Check google for zero crossing detector, etc.
This one if the many out there.
Here's a design (from the 27 May 1981 EDN) that I have built and have used in operational hardware; it works well:
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I trust this is reasonably clear, including the terminal names on the semiconductors... The 2N5062 is a small SCR; my notes say that a TIC47 is an approximate equivalent. The 4N27 is, of course, an optoisolator.
The combination of the 68k resistor and the capacitor provides a phase-shifted version of the AC input, so there is still 30-40V across the capacitor when the AC line goes through zero. When the downward zero crossing does occur, the 2N3904 turns off, the SCR's gate goes positive, and the SCR fires, dumping the capacitor's charge through the 4N27's LED and pulling the output down to ground. The output pulse has a fairly fast fall but a slow rise, and even the fall is slow by digital-logic standards; I used a Schmitt trigger to clean it up.
The great virtue of this approach is that it is virtually immune to noise. The one-shot nature of the charge dumping ensures only one output pulse per downward zero crossing. If you want a stable 60Hz source with no phase jitter, you probably want to use a phase-locked loop with a slow response to clean things up further -- that's what I did, for a complex dimmer circuit that had to be phase-locked to the AC -- but if all you want to do is count zero crossings, it's perfect as is.
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