I think you'd be way better off epoxying a small magnet to your harmonic
balancer and mounting a Hall Effect sensor to read it.
Use a piece of shielded cable to run to the sensor from the pic (make
sure it's plenum rated cable) and ground your shield.

This gives you a direct RPM reading, no division. Low noise, and no
voltage spike worries. Probably cheaper and less components too.
You can even prototype this for free if you grab the magnet and sensor
off a dead VCR's drum motor assembly.

Actually, this is how most manufacturers do it on modern cars, and in EFI
conversion kits.



On 11/30/06, Dumitru Stama wrote:
>
> Hi Guys,
> I want to make a small board-computer for an old car (1965 model) i
> own and i have a short question. I want to use a wire connected to the
> distributor/ignition to get pulses to a pic and count the rpms.
> Since this car has a 4 cylinders 4 strokes motor i will have to divide
> that number by 2 and get the real rpm.
> The question i have though is the folowing : What precautions i will
> have to take in order to have an usable product which does not crash
> when the voltage varies a lot ? I mean should i use a special 12v to
> 5v converter instead of 7805 ?
> Also i think about using the schematics for a simple zener regulator
> to get the signals from the ignition to the controller :
> http://www.asm.ro/schema.gif
> I was thinking that this way i can be sure i get a 5v max and i can
> safely use a microcontroller digital pin for this. Do you think there
> is a much better solution to this ?
>
> Thank you very much for your time
>

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