Core-less transformer for Zero-crossing detection
Since you said that it didn't need to be isolated, maybe something like this would work:



Output looks like this:


If you are on 120Vac, then change the 2.2k to ~1.1k.
Again, beware that the left hand side is not isolated from the mains.

Source: https://circuitdigest.com/electronic-circuits/zero-crossing-detector-circuit-diagram