The sticking point is the sine wave is controlled by a triac, and depending on the external system mode of operation, it can either be a full wave, a clipped waveform with only the positive half or only the negative half of the sine wave. It's not a problem to generate the logic signal from the full wave or positive clipped waveform, but I haven't cracked the code of how to do it with the negative alternation.

My current solution is an AC opto-coupler and a couple of resistors, that solves the problem, but the board footprint is larger than I like, and they are also more costly than a few discrete components if I can come up with a scheme that will work.