简单的光开关电路

简单的光开关电路,Simpleopticalswitch
IntroductionThe555isprovedtobethemostversatileandubiquitousICallovertheworld.Thisisapossibleuse:simpleinvertingschmitttrigger.CircuitexplanationWhenthephototransistorisstrokenbyIRlightitconductsandthevoltagebetweenthe1Mohmresistor(arbitrary)andthephototransdropsfromVCCtolowervalues.WhenthevoltagedropslowerthanVCC/3the555istriggeredandgoeshigh(from0TOVCC).TheamountoflightthatstrikethephototransnecessarytobringhiscollectortoVCC/3isdeterminedbytheresistor(Vdrop=Icollector*R,so,ifVdrop=2*VCC/3,theresistanceneededtosetthethresholdoncurrentisR=2*VCC/(Icollector*3)).Highsensibilityphototranswouldneedasmallerresistor,andweakerphototransistorshighervalueresistor,youcanalsouseatrimmertosettheonthresholdlevelwithprecision.Thetimeofphototransistorisn'tcritical.The555hashighcurrentcapabilityandcandrivevariousdevices,suchasBipolars,relays,bipolars+relays,mosfets,mosfets+totempole,orgivealogicoutput(seepic).Incaseyouneedtotriggersomethingwhenthegateisblocked(forexampleaburglaralarm,oramultistagecoilgun)youneedtoinverttheoutput,whichisaccomplishedusingasmallbipolartransistorwiredinaninvertingsetup(seepic)orswappingthepositionsofphototransistorwiththeresistor,sothevoltagewilldropunderVCC/3whenblocked:TheformulatodeterminetheresistancetoturnoffatIcollectorisR=VCC/(Icollector*3).