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wireless doorbell

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Wireless Doorbell An educational project using a $10.00 doorbell No kits are available for this project 9-9-2006 You cannot beat the Chinese. The project described in this article is a readymade item from a $2.00 shop! And it is made in China! It is a wireless doorbell with a cost of about $10.00. It is so cheap, you could not build it from components for $10.00! So, we have to change our attitude to building projects and use items such as this to learn how things work and modify them to suit our own requirements. This doorbell uses quality components. It's not rubbish. The circuit is quite unbelievable. You can not obtain some of the components individually and the effectiveness is magic. We have so much to learn! Transistor Type: Collector current: Max frequency 8050 NPN 1.5A 100MHz 8550 PNP 1.5A 200MHz 9014 NPN 150mA 150MHz 9018 NPN 50mA 700MHz The first thing you will notice is the clever circuitry. Some of the design goes against everything we have learnt in electronics. That's why we have to study other people's designs and realise "the more you know, the more you realise you don't know." The oscillator circuit is very interesting, but first we will look at the RF oscillator. The doorbell operates on the 303MHz band and the 30 metre range (100ft) is obtained without the use of an antenna! The circuit is actually radiating from the printed track of the tank circuit. The Tank Circuit is a singleturn coil and a small capacitor (5p & 4p in parallel). In this project we show how to add a small antenna to the circuit to get double the range plus two other improvements to increase the range. Some of the improvements will load the circuit and alter the frequency at which it operates. Others can be done without any effect on the circuit. Fortunately, the transmitting stage is what we call "tight" and is not affected by surrounding

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wireless door bell project from talking electronics

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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

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  • 5/10/2015 WirelessDoorbell

    http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html 2/6

    "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

  • 5/10/2015 WirelessDoorbell

    http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html 3/6

    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.

  • 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.

  • 5/10/2015 WirelessDoorbell

    http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html 5/6

    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.

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  • 5/10/2015 WirelessDoorbell

    http://www.talkingelectronics.com/projects/WirelessDoorbell/WirelessDoorbell.html 6/6

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