Sounds like you're asking for 2 things -- power supply and ignition signal 
pickup.  For the power supply, a 7805 can work well depending on how much 
current your circuit needs.  But no matter what power supply you use, make 
sure you protect it against positive and negative spikes/surges, which can go 
to over a few hundred volts in an automobile, especially older automobiles.

For the ignition pickup, you will need to experiment a bit as the ignition 
signal has positive and negative spikes, etc.  You'll need to attenuate the 
signal, filter it, etc.  A scope is very helpful here.  But a better option 
is to take a tach signal off an MSD or similar if the car has one.  I'm 
assuming you don't have an EFI system to get a signal from.

One other thing you'll run into on older cars is a lot of ignition noise (from 
the spark plug wires), so you'll need to protect your PIC circuit properly 
and filter/decouple the power supply lines, etc.  I've seen applications 
where this noise/EMI would shut down the PIC circuit.  You'll also want to 
place the circuit in the cockpit rather than the engine bay to help reduce 
this noise.

-Neil.




> 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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