I am working on an automotive timing circuit that uses a toyota reluctor pick
up. It's output is a pseudo sine wave that changes in frequency and amplitude
as the rpm changes. The ideal point to in the wave to send a position
interrupt is as the wave passes zero. I've tried running it through an op amp,
letting the wave swing from a 5v pedestal, and computing the midpoint between
the two peaks but this eats up too much time is an "after the event has come
and gone" solution. Anybody have a suggestion?

Bruce