wireless doorbell

5/10/2015 WirelessDoorbell

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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

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

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

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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.

22102014

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690,000

690,000

2,650,000

4,890,000

989,876

14,400,000

129,000

299,000

4,250,000

480,000

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