from the counter route.

S2 selects which output you want to use asthe reset pulse fed back via D3. Each time IC2 is

COMPONENTS

RESISTORS

RI, R9 470 ohms (2 off)R2 2M or 2M2R3, RIO, R14 -R16 10k (5 off)R4 I k

R5, R6 4k7R7, R8, R11, R12 100k (4 off)R13 1M

All 0.25W 5% carbon film.

CAPACITORS

Cl, C4C2C3C5

220n polyester (2 off)2201.IF 16V elect100n polyesterli.tF 16V elect

SEMICONDUCTORS

Dl -D7D8TR1ICIIC2

1N4148 (7 off)red -green led (see text)BC5493244017

POTENTIOMETERS

VR1 1M lin rotaryVR2, VR3 100k skeleton preset (2 off)

SWITCHES

S 1 min dpst toggleS2 1pl2w rotaryS3 min spst toggle

MISCELLANEOUS

PP3 battery clip, pcb supports (4 off), 14 -pin dil ic socket, 16 -pin dil ic socket, monojack socket, knobs (2 off), printed circuitboard, box to suit, solder and wire.

10 7 6

12

5

Fig 3. Wiring details1 BATTERY

16 15

2

VEQV)

178

20a L_J 14 19

V1S1 OUTSPEED

START/STOP01G174bi

BATTERYVE

(9V -15V1

ON /OFF

13

SHORTLEAD

21

22

reset, its '0' output goes high, and generates apulse across C4 and R13. Since C4 has a greatervalue than C3, a heavier pulse will be amplifiedby ICIb.

FLASHY BITS

Now for the flashy bit! I've used !Cie andICId as a special gating circuit driving a bi-colour led. Depending on the polarity of thevoltage across it, the led will show red, or green,or nothing at all. The polarity across the leddepends on the output states of ICIc/d. Theseare conditioned by the logic states of their inputsas controlled by R7, DI, D2 and D3. There arefour output combinations: both outputs high,both low, and either output high with the otherlow. In the first two states neither part of the ledwill be active, but in the latter two states therelevant colour will glow, depending on thepolarity. Because of the action of D3, the outputfrom IC2 pin 2 will override the output of TRI,thus each normal beat will show as one colour,with the accented beat shown by the other.Between each beat neither led will glow.

DIODE GATING

The action is as follows: Basically, bothopamps are connected as comparators with thereference level set by the voltage at R5/R6.Ignoring the action of D3 and D4 for themoment, when the collector of TRI is low so toowill be the inverting inputs of IC1c/d, via DI andD2, consequently both outputs will be high andthe leds don't glow. When TRI collector is high,Dl and D2 block the positive voltage fromreaching the opamps. Instead they are biased bythe positive voltage through R7 and R8, and thusboth outputs are low and the leds still don't glow.

However, when the '0' output of IC2 (pin 2)is low, as it will be for each unaccented beat, theinverting input of ICIc will also be held low viaD4, and that opamp output will be high. Whenthe collector of TR1 is also high the output ofIC Id will be low, therefore current will flowfrom ICIc to ICId via the respective conductingled, which will then glow. When IC2 pin 2 ishigh, on the accented beat, and TR1's collector isalso high, the output of ICIc will go low, pullingdown the inverting input of IC Id via D3. Thusthe output of IC ld will be forced low, and thecurrent will flow in the opposite direction,through the other led.

When SI is switched off, not only will TR1cease to conduct, but the counter will automaticallybe reset to zero by Slb, ensuring that the beatpattern always starts from the same point.

The unit is intended for powering by a 9Vbattery, such as a PP3 or PP9, though it may alsobe powered by any voltage up to 15Vdc.

CONSTRUCTION

The main points to observe in constructionare the correct polarity of all the semiconductors,though the led may be inserted either way round,depending on which colours you want to markthe beats. You may alternatively use two separateleds of different colours, inserted in oppositedirections.

With the switch S2, the locating washer underthe crimped washer should be adjusted so thatonly eleven positions can be selected. In

positions 1 and 11 no accent will be seen orheard - each beat will be the same.

Initially, set VR2 midway then, once youknow the circuit is functioning, readjust it untilVR1 varies the rate within the range required.

The audio output should be taken to the lineinput of an ordinary home amplifier. VR3 shouldfirst be set at minimum resistance (least gainthrough ICIb), and if necessary then increased toprovide greater pulse amplification. However, donot increase VR3 too much otherwise the beatemphasis could be lost. The settings of theamplifier tone controls could also affect theaudible difference between the marked andunmarked beats.


0 0 rYour Ed looks at some of the new books recently received.

Soft Ferrites - Properties and Applications - 2ndEdition. E.C. Snelling. Butterworths. £60. ISBN 0-40$-02760-6,Very much a book for the advanced constructor/designer. It is concernedwith the technically important properties of magnetically soft ferrites atfrequencies up to about 100MHz, and the application of these ferrites toinductors, transformers and related devices. It is primarily intended forelectronic engineers and physicists whose work involves design ordevelopment using these ferrites. The first four chapters deal with theproperties of ferrites and magnetic materials in general. The next sixchapters deal with a basic type of ferrite cored device, includinginductors, high frequency transductors, transformers and antennas. Thefinal chapter examines the properties of windings and is followed by fourappendices covering calculations, wire tables, publications and symbols.A comprehensive list of references and bibliography follows each chapter.Undoubtedly a highly informative book which has been thoroughlyupdated since the first edition of 1969.

Audio IC CircuitsManual. R.M. Marston.Heinemann Newnes. £10.95. ISBN0-434-91210-7. I am sure thatanyone who is interested in designingand building audio circuits will findthis book to be an extremely usefuladdition to the workshop library. It'sfull of circuit building blocks foraudio processing, preamps, poweramps, high -power amps, led bargraphdisplays, ccd delay -lines and powersupplies. A reasonable selection of ictypes is quoted, most of them readilyavailable, and although I know that acouple of chips listed are obsolete,havpg been deleted by themanufacturer (TDA1022 andTDA1097), circuits using alternativechips are given. I would also liked tohave seen more formulae given - not of the higher maths type, but ofthe more usual common -or -garden variety. Unless basic formulae arecontained in a book of this nature, the adventurous constructor can beat a loss for information on how to modify the circuits shown to caterfor different parameters - the filters section is one such area that wouldhave benefitted. Nonetheless, I am pleased to recommend this book toaverage -capability readers.

The Digital IC Handbook. Michael S. Morley. Tab Books£39.30. ISBN 0-8306-9302-5. "Fast, accurate information guaranteed tosimplify your search for the right digital ic", is the claim made on thecover of this book. No, Tab, I cannot accept that this statement will betrue for the majority of electronics designers in Britain. I acknowledgethat some of the text material on the nature of general logic families maybe of use to those whose understanding of electronics is still rudimentary.But the rest of the information, taking up most of the pages, seemsalmost totally irrelevant to beginners or experienced designers. The bulkof the pages principally consist of lists and more lists of device numbers,brief descriptions of their functions, a few parameters, and the price in USdollars. As one who spends much of his time designing, I believe that Iwould find little practical benefit from the data. There should at least bethe pin -out configurations shown, and what use is the dollar price to thosewho buy products in Britain? I find that there is usually no substitute forThe data books produced by chip manufacturers, and these books arereadily available and not too expensive. Most manufacturers will be onlytoo happy to sell (and sometimes to give away free) their data books toseriously interested designers. Distributors such as RS Components andElectromail also sell a good selection of data books. (If any PEadvertisers also sell them, publicise the fact by telling me.) I would farsooner have readers buy slightly outdated real data books than spend£39.30 on a book that looks impressive but which is far too short onuseful content.

AUDIO ICCIRCUITS MANUAL

R. M. MARSTON

Computers and Music.R.A.Penfold. PC Publishing.£7.95. ISBN 1-870775-07-4.Robert Penfold is, of course, oneof PE's well respected authors. Asusual, he's in top form with thisbook that should interest anyonewho wants to know more aboutcomputer controlled musicmaking. Although I felt wary ofthe statement on the back of thebook that 'No previous computerknowledge is needed', manypeople may well find thestatement to be true, for Robertgives a thorough covering of thebasics of computing, devoting fourchapters and over 80 pages to thesubject. In those pages he startsright back at the computerkeyboard, progresses through rams and eproms, discusses ports andperipherals, and takes a look at actual computers. Among them, hecomments on the Commodore 64/128, BBC Master, AmstradCPCs, Spectrum, Atari ST, Amiga, Yamaha Cl, and IBM PCs andcompatibles. The remaining three chapters then discuss specificsabout Midi, music software, and Midi instruments. Finally, threeglossaries define terms, give hexadecimal numbers, and offershopping advice on printers, computers, synthesisers and samplers,and software. An extremely useful book.

Practical Electronicsfor GCSE. Owen Bishop. JohnMurray (Publishers) Ltd. £6.95.ISBN 0-7195-4632-X. As well asbeing a frequent and highlyrespected contributor to PE, Owenis the author of very many books. Iam delighted by the way in whichhe is presenting the information inhis latest book. There is a similarityin style and illustrative presentationthat will be familiar to those whohave been following Owen's recentDigital Electronics series for PE.Many of the simpler instructionalcircuit assemblies are designed foruse with plug-in breadboards,though Veroboard is also used for some of them. The publishersclaim that the book offers the most comprehensive course of fullydocumented electronics practicals yet made available at this level. Ican well believe it, and Owen makes the subject interesting. Thebook is split in to five parts: Electronics Workspace, Practicals,Modules, Electronic Systems, Data. The first section looks atcomponents and tools, power supplies, measuring, andconstruction, with a selection of useful photographs tracing theassembly of a project. The Practicals section starts off looking atelectric current, following on with aspects such as resistance,capacitance, inductance, and then through diodes, transistors, logicgates, memories, amplifiers, and filters. Under Modules comevarious subjects, including input, process, and output modules. Partfour has a selection of useful and practical circuits to build, andpart five is a short appendix giving a few ic and transistorconnections and design data, concluding with a summary of thesymbols used in the book. This is undoubtedly a very thoroughlythought out book and will be of immense value to anyone in theearly stages of learning about electronics, even if they are not doingGCSE.

Build Your Own 80386 IBM Compatible andSave a Bundle. Aubrey Pilgrim. Tab Books. £13.20.ISBN 0-8306-3131-3. If you live in America you probablycould 'save a bundle', but otherwise it could cost you a packettrying to obtain the parts from the US suppliers quoted. In anycase, the assembly procedure only takes up a small part of thebook, and the relevant illustrations appear mainly concernedwith showing you how to insert plugs into sockets. Theremainder of the book is largely concerned with descriptions ofvarious peripheral devices, backup systems and troubleshootingtips, the latter basically of the commonsense variety "if it breaksdown try not to panic...if you can't solve the problem, call yourvendor for help".

Practical Electronics ir GCSE

OVVEN BISHOP

"-


Designing with Linear ICs. G.C.Loveday. Benchmark BookCompany. £8.75. ISBN 1-87104702-1. One aspect of this bookimmediately takes my interest, that it realistically discusses the use ofproprietary cad software for analysis of opamp circuit responses. Thebook is aimed at the serious user of linear ics but will also be of interest tothose who have only rudimentary knowledge and wish to know more.Some of the sections require an understanding of mathematics, but not ata particularly high level. There are numerous illustrative examples ofpractical circuits, mainly associated with opamps, though a whole chapteris devoted to timer ics. The fifth chapter sets some worked designproblems, dealing with the concepts, procedures and rules of thumbrequired in analogue design. Solutions to the problems are given in thefinal chapter. This appears to be an invaluable book to have on one'sbookshelf or workbench. It's also apparent that Mr Loveday should besharing his knowledge with electronic hobbyists through PE's pages -I'llbe looking into it!

Laser Experimenter'sHandbook - 2nd Edition.Delton T. Horn. Tab Books. 11.10.ISBN 0-8306-3115-1. First, AWARNING - lasers can bedangerous and the projectswithin this book are not forinexperienced constructors. Thebook gives backgroundinformation on the theory andhistory of lasers while offering anumber of practical, simpleexperiments that use relativelylow power and present minimumrisks. Those projects include asimulated laser, an infrareddetector, a continuous wave solidstate laser, and modulated laser beam transmitter and receiver. Although Iwarn about the potential danger of the projects, the overall informationgiven in this book will be of interest to those who wish to know moreabout lasers in theory and in practice. For those who are experienced andresponsible constructors, the five projects should make interestingexperimental circuits. Note, though, that the constructional information isfairly minimal and that you will probably need to purchase some of thecomponents direct from the American suppliers quoted. Within theconstraints of my warning, this book is worth reading.

The Laser Cookbook - 88 Practical Projects. GordonMcComb. Tab Books. 14.25. ISBN 0-8306-9390-4. I spent some timeconsidering whether or not I should give publicity to this book - some oftheprojects are potentially extremely dangerous. I am also horrified bythe humorous(?) phrase in the introduction about "playing 'laser -tag' withthe cat". Lasers are too dangerous to be used in any situation that is notunder very close control they can blind, and some of the power supplyvoltages can be lethal. However, there is much information in this bookwhich will be of interest to experienced and responsible constructors andit is for their benefit that I have decided not to 'censor' the book. It hasover 400 pages, and contains 24 chapters, several of which are concernedwith laser theory, safety procedures, required tools and where to obtainparts. Among the topics, the book shows how lasers can be used inholography, optics and optical experiments, laser guns, laser light shows,laser beam intrusion and detection systems, aerodynamics and airflowstudy, coherent light seismology, laser beam communication, laser andfibreoptics computer data links, and precision measurement. There is nodoubt that this is an interesting book, but if you buy it, DO TAKE CARE.

Computer Hobbyists Handbook. R.A. & J.W. Penfold.Babani. 5.95. ISBN 0-85934-196-8. Robert (of PE fame) has combinedwith his brother to produce this large format 120 page handbook. ThePenfolds say in their preface : ''The aim of this book is to provide a usefulrange of data and general information on a variety of computer topics,including such things as interfaces, computer languages, Midi, andnumbering systems. There is also a useful lexicon of computer terms, andsome helpful appendices. This book is not intended to be a course incomputing, but does provide a useful reference for data and informationin a single source where it can be quickly and easily found. It does not

.simply supply raw data, but where necessary detailed explanations arealso included. It should be equally useful to both beginners and moreexperienced hobbyists." Yes, I think I can go along with that so far asbrief examination of the book shows. It certainly appears to be a book towhich one would turn for subject definitions. However, doing a check ona few items of relevance to one of my computers, an Amstrad 1640, I wasforced to conclude that my user handbooks would still be needed.Probably the book will be of greater value to an early starter than to onewho has been involved with computing for many years.

The Handbook ofMicrocomputer interfacing Second Edition. Steve Leibson.Tab Books. 15.60. ISBN 0-8306-3101-1. This is good healthy volumeof over 300 pages which is basicallyabout how to get information into andout of computers. In other words, it'sabout interfacing. Its nine chapterscover Bits Bytes and Buses,Component -Level Busses, BackplaneBusses, Parallel Interfacing, SerialInterfacing, Interfacing to AnalogDevices, Time, Interrupts, and DirectMemory Access Interfacing. Threeappendices cover a number basecalculator program, integrate chips and is manufacturers. The bookconcludes with a parts list and index. The information presented will be ofinterest to any reader (.whichever side of the Atlantic they live!) who is after agood introduction to computer hardware and I am pleased to recommend it.

Encyclopaedia of Electronic Circuits - Volume One.Rudolf F. Graf. Tab Books. 28.15. ISBN 0-8306-1938-0. I said of VolumeTwo when I reviewed it a few months ago that I wished I had Volume One aswell. Now I have, thanks to the helpfulness of Karen Brown at John Wileyand Sons, the importers. This is as much a book for every enthusiast as itscompanion. It has over 750 pages and shows nearly 1300 circuits! Thecircuits are arranged in alphabetic subject order within 98 basic circuitcategories. There is an extremely thorough master index, and an unusualindex as well - this quotes the original source of the circuits, iemanufacturer's data sheet, electronics magazine etc, and will be of use shouldadditional information be required. I am pleased to possess both volumes ofthis encyclopaedia, and thoroughly recommend either or both of them to anyelectronics experimenter. The price is certainly not high for the quantity ofinformation offered. This is sort of book at which Tab excel, and deserves tobe on the workbench.

The Complete Electronics Career Guide. Joe Risse. TabBooks. 12.60. ISBN 0-8306-3110-0. There is no in which I can say that thisbook is suitable for the UK market. It cannot possibly be relevant when all theinformation regarding companies and qualifications is oriented to theAmerican market. There might be the occasional paragraphs that cross theAtlantic successfully, but they will be few and far between. It seems highlyirresponsible for Tab to introduce this book to the UK.

How to Draw Schematics and Design Circuit Boardswith Your IBM-PC. Steve Sokolowski. Tab Books. 11.10. ISBN 0-8306-9334-3. This certainly appears to be a useful book from Tab, and is applicableto IBM -PCs or compatibles. Purchasers of the book can order the software ondisk using a reply card supplied with the book, or they can key it in from thelengthy Basic listings published. The author shows how you can draw lines,circles and schematic symbols with your PC using the directional arrow onthe keyboard. There are four chapters in this nearly -200 page book. The firstone covers the Schematic Designer program, looking at how graphics aredisplayed, how they can be drawn and new symbols added, and a practicalcircuit drawing is illustrated, for a power supply. Chapter two gives the pcbdesigning program, discussing various aspects of it, and discussing how tomodify the program. CAE program screen displays are discussed in chapterthree. Chapter four is a series of appendices, covering such items as thehierarchy of cae reserve words, a summary of cae commands, making backupcopies, and a glossary of terms.

THE HAN131300101: MK:11::::COMI'l1TIEll

INTEllFACINC-Second Edition

PUBLISHERS' ADI RESSES

Bernard Babani (Publishing) Ltd, The Grampians, Shepherds Bush Road,London W6 7NF. 01-603 2581.

Benchmark Book Company, 18 Regency Close, Uckfield, E. Sussex TN22IDS.

Butterworth Scientific Ltd., Westbury House, Bury Street, Guildford GU25BH.

John Murray (Publishers) Ltd., 50 Albermarle Street, London W1X 4BD.

Heinemann-Newnes, 22 Bedford Square, London WC1B 3HH. 01-637 3311.

PC Publishing, 4 Brook Street, Tonbridge, Kent TN9 2PJ.

Tab Books are imported by John Wiley and Sons Ltd, Baffins Lane.Chichester, W. Sussex, P019 1UD. 0243 779777.


No. 1 LIST BAKERS DOZEN PACKSAll packs are £1 each, if you order 12 then you are entitledto another free. Please state which one you want. Notethe figure on the extreme left of the pack ref number andthe next figure is the quantity of items in the pack, finallya short description.

BD2 5 13A spurs provide a fused outlet to a ring main wheredevices such as a clock must not be switched off.

BD7 4 In flex switches with neon on/off lights, saves leaving things

switched on.BD9 2 6V to mains transformers upright mounting with fixing

clamps.BD11 1 Van speaker cabinet ideal for extensions, takes our

speaker. Ref BD137.BD13 12 30 watt reed switches, it's surprising what you can make

with these - burglar alarms, secret switches, relay, etc., etc.

BD22 2 25 way loud speaker two unit cross-overs.

BD29 1 B.O.A.C. stereo unit is wonderful breakdown value.

BD30 2 Nicad constant current chargers adapt to charge almost any

nicad battery.

BO32 2 Humidity switches, as the air beconvhs damper themembrane stretches and operates a microswitch.

BD42 5 13A rocker switch three tags so on/off, or change over with

centre off.BD45 1 24hr time switch, ex -Electricity Board, automatically adjust

for lengthening and shortening day original cost £40 each.

B049 10 Neon valves, with series resistor, these make good nightlights.

BD56 1 Mini uniselector, one use is for an electric jigsaw puzzle, wegive circuit diagram for this. One pulse into motor, movesswitch through one pole.

BD59 2 Flat solenoids - you could make your multi -tester read ACamps with this.

BD67 1 Suck or blow operated pressure switch, or it can beoperated by any low pressure variation such as water levelin water tanks.

BD91 1 Mains operated motors with gearbox. Final speed 16 rpm, 2watt rated.

BD103A 1 6V 750mA power supply, nicely cased with mains input and6V output leads.

BD120 2 Stripper boards, each contains a 400V 2A bridge rectifierand 14 other diodes and rectifiers as well as dozens ofcondensers, etc.

BD128 10 Very fine drills for pcb boards etc. Normal cost about 80peach.

BD132 2 Plastic boxes approx 3M cube with square hole through topso ideal for interrupted beam switch.

BD134 10 Motors for model aeroplanes, spin to start so needs noswitch.

BD139 6 Microphone inserts - magnetic 400 ohm also act asspeakers.

BD148 4 Reed relay kits, you get 16 reed switches and 4 coil sets with

notes on making c/o relays and other gadgets.BD149 6 Safety cover for 13A sockets -prevent those inquisitive little

fingers getting nasty shocks.

BD180 6 Neon indicators in panel mounting holders with lens.

BD193 6 5 amp 3 pin flush mounting sockets make a low cost discopanel. Need cable clips

BD196 1 in flex simmerstat - keeps your soldering iron etc. always atthe ready.

80199 1 Mains solenoid, very powerful, has tin pull or could pusn ifmodified.

BC1201 8 Keyboard switches - made for computers but have manyother applications.

60211 1 Electric clock, mains operated, put this in a box and youneed never be late.

BD221 5 12V alarms, make a noise about as loud as a car horn.Slightly soiled but OK.

BD242 2 bin x 4in speakers, 4 ohm made from Radomobile so very

good quality.BD252 1 Panostat, controls output of boiling ring from simmer to boil.

BD259 50 Leads with push-onb 1/4in tags - a must for hook-ups -mains connections etc.

BD263 2 Oblong push switches for bell or chimes, these can mainsup to 5 amps so could be foot switch if fitted into pattress.

60268 1 Mini 1 watt amp for record player. Will also change speed ofrecord player motor.

BD283 3 Mild steel boxes approx 3M x 3in x lin deep - standardelectrical.

B13293 50 Mixed silicon diodes.BD305 1 Tubular dynamic mic with optional table rest.

BD400 4 Books, useful for beginners, describes amplifiers,

equipment and kit sets.

BD653 2 Miniature driver transformers. Ref. LT44.20k to 1k centre tapped.

80553a 2 3.5V relays each with 2 pairs changeover contacts.

There are over 1,000 items in our Bakers Dozen List. If you want a completecopy, please request one when ordering.

EOUIPMEt4T WALL MOUNT It is a multi -adjustable metal bracket tht couldbe used for mounting flood light,loud speakers, TV camera, even a tan and onalmost any sort of wall or ceiling even between wall and ceiling. The mainfixing brackets rotate such that an inward or an outward corner can beaccommodated. Front panel also tilts upwards or downwards to a reasonableangle and can be easily removed separately for wiring. A very useful bracket.Regular price wuld be around £6 each. Our price only £3. Our ref 3P72. Or 2for £5. Our ref 5P152.

EXTRA SPECIAL CROC CLIPS Medium size. just right for most hook-ups.Normally sell for around 10p to 15p each. These are insulated and have alength of wire connected to them but this is very easy to snip off if you do notneed it. 20 for £1.Our ref B0117A.

COPPER CLAD PANEL for making PCB. Size approx 12in long x 81/2in wide.Double -sided on fibreglass middle which is quite thick (about1/2sin) so thiswould support quite heavy components and could even form a chassis to holda mains transformer, etc. Price £1 each. Our ref B0683.

POWERFUL IONISERGenerates approx 10 times more IONS than the ETI and similarcircuits. Will refresh your home, office, workroom, etc. Makes youfeel better and work harder -a complete mains operated kit, caseincluded. £12.50 plus £2 postage. Our ref 12P5/1.

REAL POWER AMPLIFIER for your car, it has 150 watts output. Frequencyresponse 20hz and signal to noise ratio better than 60d/b. Has built in shortcircuit protection and adjustable level to suit your existing car stereo, so needsno pm -amp. Works into speakers. Ref 30P7 described below. A real bargainat only £57.50. Order ref. 57P1.

REAL POWER CAR SPEAKERS Stereo pan output 100w eaach. 4 -Ohmimpedence and consisting of 61/2" woofer, 2" mid range and 1" tweeter. Eachset in a compact purpose built shelf mounting unit. Ideal to work with theamplifier described above. Price per pair £29.96. Order ref. 30P7.

STEREO CAR SPEAKERS Not quit so powerful - 70w per channel. 3"woofer, 2" mid range and 1" tweeter. Again, in a super purpose built shelfmounting unit. Pnce per pair £27.95. Order ref. 28P1.

ELECTRONIC SPACESHIP Sound andimpact controlled, responds to claps andshouts and reverses when it hits anything.Kit with really detailed instructions. Idealpresent for budding young electrician. Ayoungster should be able to assemble butyou may have to help with the soldering ofthe components on the pcb. Complete kit£8. Our ref 8P30.

12" HIGH RESOLUTION MONITOR Black and white screen, beautifullycased for free standing, needs only a 12v 1.5amp supply. Technical data is onits way but we understand these are TTL input. Brand new in maker's cartons.Pnce £25.00 plus £5 insured delivery. Order ref. 25P10.

14" COLOUR MONITOR made by the American Display Tek company. Useshigh resolution tube made by the famous Japanese Toshiba company.Beautifully made unit intended for console mounting, but top and sidesadequately covered by plated metal panels. Full technical spec. on its way tous. We have a limited number of these. All brand new still in maker's cartons.Price £89 each plus f6 insured carriage. Order ref. 89P/1.

BUSH RADIO MIDI SPEAKERS Stereo pair, BASS reflex system, using a fullrange 4in driver of 4ohms impedance. Mounted in very nicely made blackfronted walnut finish cabinets. Cabinet size approx 81/2in wide, 14in high and31/2in deep. Fitted with a good length of speaker flex and terminating with anormal audio plug. Price £5 the pair plus £1 post. Our ref 5P141.

31/2 FLOPPY DISC DRIVE - We still have two models in stock, Single sided80 track by Chinon. This is in the manufacturers metal case with leads and ICDconnectors. Prime £40 reference 40P1. Also a double sided 80 track. This isuncased. Price £59.50 reference 60P2. Both are brand new. Insured delivery£3 on each or both.

ATARI 65XECOMPUTER

-/ At 64k this is most powerful andsuitable for home and business.Brand new, complete with PSU. TVlead, owner's manual and six

games. Can be yours for only £45plus £3 insured delivery.

REMOTE CONTROL FOR YOUR 65XE COMPUTER. with this outfit you canbe as much as 20 feet away as you will have a joystick that can transmit anda receiver to plug into and operate your computer and TV. This is also just rightif you want to use it with a big screen TV. The joystick has two fire buttons andis of a really superior quality, with four suction cups for additional contort andone handed play. Price £15 for the radio controlled pair. Our ref 15P27.

65XE COMPENDIUM Contains: 65XE Computer, its data recorder XC12 andits joystick, with ten games for £62.50 plus £4 insured deliver.

ASTEC PSU Mains operated switch mode, so very compact. Outputs: +12v2.5A, +5v 6A, ± 5v .5A, ± 12v .5A. Size: 71/4 long x 4%in wide x 24'un high.Cased ready for use. Brand new. Normal price MO+, our price only £12.95.Our ref 13P2.

VERY POWERFUL 12 VOLT MOTORS. 1/2rd Horsepower. Made to drive theSinclair C5 electric car but adaptable to power a go-kart, a mower, a rail car,model railway, etc. Brand new. Price £20 plus £2 postage. Our ref 20P22,

PHILIPS LASERThis is helium -neon and has a power rating of 2mW. Completely safe aslong as you do not look directly into the beam when eye damage couldresult. Brand new, full spec. £30 plus £3 insured delivery. Mainsoperated power supply for this tube gives lky striking and 1.25kv at 5mArunning. Complete kit with case £15. Ditto for 12v battery. Also £15. Ourref 15P22.

ORGAN MASTER Is there a three octave musical keyboard. It is beautifullymade. has full size (piano size) keys, has gold plated contacts and is completewith ribbon cable and edge connector. Can be used with many computers. Wecan supply information sheet. Brand new. only £15 plus £3 postage. Our ref15P15.

FULL RANGE OF COMPONENTS at very keen prices are available from ourassociate company SCS COMPONENTS. You may already have theircatalogue, if not request one and we will send it FOC with your goods.

FULL RANGE OF COMPONENTS at very keen prices are availablefrom our associate company SCS COMPONENTS. You may alreadyhave their catalogue, if not request one and we will send it FOC wily yourgoods.

HIGH RESOLUTION MONITOR. In black and white, used Philips tube M24305W. Made up in a lacquered frame and has open sides. Made for use withOPD computer but suitable for most others. 5 and new. C16 plus £5 post. Ouref 16P1.

12 VOLT BRUSHLESS FAN. Japanese made. The popular square shape141/2 x 41/2 x 17in). The electronically run fuse not only consume very littlecurrent but also they do not cause interference as the brush type motors do.Ideal for cooling computers, etc. or for a caravan. £8 each. Our ref 8P28.

MINI MONO AMP on p.c.b. size 4" x 2" (app.)Fitted volume control and a hole for a tone con-trol should you require it. The amplifierhas three transistors and we estimatethe output to be 3W rms. Moretechnical data will be includedwith the amp. Brand new, perfectcondition, offered at the very lowprice of £1.15 each, or 13 for f12.00.

J & N BULL ELECTRICALDept PE, 250 PORTLAND ROAD, HOVE

BRIGHTON, SUSSEX BN3 5QT

MAIL ORDER TERMS: Cash, PO or cheque with order. Orders under£20 add £2.00 service charge. Next day delivery available £3.00 extra.Monthly account orders accepted horn schools and public companies.Access and B/card orders accepted minimum £5. Phone (0273)734648 or 203500.

POPULAR ITEMSMANY NEW THIS MONTH

SUB -MIN SWITCHES Not much bigger than a plastic transistor but doublepole. PCB mounting. Three for £1 Our ref 80688

CARTRIDGED for the Double Microdrive. Price 4 for £5. Our ref 5P146.

NICAD CHARGER UNIT Metal pronged, plastic case contains mainstransformer and rectifiers with output lead and plug -made to charge two cellsbut no doubt adaptable or wonderful spares value. ONly 50p each, two for £1.Our ref. 06386

EDGEWISE PANEL METER. If you are short of panel space then this maybeh answer. It has a FSO of 100uA and a nice full vision scale. It fits througha hole approx. 1'/sin x'Mn. Mother feature is that it has an indicator lampbehind the scale which you could light up. It would then serve as an on/offindicator. Price £1.Our ref BD700.

AA CELLS. Probably the most popular of the rechargeable NICAD types. 4for £4. Our ref 4P44.

COMPUTER SPECIAL The Porex 16meg Byte tape streamer. These arebrand new and really an exceptional bargain. A few only so hurry. Only £15.Our ref 15P29.

20 WATT 4OHM SPEAKERS With built-in tweeter. Really well made unitwhich has the power and the quality for hi-fi reproduction. 61/4in diameter.Price £5.Our ref 5P155. It is heavy so please add £1 to cover postage if notcollecting.

MINI RADIO MODULE only about 2in square with ferrate aerial and solid daituner with its own knob. It is a superhat and it operates from PP3 battery andwould drive a crystal headphone direct but be better with our mini mono amp.Price £1. Our ref B0716

BULGIN MAINS PLUG AND SOCKET The old faithful 3 pin screw terminals.The socket mounts through a 11/2in hole and the mains is brought in by theinsulated plug. Used to be quite expensive but yo can have 2 pairs for £1 or 4or either plug or socket for £1. You could make yourself a neat and compactbench panel with these. Our ref B0715 or BD715P.

MICROPHONE If you want a low cost microphone then just arrived we havea very small hand-held dynamic mic with on/off switch in the handle. Its leadterminates with one 3.5 plug and the other a 2.5 plug for remote control. Priceonly f 1. Our ref BD711

EXTENSION CABLE WITH A DEFERENCE It is flat on one side making iteasy to fi and to look tidy. It is 4 core so suitable for telephone, bell, burglaralarms, etc. 50 yard coil for £5. Our ref 5P153.

MOSFETS FOR POWER AMPUFIERS AND HIGH CURRENTDEVICES 140v 100w pair made by the famous Hitachi Company.Reference 25K413 and its compliment 25J118. Only £4 the pair. Our ref4P42.

BATTERY OPERATED TRAVEL MECHANISM On a plastic panemeasuring approx 9in x 31/2in. Is driven by a reversible 12v battery motorfitted with pulley and belt which rotates a threaded rod and causes a platformto travel backwards and forwards through a distance of approx 5in. Price £5Our ref 5P140.

MAINS OPERATED WATER VALVE with hose connection for inlet andoutlet suitable for low pressure. Auto plant watering, etc. Only £1 each. Our refBD370.

20 VOLT 4 AMP MAINS TRANSFORMER Upright mounting with fixing feet.Price £3. Our ref 3P59.

150HM PIA SPEAKERS Approx Tin x 4in. 5 watts. Offered at every low priceso you can use two in parallel to give you 10 watts at 8 ohms. £1 for the two.Our ref BD684.

EHT TRANSFORMER 4kv 2mA Ex -unused equipment. £5. Our ref 5P139.

4 CORE TINSEL COPPER LEAD As fitted to telephones, terminating with flatBT plug. 2 for £1.Our ref BD639.

EHT TRANSFORMER 8kv 3mA. £10. Our ref 10P56.

VERY USEFUL MAGNETS Flat, about lin long, 1/2in wide and Van thick. Verypowerful. 6 for f 1. Our ref BD274(a).

ACORN COMPUTER DATA RECORDER REF ALF03 Made for the Electronor BBC computers but suitable for most others. Complete with mains adaptor,leads and handbook. £10.00. £2 special packing. Ref 10P44.

SOLAR CELLS Will give a good current (depending on size) from sunlight orbright daylight. Module A gives 100mA. Price £1. Our ref BD631. Module Cgives 400mA. Price £2. Our ref 2P199. Module D gives 700mA. Price £3. Ourref 3P42.

SOLAR POWERED NI -CAD CHARGER 4 Ni-Cad batteries AA (HP7)charged in eight hours or two in only 4 hours. It is a complete, boxed ready touse unit. Price £6. Our ref. 6P3.

METAL PROJECT BOX Ideal size for battery charger, power supply etc,;sprayed grey, size Bin x 4'/.in high, ends are louvred for ventilation other sidesare flat and undrilled. Order Ref. 2P191. Price £1.

CAPACITOR BARGAIN - axial ended, 4700AF at 25V. Jap made, normally50p each, you get 4 for El . Our ref. 613.

SINGLE SCREENED FLEX 7.02 copper conductors, pvs insulated then withcooper screen, finally outer insulation. In fact quite normal screened flex. 10mfo £1.Our ref 06668.

3 -CORE FLEX BARGAIN No. 1 - Core size 1.25mm so suitable for longextension leads carrying up to 13 amps, or short leads up to 10 amps. 15mmfor £2. Ref. 2P190.

3 -CORE FLEX BARGAIN No. 2 - Core size 1.25mm so suitable for longextension leads carrying up to 13 amps, or short leads up to 25A. 10m for £2.Ref. 2P190.

ALPHA -NUMERIC KEYBOARD - This keyboard has 73 keys giving troublefree life and no contact bounce. The keys are arranged in two number pad,board size is approx. 13" x C - brand new but offered at only a fraction of itscost, namely £3 plus £1 post. Rel. 3P27.

WTH HORSEPOWER 12 VOLT MOTOR Made by Smiths, the body length ofthis is approximately 3M, the diameter 3M and the spindle Vieth of an inchdiameter. It has a centre flange for fixing or can be fixed from the end by meansof 2 nuts. A very powerful little motor which revs at 3,000 rpm. We have a largequantity of them so if you have any projects in mind then you could rely onsupplies for at least two years. Price £6. Our ref 6P1, discount for quantities of10 or more.

SUM -MIN TOGGLE SWITCH Body size Bmm x 4mm x 7mm SBDT withchrome dolly fixing nuts. 4 for £1.Our ref BD649.

52 PRACTICAL ELEC FRONICS DECEMBER 1989

APPOINTMENTS

UNDERGROUND

P

An important message forCommunications Technicians

1111111111 0 ° °0 6/ 1 )1,41 1 1 6000

11111111111A/ 111111111111

11111111 111111° igligiv\

III. I lflll I 10111111P

Advanced electronics is playing anincreasingly major role in the developmentof London Underground. As we moderniseour stations, offices and railway sites, wecan now offer more scope and opportuni-ties for Communications Technicians thanever before - installing closed circuittelevision and on -train radio, public addresssystems, telephone and data networks.

You will work a basic 38 -hour week ata weekly rate of £212 but, with theopportunities for overtime and night work,can expect to earn around £286. You willalso enjoy free travel on the Undergroundand most London Buses-plus 75%discount on British Rail for you and yourfamily. Benefits include 25 days holiday

a year-and genuine prospects forpromotion.

If you have had technical training, inany two of the specialist disciplinesmentioned above, and hold a recognisedqualification, this could be youropportunity to get ahead fast in ourmodern, forward -looking organisation.

For further information and anapplication form, please fill in the coupon,quoting Ref. CT3 and post it to: Miss AshleySezgin, London Underground Limited,Signal & Electrical Engineering (Contractors)Unit, 3rd floor, Griffith House, 280 OldMarylebone Road, London NW1 5RJ.

We are an equal opportunities employer.

COMMUNICATIONS TECHNICIANSPlease send me an application form for the post of Communication Technician:

NAME

ADDRESS

POSTCODECT3

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pages: Rates 20p per word or £8.50 per single column cm (plus VAT). All classifiedadvertisements must be pre -paid. Send your copy with the remittance (payable to Intra Pressor payment by Visa or Access accepted) to: Practical Electronics, Intra House, 193 Uxbridge

Road, London W12 9RA. Tel: 01-743 8888. Fax: 01-743-3062LET PE WORK FOR YOU!

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*Those eligible can apply for E.T. grant support*

* An equal opportunities programme *

COURSES COMMENCE

Monday 8th Jan. 1990

LONDON ELECTRONICSCOLLEGE

Dep: AA, 20 Penywern Road,

London SW5 9SU. Tel: 01-373 8721

Start training now forthe following courses.Telecomms Tech C&G 271

CI Radio Amateur Licence C&GO MicroprocessorO Introduction to Television

Send for our brochure - without obligation ortelephone us on 06267 79398 (Ref: PE12/89)Name

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OMNI ELECTRONICSstock a wide range of

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Electronic components, I.C's, diodes, LED's,capacitors, potentiometers, solar chargers,computer disks, video tapes, aerosols, etc.

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54 PRACTICAL ELEC1RONICS DECEMBER 1989

CLASSIFIEDSResistors 1/4 W 5% carbon (E12) Ip metal film 1% .. 3pResistor Pack 85 different El2 values + zero ohm link total content1000 resistors 18.95LEDs red/green 3/5mm 6p each. Yellow I 1pCable ties 75mm I p each 15.95/1,000 £49.50 per 10,000Stepping motor 4 phase 12v 7.5' step 50 ohms .......... ............. ....18.95SAA1027 stepping motor driver chip 13.95FM transmitter Kit good quality sound f7.94High quality photo resist toper clad epoxy glass boardsDimensions single sided double sided3x4 inches 10.95 f 1.074x8 inches f2.40 f2.686x12 inches £5.3712x12 inches £10.66

Special OffersComputer Grade Capacitors with screw terminals 38000uf 20v £2.508700uf 10v f 1.95, 68000uf 15v £2.95, 10000uf I6v11.507 segment Commom anode led display 12min 10.45LM2931AT5.0 Low drop out 5V regulator T0220 package 10.85BS250 P channel MOSFET 10.45, BC559 transistor ....13.95 per 10074LS05 hex inverter f 10.00 per 100, used 8748 Microcontroller 13.50Stereo LW/MW/FM Tuner pre -amp assembly complete withvolume/tone controls and tuning scale Brand new in makers carton

15.95, faulty 12.50Circuit diagram description and setting up procedure for tunerassembly described above (0.50.5 digit 6v electromagnetic counter ...

11.9510.9512.5015.00

Hour counter (used) 7 digit mains 240V AC 5011zLCD display 16 digit 7c5 dots dot matrixQuerty keyboard 58 key good quality switches

95p for P&P per order ( VAT inc )wide range of CMOS 1TL 74HC 74F Linear transistors kits

capacitors, resistors tools etc always in stockJPG Electronics 276 Chatsworth Road Chesterfield S40 2811

Access orders (0246) 211202. Callers welcome

Carbon Film Resistors 1/4W E24 series 0-51R to10M0 - 1p100 oh per value - 75p 1000 off in even hundreds per value - E7Metal Film 1/4W 10R0 totMO 5% E12 series - 2p 1% E24 series - 3pVNvatt metal/carbon film E24 series 1R0 to 10M0 -1 Watt metal/carbon film E12 series 487 to 10M0 - 5pBC107/8/9 - 12p BC547/8/9 - Op BC182L 184L - 10pBFY50/51/52 - 20p 2N3055 - 50p TIP31A,32A - 25p TIP,41,42, - 40p

Tantalum head subminiature electrolytics (Mids/Volts)0-1/35, 0-22/35, 0.47 35, 1316, 47 16 - 14p 47/35 - 15p22/35, 4.725, 10.5 - 15p 4-7 35, 68:16 - 16p 10/16,22/6 - 20p22/16 - 30p 33 10 - 30p 47 10 - 35p 100/6 - 40p

Aluminium Electrolytics (MidsNolts)1/50, 2-2/50, 4-7/25, 4-150,22/50, 47/16, 47/25, 47/50

1016, 10/25, 10/50 - 5p 22/16, 22/25 - 6p- 6p 100/16, 100/25 - 7p 100/50 - 12p

100100 - 14p 22016 - Op 220/25, 220/50 - 10p 47016, 47015 - 11p100015 - 18p 1000 35, 220 25 - 22p 4700 25 - 70p

Miniature Polyester Capacitors 250V Wkg. Vertical Mounting-01, P15, ,022, .033, -047, 168 - 4p 0-1 - 5p 0.15, -22 - 6p 0,47 - 8p

Mylar Capacitors 100V Wkg. Vertical Mounting E12 Series1000p to 8200p - 3p 01 to -068 - 4p 0-1 - 5p 0.15, 0-22 - 6p

Subminiature Ceramic Plate 100VWkg. E12 Series Vertical Mounting2%. 1P8 to 47P - 3p 56P to 330P - 4p 10% 390P to 4700P - 4pCeramic plate/disc E6 Series 50V 22P to .047 - 2p

Polystyrene Capacitors 63V Wkg. E12 Series Axial Mounting10P to 820P - 3p 1000P to 10,000 - 4p 12,000P - 5p1N4148 - 2p 1N4002 - 4p 1N5404 - 14p WO1 bridge -25p0A91 - 6p AA143 - 8p W005 - 20p 1N4006 - 6pZener diodes E24 series 3V3 to 33V 400mW - Op 1 wan - 12pL.E.D's Red, Green & Yellow 3mm & 5mm - 10p 8mm - 35p20mm fuse 0.1A to 5A quick blow - 5p Anti Surge - OpHigh Speed drills 0.8mm, 1.0mm, 1.3mm, 1.5mm, 2mm - 30pExpo Reliant drilling machines 12V d.c. with improved 3 -jaw chuck 6.50Nicads AA - 80p HP11 -E2 PP3 -E4.20 Universal Chargers -E6.50Glass reed switches single pole make contacts -8p Magnets -12p

VAT inclusive. Return postage 25p (free over 151. Lists free.

THE C.R. SUPPLY CO.,127 Chesterfield Road,

Sheffield S8 ORN. Tel. 557771.

* Series X Mixer Kits* up to 1.000 inputs* 60+100 mm faders, pots,panels and audio switches* 6-9 echo sends* versions for recording PA,radio, disco* circuit diagrams* From E9.92

Send 40p for catalogue to: K. Tek, P.O. Box 172a,Surbiton, Surrey KT8 61IN. Te1:01-399-3990

TURN YOUR SURPLUSICS transistors etc into cash, immediate

settlement. WE welcome the opportunity toquote for complete factory clearance.

Contact:COLES-HARDING & CO.,103 South Brink, Wisbech, Cambs.

ESTABLISHED 15 YEARSTel:0945 584188 - Fax: 0945 588844

-11- E 111-11

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in the display pagesof PE! Phone RICHARD

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CAMBRIDGE COMPUTER SCIENCE LTD10MByte Wntitiesters, used, 3 months Wty,

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Gould PSU 0-30V @5A Limited quantity only at

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Dual Data lead (BBC Micro to 2 Disk Drives)

Power lead (BBC Micro to Disk Drive)

Dual leads (BBC Miao to 2 Disk Drives)

74LS TTL , pick and mix, buy 10a mere fa £0.12 each

Types available: '00 '02 '04 '08 '11 '13 15' '21 26 '27 '30'32 '38 '42 '75 '83 '96 '107 '109 '122 '132 '136 '139 '145 '151'153 '157 '158 '163 '164 '166 '191 '193 '298 '365 '670

EPROMS 27128 used £1.50 each; new 27128-25 £2.50 each

6264 8K Byte SRAM -15 £3.00 each -12 0.80 each

65256 32K Byte rams £7.00 eat

8K Byte NV ram chips £300 each £10.00 four

20 pin dil low profile IC sockets £0.50 (n) - £ 400 (100)

40 pin dil low profile IC sookets £060 (ten) £ 5.00 (100)

Keyboard, 100 keys on board LCD & miao i/f £8.00 eat

Taddal mains transformer 12V 4A & 0.4A,12.0.12 @0.1A & 2A ,

9-0-9 @0.2A £4.00 each - £6.00 for 2. - £8.00 for 3

Prices include postage. Add 50p to orders below £5.00. All items

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£42.00 each

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Vr)Cooke International

DO YOU WANTSCOPES, GENERATORS, POWER

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PROGRAM NOWThe Professional Software

Developers JournalAvailable from all goodnewsagents -price £1.50

From The publishers of PracticalElectronics

741 -IC series ICs74HCOO 19p74HC04 19p74HC11 26p74HC27 31p74HC86 29p

74HC161 48p

74LS series ICs74LSOO 20p74LS30 20p741S90 30p

74LS112 23p74LS151 38p74LS245 58p

74F series ICs74F00 32p

74 F174 77p74F373 91p

ECL series ICs10101 1.15101 04 1.1 5

74HCO2 19p74HCO8 19p74HC14 39p74HC30 35p

74HC107 36p74HC175 52p

74LS02 20p74LS32 20p74LS93 30p

74LS138 35p74LS164 38p74LS374 52p

74F02 32p74F251 64p74F398 96p

74 HCO3 19p74HC10 19p74HC20 19p74HC74 38p

74HC138 41p74HC393 69p

74LS05 20p74LS51 20p

74 LS 107 23p74LS139 35p741S193 55p741S375 64p

74F64 32p74F353 64p74F399 96p

10102 1.15 10103 1.1510124 3.68 10125 3.68

A wide range of components and TTL, CMOS andECL ICs are available. We also supply PC, XT andAT spares at LOW PRICES. Write or phone for ourFREE catalogue.

Resistors at bulk prices Your MixCarbon film, 0.25W, 5% 1p each or 45p for 50Metal film. 0.25W. 1% 3p each or £1.35 for 50

VC sockets, solder type (pin:price)14:10p, 16'11p. 20:14p, 24(0.6 or 0.3):16p.

UC sockets with 0.1uF decoupler14:60p, 16:65p.

Prototyping boards (Vero)IBM XT 145.00, IBM AT £52.00

ACCESS welcome by post or phone. Callers byappointment please. VAT INCLUDED. Please add50p for post and packing. Catalogue free. No min-imum order charge.

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CLASSIFIEDS

SERVICES

MAKE YOUR OWN PCBs

9x6 U.V. Lightbox1/2 Itr. Fer is Chloride crystals80 gms Sodium Hydroxide9x6 fibre Photo board

£85 - inc. timer.£2.15£1.20

single sided £4.80double sided £5.40

All other sizes availableEuro card 160mm x 100mm single sided £2.70

double sided £3.00All other sizes available

Antex soldering iron C17 £8.50XS 25 £8.60Solder 22swg £7.50Plastic etching trays £3.50Titan mini drill 12volt £13.50

Include £1.00 P & P.Just about everything from polishing blocks to platedthrough hole equipment. Very keen prices - discount

on quantity.S.A.E. for complete price list or telephone:

Ward Electronics, 27 North Street,

Redruth, Cornwall TR15 1HJ.Tel: 0209 211050

We will make drilled PCBs to yourspecifications - one -offs to small production

runs. Cost 6p/cm2 single sided 12p/cm2 doublesided. An initial charge may be made for

artwork transfer to acetate. Send your artworkto us or for further details contact Paul Oakes,PCB Production, Intec (Inverclyde) Ltd, 5 East

Blackhall Street, GREENOCK, PA15 1HD.

For Electronic Design and PCBManufacture callIPROTO DMIGN

Unit 8, Ilford Trading EstatePaycocke Road Basildon

Essex SS14 4DRTel: 0268 289923

TECHNICAL INFO SERVICES (PE)76 Church St, Larkhall, Lanarkshire ML9 1HE

Phone 0698 884585 Mon -Fri, 9-5any other time 0699 883334 FOR FAST QUOTES

WORLD'S LARGEST COLLECTION SERVICE MANUALS Mostunobtainable elsewhere. Prices range from only £4.50 - large s.a.e. anyquotation, no obligation to buy.WORLD'S SOLE Suppliers of TV & Video Repair manuals, etc. from TVTECHNIC, Thorn etc. Every published service sheet in stock, suppliedlull size, not bits & pieces. CTV's or any combination £3.50 plus Lsae,any other single item £2.50 plus Lsae. Complete Circuit Sets for mostVideorecorders only #7 set (no sere skits made).LSAE for QUOTATIONS plus GIANT CATALOGUE NEWSLETTERS

BARGAINS FREE SISht se available.Comprehensive TV Repair Manual £9.50. Complete Radio Service andRepair Course £9.50. Complete Repair & Service Manuals Mono TV£12.50; CTV £17.00; Video £19.50. Complete Repair Data with circuit -Mono TV £9.50; CTV £12.50; Video £10.50.

C3.00 plus LSAE BRINGS THE ONLY COMPREHENSIVE SERVICESHEETS & MANUALS, CATALOGUES plus FREE CHASSIS GUIDE

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FREE Catalogue Unique Repair and Data Guides with allorders or LSAE for your copy.

MAURITON ELECTRONICS LTD (PE),8 Cherry Tree Road, Chinor, Oxoforshire OX9 4QY

Tel: (0844) 51694

R.J.E COMPONENTS,207, Godman Road,

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Free Mail order catalogue of electronic

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PCB Design -Manufacture -Assembly

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WE MANUFACTURE AND SUPPLYTOP QUALITY

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SYSTEMS SPECIALISTSNEW VHF MICROTRANSMITTER KIT,tuneable 80-115 MHz, 500 metre range,sensitive electret microphone, high quality PCB.SPECIAL OFFER complete kit ONLY £5,assembled and ready to use £8.95 post free.Access orders telephone 021 411 1821.Cheques/ P.O.'s to: Quantek Electronics Ltd,(Dept P.E.), 45a Station Road, Northfield,Birmingham, B31 3TESurveillance devices, lasers, Tesla coils,scramblers, ultrasonic and many more,over 150 designs. Send SAE to:Plancentre, Old Wharf, Dynock Road,Ledbury HR8 2HSKITS MICROTRANSMITTER, VHF/FM,received on standard radio, 25x15mm, freemicrophone included - £4.50. Telephonetransmitter, amazingly uses no batteries -£5.49. Bleeper transmitter vhf - £8.59 incp&p. SAE list. Remittance to: A.C.E. (PE),99 Greenheath, Hednesford, Staffs. -Access orders 05438 71902 - same daydispatch.

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BOOKS

73 AMATEUR RADIOWorld's leading ham radio

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JOBS

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COMPONENTS

ELECTRONICS COMPONENTS & TESTEQUIPMENT - For bargain list send SAEto: Dept. PE , 76 Wensleydale Road, GreatBarr, BirminghamB42 1PLC.S. COMPONENTS (Est. 4 years).Suppliers & stockist of electroniccomponents, electronic valves, militaryequipment and spares. Obsolete and hardto find products are our speciality. C.S.Components, Southfield House, 11Liverpool Gardens, Worthing, West SussexBN11 1RY

MISCELLANEOUS

Have a good idea?Ideas, designs andinventions wanted.

S.A.E. for more information: Martin BlissProp. MD, Adapta Plan, Dept. PE,28 Clerkenwell Crescent, Malvern,

Worcs WR14 2TX

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rif the pocket radiotelephone turns out tobe a popular consumer product, as seemslikely in the UK, it could introduce yet

another shift in the pattern of how we seeourselves in our highly technicised society.

What strikes me as significant - though it'snot an original observation - is that yourtelephone number will no longer be tied toyour home or workplace but will be roamingabout with your person. This string ofsymbols will not identify a location, aparticular assembly of bricks and mortarprobably shared by other people, but anindividual human being.

I'm referring, of course to the UK'sproposed personal communications network(pcn). The Government officially floated thisidea of a lower -cost, more accessible - what Iwould call cheap and cheerful - version of thecellular personal radiotelephone in adiscussion document early this year. Severalprospective operating companies responded

argument that, if I know what "I am aware ofelectronics" means, I must know the meaningof "I" and hence know myself. But again thisargument only leads to an intellectualrequirement, a mental model, not to a realentity. And my awareness of self -samenessthrough memory (eg, remembering havingheard the first stroke of a bell when listening tothe second) is certainly a demonstration ofpsychological continuity but doesn't tell meanything about my individual self.

So, apart from a belief that personal identityresides in a non -corporeal substance and asuggestion that it is just psychologicalcontinuity, there are no satisfactoryexplanations of what constitutes the reality ofself. It seems to me the symbol "I" can standfor any old definition of personal identity wecare to make up. Let us, however, be severelypractical.

Obviously, you can define personal identityby a lot of facts about yourself. To begin with,

"Sorry, wrong person!"enthusiastically. Now, as I write, the DTI isexamining formal applications for licences torun such pcn services.

Quite a lot of experience has been gainedalready. Cellphones have taken off rapidly inspite of their initially high cost, and the UKnow has over half a million subscribers -mainly professional and managerial ratherthan residential. Cordless telephones (whichof course use radio transceivers) arebecoming quite familiar in homes. Atelepoint service - with transceiver basestations dotted around public places - has juststarted in the UK. And next to arrive will bea pan-European digital cellular service anddigital European cordless telephone system(the present cellphone services beinganalogue and not so efficient in using thespectrum).

EFFECTIVELYPINNED

Having yourself identified by a string ofnumbers or other symbols is, of course, afamiliar part of modem life. What with youraddress wired telephone number, NationalInsurance code, bank account, credit cards,vehicle registration and many other labels,you are pretty effectively PINned down by amass of intersecting data. Speaking formyself, I'm not worried by all this. I justforget most of the numbers most of the timeand don't feel that the bureaucracies ofgovernment and commerce have any real holdon me as a person. Some people, though, arevery paranoiac about it.

But the pocket radiotelephone numbercould be different. It's not just another stringof symbols to add to the rest. It's also anexclusive key to communication with thatparticular assemblage of feelings and

By Tom Ivall

Numerology - thetelling of personal

fortunes bynumbers - looks set

to make acomeback.

thoughts, wishes and intentions, hopes andfears, pleasures and pains, that constitutes aunique human being. Communicationbetween persons - from mind to mind - iswhat distinguishes us from other animals.This new number seems to me more like aname, as a determinant of identity.

SELF ASSESSMENT

What, after all, does personal identityconsist of? We assume that there issomething essential called "I" at the core - theself, psyche, ego or soul. But what exactly isit? If I try to look for it inside myself I merelyfind a succession of different sensations,impressions, emotions, thoughts, ideas etc.These experiences seem to have some kind ofcontinuity and relatedness and I feel theymust all be part of the entity called "I". Butthis version of "I" is really nothing more thanan intellectual requirement - a necessaryconcept without which one can't understandhow the perceptions can occur.

A 'self' which is no more than anintellectual requirement doesn't have anindependent existence. There's an old

there is the particular structure of your body.This can be specified anatomically in greatdetail by its molecules - and nowadays usingthe new techinique of DNA 'fingerprinting'

Next comes the characteristic behaviour ofyour body: how you walk, sit, stand, speak,gesture, chew food, laugh and do all thosethings which collectively demonstrateconsciousness and express what is called thepersonality.

Then there is the set of names and titlesgiven to "I" by its parents and others. Thismay seem a rather contingent and extrinsicfeature of a person, but it's certainly a practicalcriterion of identity - and an enduring one,because it stays recorded somewhere after thatperson dies.

MAJORITY VOTE

I've picked out just three groups ofattributes because you need a minimum ofthree candidates to allow a 'majority vote'decision. In a life subject to change, if at leasttwo of these three groups remained unalteredthen you could say reasonably that the personalidentity was maintained. Of course, manyother attributes could be taken into account andthe majority decision could be on, say, an 80%- 20% basis.

So, in the absence of any really definiteknowledge about the essential self, apart fromintuitive insights, in practice we do rely a greatdeal on purely factual data to establishpersonal identity. An important category ofthis data is that of forenames and surnames andall the numbers and codes which get attachedto them like barnacles as we move through life.Among these names, numbers and symbols thepocket radiotelephone number must evoke adistinctive resonance. Its sequence ofnumerals are equivalent to the letters of aperson's name.

vim


Books

PRACTICAL ELECTRONICS

BOOK SERVICEHere is your Editor's choice of books he

thinks will be of interest toelectronics and

computer enthusiasts

BEGINNERS AND EARLY STARTERSM\V Mini -Matrix Boar Projects.R.A.Penfold. 112 pages. £2.50.Order Code BP99Shows a selection of 20 useful and interesting c, nitsthat can be built on a mini -matrix board of 24 holes by 10copper strips in size - an ideal book for earlyexperimenters.

WIrtV From Atoms to Amperes.F.A.Wilson. 160 pages. £2.95.Order Code BP254.For the absolute beginner, clearly explaining thefundamentals behind the whole subject of electricity andelectronics.

3a: Electronic Projects forBeginners.F.G.Rayer. 128 pages. £1.95.Order Code BP48Specially for the newcomer to electronics who is lookingfor a book containing a wide range of easily made projects.Some circuits need no soldering and many others showactual component and wiring layouts.

Electronics Build and LearnR.A.Penfold. 128 Pages. £5,95.Order Code PC 101Combining theory and practice, the book describes acircuit demonstrator unit that is used in subsequentchapters to introduce common electronic components andcircuit concepts, complete with practical experiments.

Practica E ectronic Bui ng BlocksR.A.Penfold. There are two books -Book 1 : 128 pages. £1.95.Order Code BP117Book 2 : 112 pages. £1.95.Order Code BP118Book 1 is about oscillators and gives circuits for a widerange, including sine, triangle, square, sawtooth andpulse waveforms and numerous others from voltagecontrolled to customised is types.Book 2 looks at amplifiers, ranging from low level discreteand opamp types to is power amps. A selection of mixers,filters and regulators is included.

30 Solderless Breadboard ProjectsR.A.Penfold. Two books each of 160pages. Book 1 : £2.25. Order CodeBP107. Book 2 : £2.25. Order CodeBP113.Each project is designed for building on a Veroblocbreadboard and is accompanied by a description, circuitand layout diagrams and relevant constructional notes.Many of the components are common to several projects.Book 1 covers linear devices, and Book 2 covers crnoslogic chips.

Beginners Guide to BuildingElectronic Projects R.A.Penfold. 112pages. £1.95. Order Code BP 227Shows the complete beginner how to tackle the practicalside of electronics and includes simple constructionalprojects.

am anuisikAimetGetting t e Most from YourMultimeterR.A.Penfold. 112 pages. £2.95.Order Code BP239There's more to what you can do with a meter than meetsthe casual eye. The book covers the basics of what youcan do with analogue and digital meters and discussescomponent and circuit testing.

MMW Test Equipment ConstructionR.A. Penfold £2.95.Order Code BP248Describes in detail how to construct some simple andinexpensive, but extremely useful, pieces of testequipment.

An Introduction toSatellite Television

Osci oscopesI.Hickman. £6.95.Order Code NT3Subtitled 'How to Use Them, Has They Work' the book isillustrated with diagrams and photographs and isessential reading for any one who wants to know aboutscopes, from first principles to practical applications.

How to Get Your Electronic ProjectsWorking.R.A.Penfold. 96 pages. £2.50.Order Code BP110.Essential reading for anyone who wants first-time successin project assembly. Covers tracing mechanical faults aswell as testing for failures of active and passivecomponents of most types.

YsTiVDINNONDifilit

*NEW* Satellite TV Insta at ionGuide - 2nd edition John Breeds.£11.95. Order Code STV1Full of vital information for any competent diyer whowishes to install a satellite tv antenna and obtainoptimum reception quality.

An Introduction to SatelliteTelevisionF.A.Wilson. 112 pages. £5.95.Order Code BP195Informative answers to many of the questions aboutthis communications revolution. The information ispresented on two levels, one aimed at the completebeginner, the other at professional engineers andserious amateur enthusiasts

AUDIO ANDIntroducing Digital AudioI.Sinclair. 112 pages. £5.95.Order Code PC102A non -mathematical introduction to the new digitaltechnology, discussing the principles and methodsinvolved in devices such as cd, dat and sampling.

Electronic Music ProjectsR.A.Penfold. 112 pages. £2.50.Order Code BP7424 practical constructional projects covering fuzz, wah,sustain, reverb, phasing, tremolo etc. The text is split intofour sections covering guitar, general, sound generationand accessory projects.

More Advanced Electronic MusicProjectsR.A.Penfold. 96 pages. £2.95.Order Code BP174Complementing BP74 by covering more advanced andcomplex projects including flanging, chorus, ringmodulation, plus a selection of drum, cymbal and gongcircuits.

A'I'MY Computer Music ProjectsR.A.Penfold. 112 pages. £2.95.Order Code BP173Shows how home computers can produce electronic musicand covers sequencing, analogue and Midi interfacing,digital delay lines and sound generators.

Practical Midi HandbookR.A.Penfold. 160 pages. £5.95.Order Code PC103A practical how -to -do -it book for musicians andenthusiasts who want to exploit the capabilities of Midi.Covers keyboards, drums, sequencers, effects, mixers,guitars, and computer music software.

Midi ProjectsR.A.Penfold. 112 pages. £2.95.Order Code BP182Practical details of interfacing many popular homecomputers with Midi systems, and also covering Midiinterfacing to analogue and percussion synths.

1CM Electronic SynthesiserConstruction.R.A.Penfold. 112 pages. £2.95.Order Code BP185.Even relative beginners should find the monophonicsynthesiser described here within their capabilities if thebook is thoroughly read. Individual aspects of the synthare dealt with separately and pcb designs are shown forthe main modules.


DIGITAL AND COMPUTING027A Concise Intro uction to MS-DOS.N. Kantaris. 64 pages. £2.95.Order Code BP232A ready -reference guide for those who need a quick insightinto the essential command functions of this operatingsystem, but who don't have the time to learn it fully.

An Introduction to ComputerPeripheralsR.A. and J.W. Penfold. 80 pages.£2.50. Order Code BP170Covers such items as monitors, printers, disc drives,cassettes, modems, etc, explaining what they are and howto use them with your computer and with each other.

Microprocessing Systems andCircuitsF.A.Wilson. 256 pages. £2.95.Order Code BP77A comprehensive guide to the elements of mlcroprocessingsystems, covering the fundamental principles behind thisimportant subject.

Introduction to 6800/6802Microprocessor SystemsR.J.Simpson and T.J.Terrell. 238pages. £10.95. Order Code NT9The book covers systems hardware, programmingconcepts and practical experimental work that will assistin understanding the 6800/6802 microprocessor, withadditional information on the 6802D5E evaluationsystem.

An Introduction to 68000Assembly Language.R.A. and J.W.Penfold. 112 pages.£2.95. Order Code BP184Covers the fundamentals of writing programs that willvastly increase the speed of 68000 based machines suchas the Commodore Amiga, Atari ST range, AppleMackintosh, etc.

4oWngMost From YourPrinter

RIM Electronic Science Projects.Owen Bishop. 144 pages. £2.95.Order Code BP104A bumper bundle of experimental projects ranging incomplexity and including a colour temperature meter,electronic clock, a solid state (led display) scope, an infra-red laser, a fascinating circuit for measuring the earth'selectrical field strength, and many more.

Electronic Security DevicesR.A.Penfold. 112 pages. £2.50. BP56Full of ideas for keeping your valuables safe. The circuitsinclude designs for light, infra -red, ultrasonic, gas, smoke,flood, door and baby sensors.

71M S7 More Advanced ElectronicSecurity Projects. R.A.Penfold. 112pages. £2.95. Order Code BP190Follows on from where BP56 leaves off and describes anumber of more up-to-date and sophisticated projects,such as pyro-sensors, infra -red and doppler shiftdetection, fibre -optic loops, and many others.

FM7 Electronic Projects for Carsand Boats.R.A.Penfold. 96 pages. £1.95.Order Code BP94

mgongegaingssBooks

Getting t e Most om Your PrinterJ.W.Penfold. 96 pages. £2.95.Order Code BP181How to use the features found on most dot-matrix printersfrom programs and popular wordprocessors, showingexamples of what must be typed to achieve a given effect.

Micro Interfacing CircuitsR.A.Penfold. Two books, each of 112pages.Book 1 : £2.25.Order Code BPI30.Book 2 : £2.25. Order Code BP131Both books include practical circuits and usefulbackground information though pcb layouts are notincluded. Book 1 mainly covers computer input-outputtechniques. Book 2 deals primarily with practicalapplication circuits.

MIM An Introduction to 6502Machine Code.R.A. and R.W. Penfold. 112 pages.£2.50. Order Code BP147Covers the main prindples of machine code programmingon 6502 -based machines such as the Vic -20, Orie-1 /Atrnos, Electron, BBC and Commodore 64. It assumesno previous knowledge of microprocessors or machinecode and gives illustrative programming examples.

XnWY A Z-80 Workshop Manual.E.A.Parr. 192 pages. £3.50.Order Code BP112A book for those who already know Basic but wish toexplore machine code and assembly languageprogramming on Z80 based computers.

Practical Digital ElectronicsHandbookM.Tooley. 208 pages. £6.95.Order Code PC 104Nine constructional projects introduce digital circuits,logic gates, timers, microprocessors, memory and interfacecircuits - an essential book for anyone interested in digitaldevices.

15 fairy simple projects t at can .e use wit a carand/or boat. Stripboard constructional details areincluded, as are explanations of the circuit theory.

Power Supply ProjectsR.A.Penfold. 96 pages. £2.50.Order Code BP76A selection of power supply designs, including simpleunstabilised. fixed voltage regulated and variable voltagestabilised, ni-cad charger, voltage step-up, and inverter.

More Advanced Power SupplyProjectsR.A.Penfold. 96 pages. £2.95.Order Code BP192Covers more advanced topics than BP76 and includesprecision supplies, switch mode and computer controlledsupplies, plus a selection of miscellaneous circuits.

Mg\S7 Popular Electronic Circuits.R.A.Penfold. 160 pages. £2.95.Order Code 11F'80Containing a wide range of circuit designs for experiencedconstructors who are capable of producing workingprojects direct from a circuit diagram without specificconstructional details.

DATA AND INFORMATION BOOKS

Digital IC Equivalents and PinConnectionsA.Michaels. 320 pages. £5.95.Order Code BP140Linear IC Equivalents and PinConnectionsA.Michaels. 256 pages. £5.95.Order Code BP141Between them these two books show equivalents and pinconnections of a popular user -orientated selection ofEuropean, American and Japanese ics. They alsoinclude details of functions, manufacturer and countryof origin. The Digital ICs book also quotes details ofpackaging and families.

OpampsB.Dance. £6.50.Order CodeNT2Subtitled 'Their Principles and Applications' thisinteresting book is written in a simple non -mathematicalstyle and provides a source of practical dreuits that useboth commonplace and more sophisticated opamps.

Electronic Hobbyists HandbookR.A.Penfold. 96 pages. £4.95. OrderCode BP233Provides a source of useful information that the amateurenthusiast is likely to need for day-to-day pursuance ofhobby electronics.

Practical Electronics HandbookI. Sinclair. £7.95.Order Code NT1A useful and carefully selected collection of standardcircuits, rules of thumb and design data for enthusiasts,students and engineers involved in radio, computing andgeneral electronics

Newnes Electronics Pocket BookI.E.Parr. £6.95.Order Code NTIOPresents all aspects of modem electronics in a readableand largely non -mathematical style, and is a good sourceof valuable information for enthusiasts and professionalengineers alike.

Mit7 Key Techniques for CircuitDesignG.C. Loveday. £6.95.Order Code BM 101Tackles the problems of designing circuits from scratch,introducing the concept of target specifications, thedesign sequence, device selection, rules of thumb, anduseful equivalent circuits.

HOW TO ORDERState your order code and your name

and address clearly. Enclose a

cheque, PO or international money

order (add 50p postage per book -

£1.00 for overseas surface mail), and

send to:

PE Book ServiceIntra House193 Uxbridge RoadLondon W12 9RABooks are normally deliveredwithin 10 days but please allow 28days for delivery.


INDEXJANUARY 1989 TO DECEMBER 1989

VOLUME 25Constructional Projects

64 -Line Output Port by R.Milner and A.Horsman. Multiplexes 64control lines from any 8 -bit computer. SeptemberAsk PE by Andrew Armstrong. A frequent series of answers toreader's technical questions.

Audio level remote control. SeptemberAmplifier signal feedback stability and line regulation.

OctoberExtended timing where analogue is unsuitable. NovemberLogic gates select and control extended timing. December

Camera Shutter Timer by Robert Penfold. Infrared techniques checkcamera shutter timing accuracy. MarchChristmas Flashers by Chris Bowes. Several flashing light projectsfor sprucing up the Xmas fir tree. JanuaryDigital Electronics by Owen Bishop. Tutorial features for GCSE andother students of electronics.

Part five - interfacing. JanuaryPart six - converters. February, MarchPart seven - Computer logic. AprilPart eight - More computer logic. MayPart nine - Computer systems. JunePart ten - Programming computer systems. JulyPart eleven - Applications and implications. SeptemberPart twelve - Conclusions. October

Dual Beam Oscilloscope by John Becker. Part three - the Y ampsand advice on using a scope. JanuaryEasi-Build Series by John Becker . A series of simple, useful circuits.

Compressor. Automatic audio signal level controller. SeptemberMetronome. Variable -rate flashing led and audio accented beatsetter. DecemberPower Supply. Multipurpose fixed or variable stabilised voltageregulator. JulyRemote -a -Bell. Inductive remote phone line monitor. MayVodalek. Robot -type voice modifying project. AugustVoice Operated Switch. Sound triggered remote switching circuit.

OctoberWheeby-Jeeby Siren. Multiple variable siren generator. June

Electronic Thumb by Owen Bishop. A workshop test aid for searchingout hot spots. JulyFrequency Counter and Generator by John Becker. 4MHz frequency

counter with lcd, plus dual audio and digital signal generators.September, October

Hand Clapper by Harvey Kent. Triggered and automatic hand clapeffects generator. AugustHF Scope Store by Michael Sweet. An add-on circuit for the 1988logic analyser, allowing a BBC -B to act as a storage scope. MarchHolmes Machine by Owen Bishop. Stepper -motor controlled mobilesilhouette unit for deceiving thieves. JanuaryHome Security Controller by Kevin Browne. Microprocessorcontrolled alarm system monitor. September, OctoberKirlian Camera by Don Phillips. A solid-state camera for recordingparanormal events. MayMetronome Deluxe by Robert Penfold. A precision musical beat timer.

JuneMole -Detester by Edwin Chicken. Electronic mole -deterring soundgenerator. SeptemberMono -Stereo Echo Station by John Becker. Multi -option audio delay -line unit for echo, reverb, multitracking, chorus, etc.

October, DecemberPC Multipart by Philip Small. 8 -line dual analogue/digital interface portfor Amstrads and other PCs. AprilProjector Synchroniser by Chris Bowes. Interfacing a slide projectorand stereo taperecorder. JulySiderial Clock by John Becker. For astronomical or conventionaltimekeeping. FebruarySolar Heating Controller by Andrew Williams. A control circuit usinghigh accuracy temperature sensors. JuneStepping Motor Driver by Mark Stuart. Uses the M5804 chip tointerface a home micro to motor controlled models. AugustTelescope Control Interface by David Clarke. Versatile parallel -serialcomputer interface for controlling telescopes or other devices.

October, NovemberThermal Conversion Voltmeter by Rod Cooper. A follow on from theTrue RMS Meter of June. This meter is designed for use in powerelectronics. DecemberTrue RMS Meter by Rod Cooper. Examining problems in powermeasurement, and a constructional solution. JuneVideo AGC Stabiliser by Robert Penfold. Assists in improving stabilityof poor quality hired video tapes. December

25th Anniversary FeaturesParalleling Sir Clive by John Becker. An interview with Sir CliveSinclair. NovemberIn the Beginning by Fred Bennett. PE's founding editor tells how it allbegan. NovemberThe Middle Years by Mike Kenward. A mini-memoire from PE's editor1977-85. NovemberTime Travelling Through Technology by John Becker. A nostalgictrip through PE's history. NovemberToasting Transitions by Richard Barron. Celebratory good wishesfrom PE's editor during 1986. NovemberFuture Fulfilment by John Becker. Encumbent Editor since 1987recalls his call-up and looks to PE's future. November

2014? Whew! by Wayne Green. Our American correspondent'spersonal view of technology past and future. NovemberThe Next 25 Years by Barry Fox. Speculations on technology's futurefrom PE's Leading Edge columnist. November25 Years of I.T. by Tom Ivan. A retrospective review of InformationTechnology from PE's Industry Notebook columnist. NovemberFrom PE to Prosperity by Anthony H. Smith. Building a PE projectset a reader in search of fortune - and he found it! NovemberFleming and the Valve by John Becker. Commemorating the 85thanniversary of the invention of the valve. NovemberA Party to Remember by John Becker. How PE and friendscelebrated the 25th Birthday! December


Special FeaturesAutoguide by Michael Strassen. Reporting on the status of the newautomatic traffic guidance system. MarchBoating Revolution by John Becker. Reviewing the status oftechnology for leisure craft electronic products. June - AugustDigital Multimeters by Samuel Dick . Choosing DMMs, and theiradvantages over analogue meters. MayElectronic Railways by Neil Harding. How countless electronicsystems assist rail transportation. January, FebruaryElectronic Snapshots by Barry Fox. Reviewing the status of solid-state still -photography. OctoberElectronica 88 - Munich by John Becker. A review of the World'slargest component exhibition. FebruaryEncryption by Mike Sanders. Explaining coding and decodingtechniques for linguistic security. July -SeptemberHDTV - The Great Systems Battle by Tom Ivall. The contemporarystate of high definition tv. February, MarchHF Radio by Mike Sanders. An explanation of the hows and whys ofradio reception. August - DecemberHi -Tech Timing by Anthony H. Smith. (Time and Frequency series).Atomic frequency standards. April, MayHome Automation by John Becker. Reviewing the current status ofthe major fields in this technology. April, MayMicro Genius by Jenny Welsh. A cautionary tale of what to expectfrom a Home Computer! NovemberReader Survey. Seeking readers' opinions about PE. May

Repairing the AL80 by Vivian Capel. How to keep the obsolete butmuch favoured AL80 amplifier module in good condition. DecemberSatellites by Tom Ivall

Part one - Spacecraft launching and orbiting. AprilPart two - Receiving satellite signals. JunePart three - Signal coding and polarising. July

Semiconductors by Andrew Armstrong.Part thirteen - Opamps. FebruaryPart fourteen - Audio circuits. MarchPart fifteen - Signal conditioning. April

State -Side - WCES 89 by Wayne Green. An American's personal viewof the Las Vegas Winter Consumer Electronics Show. MayTactile Deaf Aid by Vincent Johns. Reviewing a sensor which allowsthe deaf to become aware of sounds. MarchTelepointing the Way by Michael Strassen. Reviewing the new ForumTelepoint personal telephone. MayThe Thor Spot by Wayne Green. Our American correspondent looksat the current situation regarding cd-rom. MarchTime and Frequency Measurement by Anthony H. Smith.

Part three - quartz crystal timebase. FebruaryPart four - quartz crystal stability. March

Transputer Trainer Review by Robert Penfold. Reviewing FlightElectronics' transputer training system. OctoberUCT Measurements by Anthony H. Smith. How to maximise theaccuracy of counter -timer measurements. November, December

Regular FeaturesEditorial by John Becker. Monthly views and comments from theEditor.Industry Notebook by Tom Ivall. Monthly series looking at theelectronics industry.Leading Edge by Barry Fox. Monthly series looking at the technologybehind the news.Spacewatch by Dr Patrick Moore CBE. Monthly series of astronomyand space reports.

Bookmark. Frequent reviews of new books received.Catalogue Database. Monthly reviews of catalogues received.Chipcount. Monthly look at selected new chips.Countdown. Monthly list of forthcoming exhibitions.News and Marketplace. Monthly series detailing new products andservices.Readers' Letters. Frequent series expressing your views , andoffering a few replies.

CompetitionsAstra Satellite tv aerial system - competition February, results JuneAtari Folio pocket pc - competition July, results NovemberFerguson pocket lcd colour tv - competition January, results AprilForum Telepoint telephone - competition May, results AugustMaplin 20MHz oscilloscope - competition Sept, results December

Sharp IQ -7000 personal organiser - competition August, resultsNovember

Acorn Archimedes A3000 computer, Cirkit digital multimeters, 25years' subs to PE - competition November

Summary PointsDual Beam Oscilloscope (Dec 88)Fig.20 page 27, C15 should be across the +5V lines, VR3 goes to -VE(not OV); the pcb is correct. C9 1 in parts list should read C9.Easi-Build Remote -a -Bell (May 89)Fig.2, reverse polarities of D1 and D2. IC1 pin 1 is at top left. R1should be inserted in place of link wire D to M, but taking it to L insteadof M.Kirlian Camera (May 89)Fig.3, C1 polarity should be reversed. PCB point B should read as T1common. PCB point C should just read as T1+. Fig.1, the coil outputarrow should be marked as EHT to Plate. Fig.5, connection GND shouldgo to pcb point A. The EHT plate and glass may be replaced by copper -clad fibreglass with a one inch strip of copper removed from edges.Panning Mixer (Dec 88)Fig.4, C15 +VE should go to +15V line, the pcb is correct. Fig.6,second IC3 should be IC5.PC Multipart (Apr 89)Fig.3, links holes adjacent to IC9 pin 4 and IC10 pin 13, link holes nextto IC10 pins 4 and 13, link holes next to IC10 pin 5 and +5V, link hole

next to IC8 pin 1 and digital ground (anywhere will do). Fig.2, A32should read A31, B15 should read B13.Polywhatsit! (May -Jun 87)Fig.5 May 87, leads 11 & 12 should go to either of the two outertransformer secondaries. Jun 87 pcb : check that IC16 pin 9 isseparated from track to IC16 pins 8/10, and track to IC18 pin 3 isseparated from IC18 pins 4/13. As stated in Dec 88, there are minorerrors on the circuit diagram Fig.2 Jun 87, but the pcb componentlayouts and wiring diagrams are correct.Siderial Clock (Feb 89)Fig.11, VR1 should read as C5Solar Heating Controller (Nov 89)The LM335 sensors are available from STC Electronic Services (0279626777) and Viewcom Electronics (01-471 9338). C4 is 1000uF 35V.Vodalek (Aug 89)Fig.1. Bottom of VR2 goes to OV, C10 +VE goes to TR1 gate, TR1drain goes to R9/R11. Pin numbering of IC1c and IC1d should betransposed. The pcb is correct.Weather Centre (May 88) Fig.20, IC8 should be reversed.


ON A'T I. 0 NuA L

COLLEGE OFT ECHNOLOGY

Packaged ShortCourses

The National College of Technology (NCT Ltd) offers a range ofpackaged short courses in analogue electronics, digitalelectronics and fibres & optoelectronics for study at home or atwork. The advantages are that you may,

- commence at any time- work at your own pace- have a tutor (optional)

and there is no travelling involved. BTEC certificates areavailable subject to the conditions of the award. These highlypopular packed courses contain workbooks, a cassette tape,circuit board and components necessary to provide boththeoretical and practical training.

Whether you are a newcomer to electronics or have someexperience and simply need updating, there is probably apackaged short course ready for you. Write or telephone fordetails, quoting Practical Electronics, to:

NCT Ltd, Bicester Hall5 London Road, Bicester

Oxon OX6 7BUor telephone (0296) 613067 Ext. 202

INDEX TO ADVERTISERSA & G Electronics 54

A.D.M. Electronics Supplies 55

Antex Electronics. 39

Astronomy Now 47

B.K. Electronics IBC

Blackmore Electronics 55

Bull J. 52

Cambridge Computer

Science Ltd 55

Classified Ads 54-56

Coles Harding 55

Cooke International 55

Cricklewood Electronics 19

C.R. Supply Co. 55

Deansgate 54

Electronize Design. 40

Electrovalue 39

Eskan. 56

Fraser Electronics 54

Global Electronic Services 39

Greenbank Electronics 62

Hanney, L.F. 54

Hills Components 40

Intec., 56

J.P.G. Electronics 55

Keytronics. 20

K-Tek 55

London Electronics College 54

London Underground 53

Magenta Electronics 32,33

Maplin Electronics OBC

Martin Bliss 56

Mauriton Electronics 56

National College of Technology 62

National Component Club 32

Number One Systems 26

Omni 54

Phonosonics 29

Program Now 43

Proto Design 56

Radio and Telecommunications

Correspondence School 54

R.J.E. Components 56

RMOS 55

Service Trading Co 62

SM Engineering 34

Soft Machine 43

Specialist Semiconductors. 16

Stewart of Reading 40

Suma Designs 34

Tandy I FC

Technical Info Services 56

Technomatic. 10,11

T.K. Electronics 34

Ward Electronics 56

PLEASE MENTION PRACTICAL ELECTRONICSWHEN REPLYING TO ADVERTS

Interak 1SINGLE BOARD COMPUTER

"SBC-1"

A computer doesn't have o ook like you'd expect a computer to look.It doesn't have to have a keyboard and a screen and floppy disks andso on.

The SBC-1 has the bare minimum of chips a Z80 computer can haveand still be a computer: A 4 MHz Z80 -CPU chip, an EPROM chip (upto 32K), a static RAM chip (up to 32K) and a pair of 8255A I/O (inputoutput) chips giving 48 individual lines to waggle up and down. Thereare one or two additional "glue" chips included, but these are simple"74LS" or "HC" parts.

A star feature is that no special or custom chips (ie PALs, ULAs, ASICsetc) are used - and thus there are no secrets. The 280A is the fastestand best established of all the 8 -bit microprocessors - possibly t!cheapest too!

Although no serial interface is included, it is easy for a 280A to waggleone bit up oi down at the appropriate rate - the cost is a few penceworth of code in the program: why buy hardware when software will do?

Applications already identified include: Magnetic Card reader, miniprinter interface, printer buffer, push button keypad, LCD alphanumericpanel interface, 40 -zone security interface for auto sending of securityalarms, code converter (eg IBM PC keyboard codes to regular ASCII),real time clock (with plug in module), automatic horticultural irrigationcontroller.

By disabling the on -board Z8CA-CPU this card will plug into our Interak1 CP/M Plus disk -based development system, so if you don't fancyhand -assembling Z80 machine code you don't have to!

The idea is (if you are a manufacturer) you buy just one developmentsystem and then turn out the cheap SBC-1 systems by the hundred. Ifyou are really lazy we can write the program for you and assemble theSBC-1 cards so you can get on with manufacturing your product, leavinall your control problems to us.

GreenbankFor more details write or phone us:

Greenbank Electronics. Dept (E10P) 460 New Chester Road.Rock Ferry. Birkenhead. Merseyside. L42 2AE. Tel: 051-645 3391

9WIDE RANGE OF XENON FLASHTUBESWrite/Phone your enquires

WASHING MACHINE WATER PUMPBrand new 24OVAC fan cooled can be used fora varietyof purposes. Inlets 11/2 ins. Outlet 1 inch. Price includingp&p and VAT- £10.95 or 2 for £20 including p&p andVAT

SPECIAL OFFER AC CAPACITORS1.5 MFD440VE2.00 5 MFD 440V E4.002 MFD 440002.50 5.4 MFD 280V E2.004.1 MFD 440V E3.50 5 MFD 660V E3.00

12 MFD 400 £4.00pup 50p per unit plus VAT to be added to total.

TORIN CENTRIFUGAL BLOWER230V ac 2,800 RPM 0.9amp 130mm diameter impelloroutlet 63 0 37mm overall size 195 x 160 0 150mm long.Price £17.50 + £2.50 p&p (023 inc. VAT)

SHADED POLE GEAR MOTORSIn the following sizes:9 RPM 12 RPM 80 RPM 160 RPM 1100 AC or 240V ACwith capacitors (supplied). Price incl VAT& p&p E12.65

GEARED MOTORS71 RPM 20)b inch torque reversable 115V AC inputincluding capacitor and transformer for 240V ACoperation. Price incl VAT & p&p E23.00.

12 V DC COOLER EXTRACTOR FANNew brushless motor 92mm sq. Price incl VAT & p&p£11.50.

VARIABLE VOLTAGE TRANSFORMERS

INPUT 2201240V AC 50/110 OUTPUT 0-260V200W0.1 amp max E24.00 p&p E3.00 1E31.050.5KVA2.5 amp max E26.50 E3.5 1E34.791KVA 5 amp max E34.00 £4.25 1E43.992KVA 10 amp max £49.00 E5.50 (E62.683KVA 15 amp max £65.00 £6.25 1E81.945KVA 25 amp max E115.00

Carriage on request

VOLTAGE CHANGING

1

TRANSFORMER .

1250 Wan auto. Tapped 0-90V, 100, 110, 115,120, twice to obtain voltages between 90and 240V Fined in heavy duty louveredmetal case. Fused input. Price incl VATp&p £39.50.

COMPREHENSIVE RANGE OF TRANSFORMERSLT-ISOLATION & AUTO 1110-240V Auto transfercased with American socket and mains leadframe type. Available for immediate delivery.ULTRA VIOLET BLACK LIGHT FLUORESCENT4ft 40 wan E10.44 (012.00 inc VAT) Caller2h 20 watt E7.4,1 + E1.25 p&p (£9.99 inc

13in 10 watt E5.80 + 75p p&p 1E7.53 inc12in 8 watt E4.80 + 75p p&p 1E6.38 inc9in 13watt 03.60 + 50p p&p 1E5.12 incSin 4 watt C3.60 + 50p p&p 105,12 inc

inc VAT)inc VAT)inc VAT)inc VAT)inc VAT)

4.--.

&

eitheror open

TUBESonly

VAT)VAT)VAT)VAT)VAT)

230VAC BALLAST KIT for either Gin, gin or12In tubes £5.50 + 55p p&p 1E6.96 inc VAT)For 13in tubes £6.00 + 75p (AO1E775 inc VAT)

400 WATT UV LAMPOnly 034.00 +£2.50 p&p 1041.98 Inc VAT)

175 WATT SELF BALLASTED BLACK LIGHTMERCURY BULBS Available with BC or ESfitting. Price incl VAT & p&p £19.84. la

4100.

SOLID STATE EHT UNITInput 23012400 AC, Output appros 15KV. Producing10mm spark. Built-in 10 sec timer. Easily modifiedfor 20 sec. 30 sec to continuous. Designed for boilerignition. Dozens of uses in the field of physics andelectronics. eg supplying neon or argon tubes etc.Priceless use E8.50 + £1.00 P&PIE10.93 inc VAT) NMS

12 VOLT BILGE PUMPSt-..-...1

Buy direct from the importers IU500 GPH 1511 heed 3 amp %1/4,....£16.00 inc.

Orni1750 GPH 15ft head 9 amp£19.25 + £2.00 p&p (025.00 inc VAT) L

COOLING FANS -BRAND NEW!200/400V AC American Boxer 'Peewee' 7 -bladed highefficiency cooling unit 80mm eq zx 40mm deeps 400n,approx. Price incl VAT & p&p £10.35.

EX -EQUIPMENT FANS 120mm eq x 38Inm deep ineither 115V or 230V AC Tested and guaranteed. Price

Inc' VATS p&p E776.Large selection of various speed greased motors fromstock. Phone or write for dwells.

EPROM ERASURE KITBuild your aen EPROM ERASURE fora medial of he ono of arredeup wit K4 of pets ISu as minks 12n 8 settM InesTube Rush unit pair of h On leads neon meddler wool erntchssleV rnicromith and circuit 014.00 . 0.00 pap (011.40 inc VAT).

From stock at prices that defy competitionC/F Blowers Program TimersMicroswitches Synch Motors

write/phone your enquiries

SUPER HY-LIGHT STROBE KITDesigned for Disco. Theatrical users etc.Approx 16 joules. Adjustable speed E48.00 + £2.00 p&p1E57.50 inc. VAT)

Case end reflector £22.00 + E2.00 p&p 1E27.60 inc VATISAE for further details including Hy/Light andindustrial Strobe Kits.

NMS = NEW MANUF SURPLUSR&T = RECONDITIONED AND TESTED

AmpleParking Space SERVICE TRADING CO

57 BRIDGMAN ROAD, CHISWICK, LONDON W4 5BB AkShowroom open 01-995 1560Montlay/Friday ACCOUNT CUSTOMERS MIN. ORDER £10


POWER AMPLIFIER MODULES -TURNTABLES -DIMMERS -LOUDSPEAKERS -19 INCH STEREO RACK AMPLIFIERS

PRICES INCLUDE V.A.T.* PROMPT DELIVERIES FRIENDLY SERVICELARGE S.A.E., 30p STAMPED FOR CURRENT LIST.

OMP POWER AMPLIFIER MODULES Supplied ready built and tested.

OMP POWER AMPLIFIER MODULES Now enjoy a world-wide reputation for quality, reliability andperformance al a realistic price. Four models available to suit the needs of the professional and hobby market. i.e.. Industry.

Leisure. Instrumental and Hi-Ei etc. When companng prices. NOTE all models include Toroidal power supply, Integral heat sink.

Glass fibre PCB.. and Dnve circuits to power compatible Vu meter. Open and short circuit proof.

THOUSANDS OF MODULES PURCHASED BY PROFESSIONAL USERS

OMP100 Mk 11 Bi-Polar Output power 110 wattsR.M.S. into 4 ohms, Frequency Response 15Hz -30KHz -3dB, T.H.D. 0.01%, S.N.R. -118dB, Sens. forMax. output 500mV at 10K, Size 355 x 115x65mm.PRICE £33.99 + £3.00 P&P.

NEW SERIES II MOS-FET MODULES

OMP/MF 100 Mos-Fet Output power 110 watts R.M.S.into 4 ohms, Frequency Response 1Hz - 100KHz- 3dB, Damping Factor, >300, Slew Rate 45V/uS,T.H.D. Typical 0.002%, Input Sensitivity 500mV, S.N.R.-125dB. Size 300 x 123 x 60mm.PRICE £39.99 + £3.00 P&P.

OMP/MF200 Mos-Fet Output power 200 watts R.M.S.into 4 ohms, Frequency Response 1Hz - 100KHz-3dB, Damping Factor >300, Slew Rate 50V/uS,T.H.D. Typical 0.001%, Input Sensitivity 500mV, S.N.R.-130dB. Size 300 x 155 x 100mm.PRICE £62.99 + £3.50 P&P.

OMP/MF300 Mos-Fet Output power 300 watts R.M.S.into 4 ohms, Frequency Response 1Hz - 100KHz-3dB, Damping Factor >300, Slew Rate 60V/uS,T.H.D. Typical 0.0008%, Input Sensitivity 500mV,S.N.R. -130dB. Size 330 x 175 x 100mm.PRICE £79.99 + £4.50 P&P.

NOTE, MOS-FET MODULES ARE AVAILABLE IN TWO VERSIONS, STANDARD - INPUT SENS. 500mV BAND WIDTH 100KHz.PEC (PROFESSIONAL EQUIPMENT COMPATABLE) - INPUT SENS. 775mV. BAND WIDTH 50KHz, ORDER STANDARD OR PEC

Vu METER Compatible with our four amplifiers detailed above. A very accurate visualdisplay employing 11 L.E.D. diodes (7 green, 4 red) plus an additional onioff indicator.Sophisticated logic control circuits for very fast rise and decay times. Tough moulded plasticcase, with tinted acrylic front. Size 84 x 27 x 45mm.PRICE £8.50 + 50p P&P.

LOUDSPEAKERS

OMP VARISPEED TURNTABLE CHASSIS.

OPTIONAL MAGNETIC CARTRIDGES

* MANUAL ARM * STEEL CHASSIS * ELECTRONIC SPEED CON-TROL 33 & 45 * VARI PITCH CONTROL * HIGH TORQUE SERVODRIVEN DC MOTOR * TRANSIT SCREWS * 17 DIE CAST PLATTER *NEON STROBE * CALIBRATED BAL WEIGHT * REMOVABLE HEADSHELL * 'h. CARTRIDGE FIXINGS * CUE LEVER * POWER 220 240V5060Hz * 390x305mm * SUPPLIED WITH MOUNTING CUT-OUTTEMPLATE.

PRICE £59.99 + £3.50 P&P.

STANTON AL500 GOLDRING G850PRICE £16.99 4- SOD P&P PRICE £6.99 50o PAP

LARGE SELECTION OF SPECIALIST LOUDSPEAKERSAVAILABLE, INCLUDING CABINET FITTINGS, SPEAKERGRILLES, CROSS-OVERS AND HIGH POWER, HIGH FRE-QUENCY BULLETS AND HORNS, LARGE S.A.E. (30pSTAMPED) FOR COMPLETE LIST.

McKENZIE:- INSTRUMENTS, P.A., DISCO, ETC.ALL McKENZIE UNITS 8 OHMS IMPEDENCE8..100 WATT C8100GPM GEN. PURPOSE. LEAD GUITAR, EXCELLENT MID., DISCO.RES, FR EQ, 80Hz FREQ, RESP, TO 14KHz. SENS, 99dB. PRICE £28.59 + £2.00 P&P.10' 100 WATT C10100GP GUITAR, VOICE, ORGAN, KEYBOARD, DISCO, EXCELLENT MID.RES, FREQ, 70Hz. FREQ. RESP, TO 6KHz. SENS, 100dB. PRICE £34.70 + £2.50 P&P.10" 200 WATT C10200GP GUITAR, KEYBOARD, DISCO, EXCELLENT HIGH POWER MID.RES, FREO, 45Hz. FREQ, RESP, TO 7KHz. SENS, 103dB. PRICE £47.48 + £2.50 P&P.17 100 WATT C12100GP HIGH POWER GEN, PURPOSE, LEAD GUITAR, DISCO.RES, FREQ, 45Hz. FREQ, RESP, TO 7KHz. SENS, 98dB. PRICE £36.66 + £3.50 P&P.17 100 WATT C12100TC TWIN CONE) HIGH POWER WIDE RESPONSE, PA., VOICE, DISCO.RES, FREQ, 45Hz. FREQ, RESP, TO 14KHz. SENS, 100dB. PRICE £37.63 + £3.50 P&P.12' 200 WATT C122008 HIGH POWER BASS, KEYBOARDS, DISCO, P.A.RES, FREQ, 40Hz FREQ, RESP, TO 7KHz. SENS, 100dB. PRICE £64.17 + £3.50 P&P.12" 300 WATT C12300GP HIGH POWER BASS LEAD GUITAR, KEYBOARDS, DISCO, ETC.RES, FREQ, 45Hz. FREQ, RESP, TO 5KHz. SENS, 100dB. PRICE £85.79 + £3.50 P&P.15" 100 WATT C15100BS BASS GUITAR, LOW FREQUENCY, PA., DISCO.RES, FREQ, 40Hz. FREQ. RESP, TO 5KHz. SENS, 98dB. PRICE £53.70 + £4.001.8 P.15" 200 WATT C15200BS VERY HIGH POWER BASS.RES, FREQ, 40Hz. FREQ, RESP, TO 4KHz. SENS, 99dB. PRICE £73.26 + £4.00 P&P.15" 250 WATT C15250BS VERY HIGH POWER BASS.RES. FREQ 40Hz. FREQ. RESP TO 4KHz. SENS, 99dB. PRICE £80.53 + £4.50 P&P.15" 400 WATT C15400BS VERY HIGH POWER, LOW FREQUENCY BASS.RES, FREQ, 40Hz. FREQ, RESP, TO 4KHz. SENS, 102dB. PRICE £94.12 + £4.50 P&P.18" 400 WATT C18404BS EXTREMELY HIGH POWER, LOW FREQUENCY BASS.RES, FREQ. 27Hz. FREQ, RESP, TO 3KHz. SENS, 99dB .. PRICE £167.85 + £5.00 P&P.

EARBENDERS:- HI-FI, STUDIO, IN -CAR, ETC.ALL EARB ENDER UNITS 8 OHMS EXCEPT EB8-50 AND EB10-50 DUAL 4 AND 8 OHM.BASS, SINGLE CONE, HIGH COMPLIANCE, ROLLED FOAM SURROUND8" 50 WATT EB8-50 DUAL IMPEDENCE, TAPPED 4/8 OHM BASS, HI-FI, IN -CAR.RES, FREQ, 40Hz. FREQ, RESP, TO 7KHz. SENS, 97dB. PRICE £8.90 + £2.00 P&P.10" 50 WATT EB10-50 DUAL IMPEDENCE, TAPPED 4/8 OHM BASS, HI-FI, IN -CAR.RES, FREQ,40HZ. FREQ, RESP, TO 5KHz. SENS, 99dB PRICE £12.00 + £2.50 P&P.10' 100 WATT EB10-100 BASS, HI-FI, STUDIO.RES, FREQ, 35Hz. FREQ, RESP, TO 3KHz. SENS, 96dB. PRICE £27.50 + £3.50 P&P.12" 60 WATT EB12-60 BASS, HI-FI, STUDIO.RES, FREQ, 28Hz. FREO, RESP, TO 3KHz. SENS, 92dB. PRICE £21.00 + £3.00 P&P.17 100 WATT EB12-100 BASS, STUDIO, HI-FI, EXCELLENT DISCORES, FREQ, 26Hz. FREQ. RESP, TO 3KHz. SENS, 93dB. PRICE £32.00 + £3.50 P&P.FULL RANGE TWIN CONE, HIGH COMPLIANCE, ROLLED SURROUND51/4" 60 WATT E135-60TC (TWIN CONE) HI-FI, MULTI -ARRAY DISCO ETC.RES, FREQ, 63Hz. FREQ, RESP, TO 20KHz. SENS, 92dB. PRICE £9.99 + £1.50 P&P.6W' 60 WATT EB6-6OTC (TWIN CONE) HI-FI, MULTI -ARRAY DISCO ETC.RES, FREQ, 38Hz. FREQ, RESP, TO 20KHz. SENS, 94dB. PRICE £10.99 + £1.50 P&P.8" 60 WATT EB8-60TC (TWIN CONE) HI-FI, MULTI -ARRAY DISCO ETC.RES, FREQ, 40Hz. FRE°, RESP, TO 18KHz. SENS, 89dB. PRICE £12.99 + £1.50 P&P.10" 60 WATT EB10-60TC (TWIN CONE) HI-FI, MULTI -ARRAY DISCO ETCRES. FREQ. 35Hz FREQ. RESP, TO 12KHz. SENS, 86dB PRICE £16.49 + £2.00 P&P.

TRANSMITTER HOBBY KITS

OMP MOS-FET POWER AMPLIFIERS,HIGH POWER. TWO CHANNEL 19 INCH RACK

THOUSANDS PURCHASEDBY PROFESSIONAL USERS

NEW MXF SERIES OF POWER AMPLIFIERSTHREE MODELS:- MXF200 (100w + 100w)

MXF400 (200w + 200w) MXF600 (300w + 300w)All power ratings R.M.S. Into 4 ohms.

FEATURES: * Independent power supplies with two Toroidal Transformers * Twin L.E.D. Vu meters * Rotaryindended level controls * Illuminated on/off switch * XLR connectors * Standard 775mV inputs * Open and shortcircuit proof * Latest Mos-Fets for stress free power delivery into virtually any load * High slew rate * Very lowdistortion * Aluminium cases * MXF600 Fan Cooled with D.C. Loudspeaker and Thermal Protection.

USED THE WORLD OVER IN CLUBS, PUBS, CINEMAS, DISCOS ETC.

SIZES:- MXF 200 W19" x Ha1/4" (2U x1311"MXF 400 w19"x H51/4" (3U x D12"MXF 600 W19" x H51/4" (3U xim"

MXF200 £171.35PRICES: MXF400 £228.85

MXF600 £322.00SECURICOR DELIVERY £12.00 EACH

OMP LINNET LOUDSPEAKERSTHE VERY BEST IN QUALITY AND VALUE

MADE ESPECIALLY TO SUITTODAY'S NEED FOR COM-PACTNESS WITH HIGH OUTPUTSOUND LEVELS, FINISHED INHARDWEARING BLACK VYNIDEWITH PROTECTIVE CORNERS,GRILLE AND CARRYING HANDLE,INCORPORATES 17 DRIVER PLUSHIGH FREQ. HORN FOR FULLFREQ. RANGE: 45Hz-20KHz BOTHMODELS 8 OHM, SIZE H18" x W15"x 017.

CHOICE OF TWO MODELS

POWER RATINGS QUOTED IN WATTS RMS FOR EACH CABINET

OMP 12-100 (100W 100dB) PRICE £159.99 PER PAIROMP 12-200 (200W 102dB) PRICE £209.99 PER PAIR

SECURICOR DEL.. £12.00 PER PAIR

PIEZO ELECTRIC TWEETERS -MOTOROLA

OMP SLIDE DIMMER1K WATT & 2.5K WATT

CONTROLS LOADS UP TO IKW a 2 5KWSUITABLE FOR RESISTIVE AND INDUC-TIVE LOADS BLACK ANODISED CASEREADILY FLUSH MOUNTED THROUGHPANELCABINET CUT-OUTS. ADVANCEDFEATURES INCLUDE:-

FULL 65rnrn SLIDE

TRAVELNEON

MONITORINDICATOR* FLASH OVERRIDEBUTTON* HIGH & LOW LEVELPRESETS

* FULLY SUPPRESSEDTO BS BOO

fans: -110N H128xW40.055mm2 5KW H128xW76.079rnm

PRICES:- 1K WATT £15.992.5K WATT f 24 99 L- 60p P&P

PIEZO ELECTRIC TWEETERS - MOTOROLAJoin the Reza revolution. The low dynamic mass (no voice coil) of a Piezo tweeter produces an improved transientresponse with a lower distortion level than ordinary dynamic tweeters. As a crossover is not required these units canbe added to existing speaker systems of up to 100 watts (more if 2 put in series). FREE EXPLANATORY LEAFLETSSUPPLIED WITH EACH TWEETER. TYPE 'A' (KSN2036A) 3" round with protective wire

mesh, ideal for bookshelf and medium sized Hi-fispeakers. Price £4.90 each + 50p P&P.TYPE 'B' (KSN1005a) 31/2" super horn. For generalpurpose speakers, disco and P.A. systems etc. Price£5.00 each + 50p P&P.TYPE 'C' (KSN6016A) 2"x5" wide dispersion horn, Forquality Hi-fi systems and quality discos etc. Price £6.99each + 50p P&P.TYPE 'D' (KSN1025A) 2"x6" wide dispersion horn.Upper frequency response retained extending down tomid range (2KHz). Suitable for high quality Hi-fi systemsand quality discos. Price £9.99 each + sop P&P.TYPE'E' (KSN1038A)33/4" horn tweeter with attractivesilver finish trim. Suitable for Hi-fi monitor systems etc.Price £5.99 each + 50p P&P.LEVEL CONTROL Combines on a recessed mountingplate, level control and cabinet input jack socket.85 El5mm. Price £3.99 + 50p P&P.

STEREO DISCO MIXER

PROVEN TRANSMITTER DESIGNS INCLUDING GLASS FIBRERIVED CIRCUIT BOARD AND HIGH DUALITY COMPONENTS

COMPLETE WITH CIRCUIT AND INSTRUCTIONS

3W FM TRANSMITTER 130-108MHz, VARICAP CONTROLLED PROFESSIONAL PER-

FORMANCE, RANGE UP TO 3 MILES, SIZE 38 x 123mm, SUPPLY 12V (a 0.5AMP.

PRICE 114.49 + £1.00 P&PFM MICRO TRANSMITTER (BUG( 100-108MHz VARICAP TUNED COMPLETE WITH

VERY SENS FET MIC, RANGE 100300m, SIZE 56 x 46mm. SUPPLY 9V BAIT, PRICE

£8.62 + £1.00 P&P

3 watt FMTransmitter

POSTAL CHARGES PER ORDER D60 MINIMUM. OFFICIAL ORDERS WELCOME FROMj SCHOOLS, COLLEGES, GOVT. BODIES, ETC. PRICES INCLUSIVE OF V.A.7 SALES COUNTER.

VISA ACCESS ACCEPTED BY POST, PHONE OR FM.

8ARGAYEARD

STEREO DISCO MIXER with 2 x 5 band L& Rgraphic equalisers and twin 10 segment L.E.D.Vu Meters. Many outstanding features 5 Inputswith individual faders providing a useful com-bination of the following: -3 Turntables (Mag). 3 Mics. 4 Line including CDplus Mic with talk over switch Headphone Moni-tor. Pan Pot L. & R. Master Output controls.Output 775mV. Size 360x 280x 90mm. Supply220-240v.

Price £134.99 - £4.00 PAP

B. K. ELECTRONICS Dept PEUNIT 5, COMET WAY, SOUTHEND-ON-SEA, ESSEX. SS2 6TR

TEL 0702-527572 FAX 0702-420243


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PRACTICAL · VOL 25 NO 12 DECEMBER 1989 CONTENTS COMPETITION RESULTS MAPLINOSCOPY WINNERS! 45 All...

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Transcript of PRACTICAL · VOL 25 NO 12 DECEMBER 1989 CONTENTS COMPETITION RESULTS MAPLINOSCOPY WINNERS! 45 All...