Bruce,
What you want to do is run the pseudo sine wave into the input of a
comparator. Ground the inverting input and feed the signal to the
non-inverting input. The output will be a 0 to Vdd square wave of the same
frequency. If you use a single supply rail-to-rail type you should
be in business. You could also use an op amp connected as a
comparator if you can't get your hands on a 'real' comparator.

If using a PIC, you can use the signal to bump TMR0, or you can
use the edge to produce an interrupt.

Hope this helps.
Fr. Tom McGahee

----------
> From: Bruce Turrentine <Turrbie@AOL.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Zero crossing dection circuit.
> Date: Thursday, June 25, 1998 10:40 AM
>
> 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