Hi
over Amplify the sine wave and run it through a comparitor
Something like this. Experiment with the circuit values to find the best for
your application. The adjustment vr1 allows you to set the point that the
circuit changes state, from slightly before to slightly after true 0


                         r2                      r5
                    .--\/\/\/\--.-------------\/\/\/\--.
                    |           |                      |
             r1     |    |\     |                      |
 Sine in  -\/\/\/\--^----| \    |    r4      |\        |
                         |  >---^--\/\/\/\---| \       |
         Gnd -\/\/\/\----| /                 |  >------+---- output
                r3       |/      .-\/\/\/\---| /       |
                                 |    r6     |/        |
                                 |        r7           |
                                 +---\/\/\/\-----------'
                                 |
                      r8         V          r9
           +vcc ---\/\/\/\----\/\/\/\----\/\/\/\--- -vcc
                                vr1

r1 =   1k  r2/r1 sets the gain in this case 100 adjust as necessary
r2 = 100k
r3 =   1k  1/(1/r1+1/r2)=990 ohm 1k is close enough
r4 =  10k  r5/r4 sets the gain in this case 10 adjust as necessary
r5 = 100k
r6 =  10k  1/(1/r4+1/r5)=9090 ohm 10k is close enough
r7 =   5m  (5000k)
r8 =   4k  sets trigger range adjust as necessary
r9 =   4k  sets trigger range adjust as necessary
vr1=  4.7k allows you to adjust the trigger point ahead or behind true dead
center
op1=  any op amp, the lm324 comes to mind it is a quad package
NOTES:

If the input sine wave is large enough the first op amp, r1, r2 and r3 may
not be needed

r7 this resistor adds positive feed back to so that a noisy signal does not
create multiple triggers. Experiment to find the best value. More noise
needs a lower resistor

r3 and r6 are used to null op amp offset. If absolute precision is not
needed replace these resistors with a wire


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Bruce Turrentine
> Sent: Thursday, June 25, 1998 7:40 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Zero crossing dection circuit.
>
>
> 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
>