hysteresis of 74HC04

If you are looking for a "missing pulse" (sine half cycle) you
would like to do that at the peak, not the zero.

Way back when, I worked on motor controls where first we
would differentiate the line sine (after xfmr for safety) so
that -its- zero crossing indicated line peak. We used that
for phase-cut SCR control trigger (must trigger -after- peak
and always quit before zero, control was by delay-from-peak
to fire the SCRs onto the motor winding).

So you could get the peal location and at that location, sample
and compare (or just compare, if quick and clean enough) to
say whether you got a pulse or you didn't.

But trying to do that kind of triggering on a raggedy sine might
give you more than you wanted. The good part is, you'd be
working where the signal "should" be robust, and can set a
threshold well above "nothing at all" to declare a malformed
sine cycle - like if your xfmr secondary is 24VAC, call anything
less than 10V at peak "not right". Or maybe it's 5 or 2 or 1V,
but still not however many mV of line trash you might be
bothered by when looking for zero, one measure and done.