1、Digital VoltmeterI.IntroductionOur goal here isto build a voltmeter. We will do this using almost all digital circuitryratherthan analog. Thecircuitworkslikemanyother digital measurement circuitsinthatitconverts thequantity to be measured intoa timeinterval, then m
2、easuresthattime.Call the voltage we wish to measure Vin. Say we charge a capacitor linearly with time(that is, at a constant chargingcurrentI), starting at zero volts.Itwill take atime t= Vin/ (dVin /dt) =VinC / (dQ / dt) = VinC/I forthecapacitor to charge up toVin
3、. So if we can measure thistime interval,wecan determineVin. To measure time in digital electronics, we produce ahighfrequencypulse train andcountthe pulsesoccurringduring thetimeinterval to be measured.The number of pulses counted is ameasure of the timeinterval,
5、wn on thelast page of this writeup (don't let it frightenyou!). On the secondtolastpageisthe samecircuitwith itsfunctional sections outlinedand labeled.The sections needed for thisproject are:A)A pulsetrain generator. In electronics, thisiscalled aclock. We havea
6、lreadylearnedhowto build this using the555 timerchip.This is theclock generation section.B)A way to monitor thevoltageacross thecapacitorand generate signals when thecharging starts at zerovolts and when thevoltagebecomes equal to thevoltagetobemeasured, Vin. I call
7、 this the sectioncontrol and gating.C)A way to “count” the number of clockpulses that occur between thestartand stopcharging signals.Thisisthecounting section.D)A constant current capacitorcharging circuit that can be turned on and offand reset tozerovolts by the st
8、artand stopcharging signals.analog input/transducerE) Ahumanreadable way to display thenumberofpulses counted during thisinterval.This is
