In article , John Fields wrote:

if that 10-50V ac has an unstable amplitude
some sort of schmitt circuit with a variable offset to allow you to get it
to trigger on the zero crossing (in one direction) may probably help you to
match the phase better...

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Why not just say, "Use a PIC."?

I wouldn't say that, an AVR maybe, but I have no erperience with PICs

I was thinking more along the lines of a potentiometer between +V and -V
wity the output to one of the op-amp inputs so that one of the switching
points of the schmitt could be positioned at 0V...

Bye.
Jasen