i have a 100hz sinus signal
i have a sinus voltage witch 5volt peak, and want to know when its zero volt.
This is why we can't help.i want to use it for 120v ac
Here we have 110VAC 50 or 60hz. Not 100hz. Not 5V.
The output is 0 or 5v to go to the computer.
I can build a circuit that works like this but you will get a (+)edge at one crossing and a (-)edge at the other.
This next circuit will give you a 200nS pulse (interrupt) at any zero crossing.
Now about phase:
In software you will need to measure the frequency. (you need to know 50 or 60hz) Depends on where you live.
Because you get two pulses/cycle then you really will get numbers like 100 or 120 hz.
10mS or 8.3mS
I do not know what you want to do and you are not going to say.....but
I will pretend you want to turn on a SCR like a lamp dimmer.
Measure from one pulse to another = 10 (8.3mS).
If you want the lamp at 1/2 power them after the interrupt, delay 5mS (4.15ms) and turn on the SCR.
This is how you set the phase. (measure time)