Yes that is general idea.
I do not need any detail regardin AC signal. I just need to detect AC signal whenever it is on positive side.
I do not care about the value of the detected signal.
I cara only about the time that signal is present.
So when I press interphone button Arduino starts measuring the time it is being pressed.
THanks for suggestions. If I come up with a code that suits my idea will post it for a code review.