Wireless Doorbell
description
Transcript of Wireless Doorbell
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5/10/2015 WirelessDoorbell
http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html 1/6
WirelessDoorbell
Aneducationalprojectusinga$10.00doorbell
Nokitsareavailableforthisproject
9-9-2006
YoucannotbeattheChinese.Theprojectdescribedinthisarticleisareadymadeitemfroma$2.00shop!AnditismadeinChina!Itisawirelessdoorbellwithacostofabout$10.00.Itissocheap,youcouldnotbuilditfromcomponentsfor$10.00!So,wehavetochangeourattitudetobuildingprojectsanduseitemssuchasthistolearnhowthingsworkandmodifythemtosuitourownrequirements.Thisdoorbellusesqualitycomponents.It'snotrubbish.Thecircuitisquiteunbelievable.Youcannotobtainsomeofthecomponentsindividuallyandtheeffectivenessismagic.Wehavesomuchtolearn!
TransistorType:
Collectorcurrent:
Maxfrequency
8050NPN 1.5A 100MHz8550PNP 1.5A 200MHz9014NPN 150mA 150MHz9018NPN 50mA 700MHz
Thefirstthingyouwillnoticeistheclevercircuitry.Someofthedesigngoesagainsteverythingwehavelearntinelectronics.That'swhywehavetostudyotherpeople'sdesignsandrealise"themoreyouknow,themoreyourealiseyoudon'tknow."Theoscillatorcircuitisveryinteresting,butfirstwewilllookattheRFoscillator.Thedoorbelloperatesonthe303MHzbandandthe30metrerange(100ft)isobtainedwithouttheuseofanantenna!Thecircuitisactuallyradiatingfromtheprintedtrackofthetankcircuit.TheTankCircuitisasingleturncoilandasmallcapacitor(5p&4pinparallel).
Inthisprojectweshowhowtoaddasmallantennatothecircuittogetdoubletherangeplustwootherimprovementstoincreasetherange.Someoftheimprovementswillloadthecircuitandalterthefrequencyatwhichitoperates.Otherscanbedonewithoutanyeffectonthecircuit.Fortunately,thetransmittingstageiswhatwecall"tight"andisnotaffectedbysurrounding
http://www.googleadservices.com/pagead/aclk?sa=L&ai=CpZAeD7hOVeG3JYyQogOvt4DoDqKOpcVCgsHk7OYB_4KOvd0CEAEgkNTFB2Dl8uODmA6gAaaNmeoDyAEBqAMByAPDhIACqgS_AU_QmWLJxkviS03d0bqxt7c-xsa7FqE8MDvqtCaqa3JY1QGb33eIKBmBYltcpMzn5Ro41-pTZn5LM1Ij4AxxoBrmG6c0QnPhgLVcrmaIGTXQz7t3gXnRZg3OiBe_BLvRDoWtjH8hjQUhj9k8cYjd1jWN5FtOilegpdFZrGtEBxrY07d_tVrrn2Bd7lczutfi8cUegjT51EDmhbuPZvUjTmHU9Q46b8naXC1M2Xz2mtatZRqJkAzqFwRzYQhn56mMiAYBgAfC8uYV2AcB&num=1&cid=5GiiMSrEJ0pCSzKIlmjMKk_r&sig=AOD64_0sjuaWk-GbkUFUQ-glSZGgrjZNBg&client=ca-pub-1453190872429143&adurl=https://ad.doubleclick.net/ddm/clk/286993286%3B113946558%3Behttp://www.googleadservices.com/pagead/aclk?sa=L&ai=CpZAeD7hOVeG3JYyQogOvt4DoDqKOpcVCgsHk7OYB_4KOvd0CEAEgkNTFB2Dl8uODmA6gAaaNmeoDyAEBqAMByAPDhIACqgS_AU_QmWLJxkviS03d0bqxt7c-xsa7FqE8MDvqtCaqa3JY1QGb33eIKBmBYltcpMzn5Ro41-pTZn5LM1Ij4AxxoBrmG6c0QnPhgLVcrmaIGTXQz7t3gXnRZg3OiBe_BLvRDoWtjH8hjQUhj9k8cYjd1jWN5FtOilegpdFZrGtEBxrY07d_tVrrn2Bd7lczutfi8cUegjT51EDmhbuPZvUjTmHU9Q46b8naXC1M2Xz2mtatZRqJkAzqFwRzYQhn56mMiAYBgAfC8uYV2AcB&num=1&cid=5GiiMSrEJ0pCSzKIlmjMKk_r&sig=AOD64_0sjuaWk-GbkUFUQ-glSZGgrjZNBg&client=ca-pub-1453190872429143&adurl=https://ad.doubleclick.net/ddm/clk/286993286%3B113946558%3Behttp://adclick.g.doubleclick.net/aclk?sa=l&ai=CTmZLD7hOVd38HJGpoAP-tYHICqzSkYkHzIn6zKoBrK-j34wCEAEgkNTFB2Dl8uODmA6gAazlutUDyAEJqAMBqgS_AU_QzQBbeH7OPM1Ohc36GCYtSIe8JCiuUYnz6DPawmgqBMCwGuxF5yrDVCMKBt5HI_xjCRMAPl-Y6OfuqV5IM3hV-LA7TDzSBP7sOY1pG3Ekfx1IzbgZc32uB25Ey-CWhEh4XAjB4UmnaMCjAxv7I-G0KCrxapusAvnnuEJQQIFCawYyXgoJehjPHJ2wIQXbeUjNkIEmbvflDkjmxHYOP4gOvwpytP4JG_uBPtD4374a2VcwI8tMykw-NvPZ436d-gUGCCUQARgAoAYugAe8msUq2AcA&num=1&sig=AOD64_2Zq4H44QSbK20lTvKimwYz8jOMxQ&adurl=http://www.amazon.in/Gadget-Heros-Gradienter-Standard-Hotshoe/dp/B00PAUIPZ2%3Ftag%3Dgooginremarketing16-21&client=ca-pub-1453190872429143 -
5/10/2015 WirelessDoorbell
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"straycapacitance."Normally,thisstraycapacitanceisapersonshandorbody,touchingorcomingnearthetransmitting(output)stageandalteringthefrequency.Thecircuithasbeenkeptnearthepowerrailsbytheuseofachokeinthepositiverail.Thepositiverailisthenreflectedtothenegativerailviathebattery.Thisfeaturehelpsuswhenwewanttoaddanantenna.A7cmlengthoftinnedcopperwireisconnectedtothecollectorofthetransistorandbentaroundtheboardsothateverythingcanbeputbackintothecase.Whentheprojectwastestedinsidetheauthorshouse,therangewasincreasedtodouble.Whenthetransmitterwastakenoutside,therangewasover60metres(200ft)andthefullrangecouldnotbetestedasthesoundfromthedoorbellwastoofainttobeheard!
Wecanlearnsomuchfromaproductthatisalreadyonthemarket.Firstly,youknowitworks,itisreliableandyouknowwhatcanbebuiltfor$10.00.Secondly,youknowwhattypeofcomponentscanbepurchasedcheaplyandwhattoexpectfromthem.Inthiscasethetransmittingtransistorhasthehighestgainsotheyhavetakenaspecialefforttogetagoodqualitytransistor.Now,let'slookatthetransmittercircuit:
THETRANSMITTERCIRCUITThetransmittercircuitismadeupoftwobuildingblocksthe303MHzRFoscillatorandthe32kHzcrystalcontrolledoscillator.
The303MHzoscillatorconsistsofaselfoscillatingcircuitmadeupofthecoilonthePCboardanda9p(9puff)capacitor(actually4pand5pinparallel).Thecircuitstartsupbythetransistorproducingnoise.Thisrisingandfallingsignalonthecollectorispassedtotheparalleltunedcircuit(thetankcircuit)andthebaseseesaverysmoothsinewaveatafrequencyof303MHz.Thissinewaveisthenamplifiedbythetransistorandthisishowthe303MHzfrequencyisgenerated.Nowwecometothepurposeofthe15microhenrychokeonthetankcircuit.Whenthecircuitoscillates,ittakesalargerandsmallamountofcurrent.Thiscurrentpassesthroughthechokeandtheturnsproduceabackemforbackvoltagethatfightsagainsttheflow(change)incurrent.Theendeffectisavoltagecreatedatthepointwherethechokeisconnectedtothetrackworkontheboard.Thiseffectivelyallowsthetrackworktoproduceawaveformandsincethefrequencyofthiswaveisveryhigh,apercentageoftheenergyisradiatedintotheairaselectromagneticenergy.Thechokeallowsthetrackworktoeffectivelyriseandfallwhileprovidingaverylowresistancepathfortheflowofcurrentduringcertainpartsofthecycle.
Thesecondbuildingblockisthecrystaloscillator.ThisismadeupofatwostageDCcoupledamplifierwithfeedbackviathe2n2andcrystal.Ifthecrystalisremoved,theoscillatorisseenasproducingverynarrowspikeswithafrequencydeterminedbythe2n2,asshowninthediagrambelow:
Whenthecrystalisadded,thefrequencyincreases(becausetheeffectofthe2n2andcrystalinseriescreatesalowercapacitancethan2n2)andasitrises,theamplitudeofthefeedbacksignalincreasesuntilitreachesamaximumattheresonantfrequencyofthecrystal.Thecrystalexhibitsthelowestimpedance(thehighestcapacitance)attheresonantfrequency.Thisishowthecircuitstabilizesatthefrequencyofthecrystal.Whenthedeviceisturnedon,the150konthebaseofthesecondtransistorturnsthetransistoron.Thethirdtransistorhas0.65vonthecollectorandthebasealsoreceivesverycloseto0.65v,viathe220kresistor.Thethirdtransistorisnotfullyturnedonanditproducesasmallamountofnoise.Thisnoiseispassedtothesecondtransistorandappearsonthecollector.Thecollectorpassesthisnoisetothebaseofthethirdtransistorandthenoiseveryrapidlyincreasestoamaximum.Itcomestoapointwherethewaveformaboveisgeneratedandthereasonwhythespikesaresonarrowiseasytoexplain.WhenthemiddletransistorchangesfromanOFFstatetoanONstate,thecapacitorwillbe
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partiallychargedandthevoltageontheendconnectedtothebaseofthethirdtransistorwilldropabout6vandputanegativevoltageonthebaseofthethirdtransistor.ThiswillkeepitoffandthemiddletransistorwillbekeptONviathe150kbaseresistor.Thecapacitorwillgraduallychargeintheoppositedirectionviathe220kand150kandwhenthebaseofthethirdtransistorseesabout0.6v,itbeginstoturnON.ThiscausesthemiddletransistortoturnOFFandthecollectorvoltagerises.Thiscausesthecapacitortochargeandcreateacurrentflowinthebaseofthethirdtransistor.BothtransistorsarenowturnedONandthecapacitorchargesveryquicklyviathe12kandbaseemitterjunctionofthethirdtransistor.Thiscreatestheverynarrowhighperiodinthewaveform.
Whenthepushbuttonispressed,thecircuitproducesa303MHzcarrierwitha32kHztone.Thereceiverdetectsthe32kHzandturnsonaSOUNDchip.
THERECEIVERCIRCUITThereceivercircuitconsistsofanumberofstagesandwewillgothrougheachoneseparately.
Thefirststageisactuallya303MHzoscillatorthatisoperatingallthetime.Itproducesaclean303MHzsignalandthisfrequencyistoohightobedetectedorprocessedbythe4069chip,asthechipwillonlyoperatetoabout1MHz.Thetheorybehindusingthistypeofstageisquitesimple.Itiseasierto"upset"ormodifyastagethatisalreadyoscillating,ratherthangetanonoscillatingstagetobeginoscillating.Thereareallsortsofelectromagneticradiationat300MHzandthe2turncoilpicksupallthisnoise.The303MHzoscillatorisfirstlysetintooperationbythenoiseproducedbythetransistorandthisispassedtothetankcircuitmadeupofthe2turncoiland2pcapacitorasaparalleltunedcircuit.Thetransistorkeepsamplifyingthisuntilitgetstoastabilizedpointwherethecollectorisproducing"hash"(junk)ofabout300mV.Whenthetransmitterisactivated,thereceivercircuitwilldetectasignalassmallasafewmicrovoltsandthe32kHzsignalwillbeincludedwithalltheothernoise.Thereisalittlebitmorebehindthisreceivingstage,thanfirstmeetstheeye.Thestageisactuallyatransmitter,butwewillstillcallitthereceivercircuit.Yes,itisaveryweaktransmitteranditfillsthesurroundingwithaclean303MHzsignal.Whenthe303MHzsignalfromthetransmitterentersthisspace,thesignalsinterferewithoneanotherandthereceivertakesmoreandlesscurrentasittriestomaintainthesignalstrength.Whenthe32kHzsignalispresent,thereceivertakesavaryingcurrentthatcorrespondstothe32kHzsignalandthisishowthereceivercircuitproducesthewaveformtocorrespondtothe32KHz.
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5/10/2015 WirelessDoorbell
http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html 4/6
Alowimpedancepathtothe0vrailisprovidedfortheemitterbyusinga82uHchokeanda2n2capacitoracrossa560Rresistor.Thislowimpedancepathisneededsothatthetransistorhasahighgain.Thecircuitisputintoverydelicateoscillationbyusinga1k5fromthepositiverail.Itoperatesfrom3vandthecurrenttakenbythisstageislessthan1mA.
THEBRASSTUNINGSLUGThecoilinthetankcircuitistunedviaabrassscrew(orslugorcore).Thisisaltersthefrequencyatwhichthecircuitoperates,whenitisturnedviaasmallscrewdriver.At303MHz,youcannotuseaferritecoreasitwillcompletelyabsorbthemagneticradiationproducedbythecoilandpreventthecircuitoperating.Anotherchoiceisair.Ifyouuseanaircoredcoil,youcanuseatrimmercapacitortoadjustthefrequency.Acheaperapproachistouseabrasscore.Brasshasapermeabilityveryclosetoair(0isthepermeabilityoffreespace)andithasverylittleeffectonconcentratingthemagneticlinesoffluxormovingthemapart.Materialsthatcausethelinesoffluxtomovefartherapart,resultinginadecreaseinmagneticfluxdensitycomparedwithavacuum,arecalleddiamagnetic.Materialsthatconcentratemagneticfluxbyafactorofmorethanonebutlessthanorequaltotenarecalledparamagneticmaterialsthatconcentratethefluxbyafactorofmorethantenarecalledferromagnetic.However,atveryhighfrequencies,suchas303MHz,themagneticfluxcauseseddycurrentstoflowinthebrassandthisdecreasestheavailablefluxsothatinsertingabrasscorecausesthefrequencytodrop.Thisishowthereceiveristunedtoexactlythesamefrequencyasthetransmitter.Themostcriticalcapacitorinthereceiveristhe2p.Thissetsthefrequency.The4pismerelyafeedbackcapacitor.The2n2and1narecalled"pass"capacitorsandallowhighfrequencysignalstopassthroughthem.The1nactuallykeepsthepositiveandnegativerailsrigidwhilethe2n2preventstheemittermovingupanddownwhenamplifyingthe303MHzsignal.Thesignalonthecollectorpassestothefirstinputofthechipviaa5k6resistorand1n(passcapacitor).Alotofthehighfrequencycomponentisremovedwiththe1ncapacitorconnectedtothe0vrail.
Thefirstinverterhasa1Mconnectedbetweentheoutputandinputtosetittomidrailsoitbecomesahighgainamplifier.Thesecondandthirdinvertersalsoamplifythesignalandonpin6wehaveasignalgreaterthan0.6vcontainingalotofnoiseandanidentifiable32kHzwaveform.The32kHzcrystalonlyallowsthe32kHzsignaltopassandthebaseofthetransistorseesaverycleansignal.Anyotherfrequencieswillnotappearonthebaseofthetransistor.The32kHzisfurtheramplifiedwithtwomorestagesandappearsatpin10ofthechip.Itisthenpassedtoadiodepumpthatchargesa47uelectrolytic.Normally,thiselectrolyticisunchargedandpin8isHIGH.ThePNPtransistorisnotturnedonandthesoundchipissilent.Butwhentheelectrolyticcharges,thetransistorturnsonandthesoundchipoperates.
GOINGFURTHEROneofthemainreasonsforpresentingthisprojectistoshowhowtogetthebestrangefromatransmitter.Normallyyouneedveryexpensiveequipmenttohelpyou,butaverycleveralternativeistouseourmethod.Allyouhavetodoisplacethereceiverabout15metresfromthetransmitterandinaverypoorreceptionarea.Theaimistogetthereceivertobeattheextremeendoftherangesothatifyoumovethetransmitterawaybyaslittleasametre,thereceiverwillnotdetectthesignal.
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5/10/2015 WirelessDoorbell
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NowthereceiverisaverysensitiveRFindicator.Bymovingthetransmittertodifferentplaces,thereceivershouldnotdetectthesignal.Youarenowreadytoaddanantennatothetransmitteranddetermineitseffectiveness.Wehavealreadymentionedthetransmittercircuitisclassifiedas"tight"andaddinganantennashouldnotshiftthefrequency.
ADDINGANANTENNAThe7cmantennaisaddedtothepointwherethechoketouchesthePCcoilontheboard.
Addingthe7cmantenna
Thispointishighlyactivebutitdoesnotinterferewiththeoperationofthecircuit,whentheantennaisconnected.Bendthewirearoundtheedgeoftheboardandabout0.5cmaboveitandtestthetransmitter.Youshouldgetanincreaseintherange,uptodouble.Iftherangedoesnotincreaseappreciably,thereceivermaynotbetunedexactlytothetransmitter.Todothis,adjustthebrassscrewinthetankcircuitofthereceiver.Beforemakinganyadjustment,makesureyouknowtheoriginalpositionbynotingthealignmentoftheslot.Onlyturnthescrewabout15ineitherdirectionandtakea"fieldtest."Iftherangereduces,youknowthescrewhasbeenturnedinthewrongdirection.
INCREASINGTHEOUTPUTAnotherthingthatwillincreasetheoutputofthetransmitterinvolvesrewiringtheLED.AtthemomenttheLEDisinserieswiththepositiverailofthetransmitter.Thiseffectivelydropsabout1.7vandthecircuitonlygets7.3vfromafreshbattery.ByconnectingtheLEDbetweenthepositiveand0vrail,witha470Rloadresistor,thetransmittingsectionwillsee9vandthiswillincreasetherange.Onemorethingthatwillincreasetheoutputistoadda220Racrossthe220Remitterresistorofthetransmittertransistor.Thisincreasedtherangeinourprototype,butthefrequencyadjustingscrewonthereceiverhadtobeturnedabout15clockwisetocompensatefortheslightchangeinfrequency.
22102014
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690,000
690,000
2,650,000
4,890,000
989,876
14,400,000
129,000
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4,250,000
480,000
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5/10/2015 WirelessDoorbell
http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html 6/6
690,000 14,400,000 129,000 12,990,000 8,550,000