全面了解红外遥控器.docx

全面了解红外遥控器.docx

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全面了解红外遥控器

全面了解红外遥控

作者：

SanBergmans

IRRemoteControlTheory

ThecheapestwaytoremotelycontroladevicewithinavisiblerangeisviaInfra-Redlight.Almostallaudioandvideoequipmentcanbecontrolledthiswaynowadays.Duetothiswidespreadusetherequiredcomponentsarequitecheap.Thusmakingitidealforushobbyiststouseforourownprojects.

ThispartofmyknowledgebasewillexplainthetheoryofoperationofIRremotecontrol,andsomeoftheprotocolsthatareinuseinconsumerelectronics.

Infra-RedLight

Infra-Redactuallyisnormallightwithaparticularcolour.Ushumanscan'tseethiscolourbecauseit'swavelengthof950nmisbelowthevisiblespectrum.That'soneofthereasonswhyIRischosenforremotecontrolpurposes,wewanttouseitbutwe'renotinterestedinseeingit.AnotherreasonisbecauseIRLEDsarequiteeasytomake,andthereforcanbeverycheap.

Althoughwehumanscan'tseetheInfra-Redlightemittedfromaremotecontroldoesn'tmeanwecan'tmakeitvisible.

Avideocameraordigitalcameracan"see"theInfra-Redlightasyoucanseeinthispicture.

UnfortunatelyforustherearemanymoresourcesofInfra-Redlight.Thesunisthebrightestsourceofall,buttherearemanyothers,like:

lightbulbs,candles,centralheatingsystem,andevenourbodyradiatesInfra-Redlight.Infacteverythingthatradiatesheat,alsoradiatesInfra-Redlight.

ThereforwehavetotakesomeprecautionstoguaranteethatourIRmessagegetsacrosstothereceiverwithouterrors.

Modulation

Modulationistheanswertomakeoursignalstandoutabovethenoise.WithmodulationwemaketheIRlightsourceblinkinaparticularfrequency.TheIRreceiverwillbetunedtothatfrequency,soitcanignoreeverythingelse.

Youcanthinkofthisblinkingasattractingthereceiver'sattention.Wehumansalsonoticetheblinkingofyellowlightsatconstructionsitesinstantly,eveninbroughtdaylight.

InthepictureaboveyoucanseeamodulatedsignaldrivingtheIRLEDofthetransmitterontheleftside.Thedetectedsignaliscomingoutofthereceiverattheotherside.

Inserialcommunicationweusuallyspeakof'marks'and'spaces'.The'space'isthedefaultsignal,whichistheoffstateinthetransmittercase.Nolightisemittedduringthe'space'state.Duringthe'mark'stateofthesignaltheIRlightispulsedonandoffataparticularfrequency.Frequenciesbetween30kHzand60kHzarecommonlyusedinconsumerelectronics.

Atthereceiversidea'space'isrepresentedbyahighlevelofthereceiver'soutput.A'mark'isthenautomaticallyrepresentedbyalowlevel.

Pleasenotethatthe'marks'and'spaces'arenotthe1-sand0-swewanttotransmit.Therealrelationshipbetweenthe'marks'and'spaces'andthe1-sand0-sdependsontheprotocolthat'sbeingused.Moreinformationaboutthatcanbefoundonthepagesthatdescribetheprotocols.

TheTransmitter

Thetransmitterusuallyisabatterypoweredhandset.Itshouldconsumeaslittlepoweraspossible,andtheIRsignalshouldalsobeasstrongaspossibletoachieveanacceptablecontroldistance.Preferablyitshouldbeshockproofaswell.

ManychipsaredesignedtobeusedasIRtransmitters.Theolderchipswerededicatedtoonlyoneofthemanyprotocolsthatwereinvented.NowadaysverylowpowermicrocontrollersareusedinIRtransmittersforthesimplereasonthattheyaremoreflexibleintheiruse.Whennobuttonispressedtheyareinaverylowpowersleepmode,inwhichhardlyanycurrentisconsumed.TheprocessorwakesuptotransmittheappropriateIRcommandonlywhenakeyispressed.

Quartzcrystalsareseldomusedinsuchhandsets.Theyareveryfragileandtendtobreakeasilywhenthehandsetisdropped.Ceramicresonatorsaremuchmoresuitablehere,becausetheycanwithstandlargerphysicalshocks.Thefactthattheyarealittlelessaccurateisnotimportant.

ThecurrentthroughtheLED（orLEDs）canvaryfrom100mAtowellover1A!

InordertogetanacceptablecontroldistancetheLEDcurrentshavetobeashighaspossible.Atrade-offshouldbemadebetweenLEDparameters,batterylifetimeandmaximumcontroldistance.LEDcurrentscanbethathighbecausethepulsesdrivingtheLEDsareveryshort.AveragepowerdissipationoftheLEDshouldnotexceedthemaximumvaluethough.YoushouldalsoseetoitthatthemaximumpeekcurrentfortheLEDisnotexceeded.AlltheseparameterscanbefoundintheLED'sdatasheet.

AsimpletransistorcircuitcanbeusedtodrivetheLED.AtransistorwithasuitableHFEandswitchingspeedshouldbeselectedforthispurpose.

TheresistorvaluescansimplybecalculatedusingOhm'slaw.RememberthatthenominalvoltagedropoveranIRLEDisapproximately1.1V.

Thenormaldriver,describedabove,hasonedisadvantage.Asthebatteryvoltagedrops,thecurrentthroughtheLEDwilldecreaseaswell.Thiswillresultinashortercontroldistancethatcanbecovered.

Anemitterfollowercircuitcanavoidthis.The2diodesinserieswilllimitthepulsesonthebaseofthetransistorto1.2V.Thebase-emittervoltageofthetransistorsubtracts0.6Vfromthat,resultinginaconstantamplitudeof0.6Vattheemitter.Thisconstantamplitudeacrossaconstantresistorresultsincurrentpulsesofaconstantmagnitude.CalculatingthecurrentthroughtheLEDissimplyapplyingOhm'slawagain.

TheReceiver

Manydifferentreceivercircuitsexistonthemarket.Themostimportantselectioncriteriaarethemodulationfrequencyusedandtheavailabilityinyouregion.

InthepictureaboveyoucanseeatypicalblockdiagramofsuchanIRreceiver.Don'tbealarmedifyoudon'tunderstandthispartofthedescription,foreverythingisbuiltintoonesingleelectroniccomponent.

ThereceivedIRsignalispickedupbytheIRdetectiondiodeontheleftsideofthediagram.Thissignalisamplifiedandlimitedbythefirst2stages.ThelimiteractsasanAGCcircuittogetaconstantpulselevel,regardlessofthedistancetothehandset.

AsyoucanseeonlytheACsignalissenttotheBandPassFilter.TheBandPassFilteristunedtothemodulationfrequencyofthehandsetunit.Commonfrequenciesrangefrom30kHzto60kHzinconsumerelectronics.

Thenextstagesareadetector,integratorandcomparator.Thepurposeofthesethreeblocksistodetectthepresenceofthemodulationfrequency.Ifthismodulationfrequencyispresenttheoutputofthecomparatorwillbelow.

AsIsaidbefore,alltheseblocksareintegratedintoasingleelectroniccomponent.Therearemanydifferentmanufacturersofthesecomponentsonthemarket.Andmostdevicesareavailableinseveralversionseachofwhicharetunedtoaparticularmodulationfrequency.

Pleasenotethattheamplifierissettoaveryhighgain.Thereforethesystemtendstostartoscillatingveryeasily.Placingalargecapacitorofatleast22µFclosetothereceiver'spowerconnectionsismandatorytodecouplethepowerlines.Somedatasheetsrecommendaresistorof330Ohmsinserieswiththepowersupplytofurtherdecouplethepowersupplyfromtherestofthecircuit.

ThereareseveralmanufacturersofIRreceiversonthemarket.Siemens,VishayandTelefunkenarethemainsuppliershereinEurope.SiemenshasitsSFH506-xxseries,wherexxdenotesthemodulationfrequencyof30,33,36,38,40or56kHz.TelefunkenhaditsTFMS5xx0andTK18xxseries,wherexxagainindicatesthemodulationfrequencythedeviceistunedto.Itappearsthatthesepartshavenowbecomeobsolete.TheyarereplacedbytheVishayTSOP12xx,TSOP48xxandTSOP62xxproductseries.

Sharp,XiamenHualianandJapaneseElectricare3AsianIRreceiverproducingcompanies.SharphasdeviceswithverycrypticIDnames,like:

GP1UD26xK,GP1UD27xKandGP1UD28xK,wherexisrelatedtothemodulationfrequency.Hualianhasit'sHRMxx00series,liketheHRM3700andHRM3800.JapaneseElectrichasaseriesofdevicesthatdon'tincludethemodulationfrequencyinthepart'sID.ThePIC-12042LMistunedto36.7kHz,andthePIC12043LMistunedto37.9kHz.

TheEnd?

ThisconcludesthetheoryofoperationforIRremotecontrolsystemsintendedforuseinconsumerelectronics.IrealisethatotherwaysexisttoimplementIRcontrol,butIwilllimitmyselftothedescriptionabove.Oneoftheissuesnotcoveredhereissecurity.SecurityisofnoimportanceifIwanttocontrolmyVCRorTVset.Butwhenitcomestoopeningdoorsorcarsitliterallybecomesa'key'feature!

MaybeIwillcoverthisissuelater,butnotfornow.

Ialsorealisethatmysmalllistofmanufacturersisfarfrombeingcomplete.Itishardlypossibletolisteverymanufacturerhere.

ThispageonlydescribedthebasictheoryofoperationofIRremotecontrol.Itdidnotdescribetheprotocolsthatareinvolvedincommunicationbetweentransmitterandreceiver.Manyprotocolsaredesignedbydifferentmanufacturers.Youcanfindtheprotocolsofsomemanufacturersinthelinksectionatthetopofthispage.

ITTProtocol

TheITTIRprotocolisaveryoldone.ItdiffersfromotherprotocolsinthatitdoesnotuseamodulatedcarrierfrequencytosendtheIRmessages.Asinglecommandistransmittedbyatotalof14pulseswithawidthof10µseach.Thecommandisencodedbythedistancebetweenthepulses.

Thisprotocolisveryreliableandconsumesverylittlepowerensuringlongbatterylife.

ManyconsumerelectronicsbrandsusedthisprotocolinEurope.Amongthemwere:

ITT,Greatz,Schaub-Lorenz,Finlux,Luxor,Salora,OceanicandlateralsoNokia,tonamebutafew.

Features

∙Only14veryshortIRpulsespermessage

∙Pulsedistanceencoding

∙Longbatterylife

∙4bitaddress,6bitcommandlength