how to measure pluse volt with adc ?

i want measure a 100hz pulse volt with pic 16f72 with pin ra0.(without use of timer )it pulse comming form 4n35 with full bridge rect.
please anybody better suggest me

thanks

100 Hz pulse has spectrum that spans many harmonics, depending on accuracy desired an ADC must operate at minimum of 2x highest frequency and have an anti-aliasing Nyquist filter. Consider >>10x rate or 1kHz if you want to reconstruct input signal from ADC data and find peak.

Otherwise if that is all you need, just add a Cap and charge to peak value and measure DC. If that interferes with circuit, then add a diode and small cap instead and measure with ADC. Observe RC time constant by design.

thank you sir for reply .Sir i can,t modity harware because hardware is fixed my customber not want to change it.
it is good working its origional firmware.i want to write my code for it
104pf is already on it

You need to add a capacitor between ADC input pin and GND.

sir my circuit is attached .actually i need read only two point for low cut off of main and low restore of main for inverter

can possible it with timer or without timer. please help me i writen all code for my project but i stuck here

That is horribly drawn!

Basically, it's a bridge rectifier (with one diode in one arm and two in series in the other for some reason) driving two opto-coupler LEDs. One opto-coupler does absolutely nothing, the other charges C1 while it is conducting. The high value variable resistor sets the time constant by providing a discharge path while the opto isn't conducting.

It isn't the most clever of circuits because C1 will be in charging up state for almost all the AC cycle. However, I doubt you can use it to measure pulses accurately. If you want to do that, you need clean pulses at the output of the optocoupler so short out R1 and replace RV1 with a fixed 10K resistor.

As it is, there will be an analog voltage across C1 which varies according to the value of RV1 and will only change as its value is made quite small. You can measure the voltage using the ADC but it wont tell you much about the pulses themselves.

Brian.

thank you sir for reply...how i can measure these volts whithout and hardware modification ?
i need only measure volt. pulse is not nessasary for me.i need volts for main low cut off for my inverter project