555定时器详细说明英文.docx

555定时器详细说明英文.docx

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555定时器详细说明英文

AnOverviewofthe555Timer

The555IntegratedCircuit（IC）isaneasytousetimerthathasmanyapplications.Itiswidelyusedinelectroniccircuitsandthispopularitymeansitisalsoverycheaptopurchase,typicallycostingaround30p.A'dual'versioncalledthe556isalsoavailablewhichincludestwoindependent555ICsinonepackage. 

Thefollowingillustrationshowsboththe555（8-pin）andthe556（14-pin）. 

Inacircuitdiagramthe555timerchipisoftendrawnliketheillustrationbelow.Noticehowthepinsarenotinthesameorderastheactualchip,thisisbecauseitismucheasiertorecognizethefunctionofeachpin,andmakesdrawingcircuitdiagramsmucheasier. 

Forthe555tofunctionitreliesonbothanalogueanddigitalelectronictechniques,butifweconsideritsoutputonly,itcanbethoughtofasadigitaldevice.Theoutputcanbeinoneoftwostatesatanytime,thefirststateisthe'low'state,whichis0v.Thesecondstateisthe'high'state,whichisthevoltageVs（Thevoltageofyourpowersupplywhichcanbeanythingfrom4.5to15v.18vabsolutemaximum）.Themostcommontypesofoutputscanbecategorizedbythefollowing（theirnamesgiveyouaclueastotheirfunctions）:

∙Monostablemode:

inthismode,the555functionsasa"one-shot".Applicationsincludetimers,missingpulsedetection,bouncefreeswitches,touchswitches,frequencydivider,capacitancemeasurement,pulse-widthmodulation（PWM）etc

∙Astable-freerunningmode:

the555canoperateasanoscillator.UsesincludeLEDandlampflashers,pulsegeneration,logicclocks,tonegeneration,securityalarms,pulsepositionmodulation,etc.

∙BistablemodeorSchmitttrigger:

the555canoperateasaflip-flop,iftheDISpinisnotconnectedandnocapacitorisused.Usesincludebouncefreelatchedswitches,etc.

∙PinConfigurationofthe555Timer

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Hereistheidentificationforeachpin:

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Whendrawingacircuitdiagram,alwaysdrawthe555asabuildingblock,asshownbelowwiththepinsinthefollowinglocations.Thiswillhelpyouinstantlyrecognisethefunctionofeachpin:

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Pin1（Ground）:

Connectstothe0vpowersupply. 

Pin2（Trigger）:

Detects1/3ofrailvoltagetomakeoutputHIGH.Pin2hascontroloverpin6.Ifpin2isLOW,andpin6LOW, outputgoesandstaysHIGH.Ifpin6HIGH,andpin2goesLOW,outputgoesLOWwhilepin2LOW.Thispinhasaveryhighimpedance（about10M）andwilltriggerwithabout1uA. 

Pin3（Output）:

（Pins3and7are"inphase."）GoesHIGH（about2vlessthanrail）andLOW（about0.5vlessthan0v）andwilldeliverupto200mA. 

Pin4（Reset）:

InternallyconnectedHIGHvia100k.Mustbetakenbelow0.8vtoresetthechip. 

Pin5（Control）:

AvoltageappliedtothispinwillvarythetimingoftheRCnetwork（quiteconsiderably）. 

Pin6（Threshold）:

Detects2/3ofrailvoltagetomakeoutputLOWonlyifpin2isHIGH.Thispinhasaveryhighimpedance（about10M）andwilltriggerwithabout0.2uA. 

Pin7（Discharge）:

GoesLOWwhenpin6detects2/3railvoltagebutpin2mustbeHIGH.Ifpin2isHIGH,pin6canbeHIGHorLOWandpin7remainsLOW.GoesOPEN（HIGH）andstaysHIGHwhenpin2detects1/3railvoltage（evenasaLOWpulse）whenpin6isLOW. （Pins7and3are"inphase."）Pin7isequaltopin3butpin7doesnotgohigh-itgoesOPEN. ButitgoesLOWandwillsinkabout200mA. 

Pin8（Supply）:

Connectstothepositivepowersupply（Vs）.Thiscanbeanyvoltagebetween4.5Vand15VDC,butiscommonly5VDCwhenworkingwithdigitalICs. 

∙Insidethe555Timer

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Youmaywonderwhatisinsidethe555timerchiporwhatmakesitwork.Well,the555timerchipanIntergratedCircuit（IC）andthereforeitcontainsaminiturizedcircuitsurroundedbysilicon.Eachofthepinsisconnectedtothecircuitwhichconsistsofover20transistors,2diodesand15resistors. 

Theillustrationbelowshowsthefunctionalblockdiagramofthe555timerIC. 

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Doyounoticethethree5kresistors?

Thisishowthechipgotit'sname. 

555TimerOperatingModes

Astablemode 

An AstableCircuit hasnostablestate-hencethename"astable".Theoutputcontinuallyswitchesstatebetweenhighandlowwithoutwithoutanyinterventionfromtheuser,calleda'square'wave.Thistypeofcircuitcouldbeusedtogiveamechanismintermittentmotionbyswitchingamotoronandoffatregularintervals.ItcanalsobeusedtoflashlampsandLEDs,andisusefulasa'clock'pulseforotherdigitalICsandcircuits. 

Monostablemode 

A MonostableCircuit producesonepulseofasetlengthinresponsetoatriggerinputsuchasapushbutton.Theoutputofthecircuitstaysinthelowstateuntilthereisatriggerinput,hencethename"monostable"meaning"onestablestate".histypeofcircuitisidealforuseina"pushtooperate"systemforamodeldisplayedatexhibitions.Avisitorcanpushabuttontostartamodel'smechanismmoving,andthemechanismwillautomaticallyswitchoffafterasettime. 

BistableMode（orSchmittTrigger） 

A BistableMode orwhatissometimescalleda SchmittTrigger,hastwostablestates,highandlow.TakingtheTriggerinputlowmakestheoutputofthecircuitgointothehighstate.TakingtheResetinputlowmakestheoutputofthecircuitgointothelowstate.Thistypeofcircuitisidealforuseinanautomatedmodelrailwaysystemwherethetrainisrequiredtorunbackandforthoverthesamepieceoftrack.Apushbutton（orreedswitchwithamagnetontheundersideofthetrain）wouldbeplacedateachendofthetracksothatwhenoneishitbythetrain,itwilleithertriggerorresetthebistable.Theoutputofthe555wouldcontrolaDPDTrelaywhichwouldbewiredasareversingswitchtoreversethedirectionofcurrenttothetrack,therebyreversingthedirectionofthetrain. 

UsingtheOutputofa555Timer

Theoutput（Pin3）ofthe555canbeinoneoftwostatesatanytime,whichmeansitisadigitaloutput.ItcanbeconnecteddirectlytotheinputsofotherdigitalICs,oritcancontrolotherdeviceswiththehelpofafewextracomponents.Thefirststateisthe'low'state,whichisthevoltage0Vatthepowersupply.Thesecondstateisthe'high'state,whichisthevoltageVccatthepowersupply. 

SinkingandSourcing 

WhentheOutputgoeslow,currentwillflowthroughthedeviceandswitchiton.Thisiscalled'sinking'currentbecausethecurrentissourcedfromVsandflowsthroughthedeviceandthe555to0V. 

WhentheOutputgoeshigh,currentwillflowthroughthedeviceandswitchiton.Thisiscalled'sourcing'currentbecausethecurrentissourcedfromthe555andflowsthroughthedeviceto0V. 

Sinkingandsourcingcanalsobeusedtogethersothattwodevicescanbealternatelyswitchedonandoff. 

Thedevice（s）couldbeanythingthatcanbeswitchedonandoff,suchasLEDs,lamps,relays,motorsorelectromagnets.Unfortunately,thesedeviceshavetobeconnectedtotheOutputindifferentwaysbecausetheOutputofthe555canonlysourceorsinkacurrentofupto200mA.Makesureyourpowersupplycanprovideenoughcurrentforboththedeviceandthe555,otherwisethetimingofthe555willbeaffected. 

555TimerCalculator

Thispageisveryusefulwhencreating555timercircuits.itwillallowyoutocalculatethefrequencyanddutycycle. 

Astable555SquareWaveCalculator 

ComingSoon 

Monostable555TimeoutCalculator 

ComingSoon 

CommonMistakesWhenUsinga555Timer

Herearesomemistakestoavoid:

1.Pin7getsconnectedtothe0vrailviaatransistorinsidethechipduringpartoftheoperationofthe555.Ifthepotisturnedtoverylowresistanceinthefollowingcircuit,ahighcurrentwillflowthroughthepotanditwillbedamaged:

2.Theimpedanceofthe100uelectrolyticwillallowaveryhighcurrenttoflowandthechipwillgetveryhot.  Use10umaximumwhenusing8Rspeaker. 

3.Theresetpin（pin4）isinternallytiedHIGHviaapprox100kbutitshouldnotbeleftfloatingasstraypulsesmayresetthechip. 

4.Donotdraw555circuitsasshowninthefollowingdiagram.Keeptoastandardlayoutsothecircuitiseasytofollow. 

5.Here'sanexamplefromtheweb.Ittakesalotoftimetoworkoutwhatthecircuitisdoing:

Theaimittolay-outacircuitsothatitshowsinstantlywhatishappening.That'swhyeverythingmustbeinrecognisedlocations. 

Hereisthecorrectedcircuit:

Fromthisdiagramitisobviousthecircuitisanoscillator（andnotaone-shotetc）. 

6.Don'tusehighvalueelectrolyticsandhighresistancestoproducelongdelays.The555isveryunreliablewithtimingvaluesabove5-10minutes.Thereasonissimple.Thechargingcurrentfortheelectrolyticisbetween1-3microampinthefollowingdiagram（whentheelectroisbeginningtocharge）anddropstolessthan1microampwhentheelectroisnearlycharged. 

Iftheleakageoftheelectrois1microamp,itwillneverfullychargeandallowthe555to"time-out."

7.DonotconnectaPNPtotheoutputofa555asshowninthefollowingdiagram.Pin3doesnotrisehighenoughtoturnoffthetransistorandthecurrenttakenbythecircuitwillbeexcessive.UseanNPNdriver.