Hooking a Car Distributor Tach Pulse to an Arduino

I am trying to read the tach pulse coming from the distributor of my car.

I have the appropriate circuit to convert the 12v to 5v to read on the digital input. However, there is a huge spike coming down the tach pulse line (+400V), see attached picture. How would I filter this spike?

Thanks!
Chris

wyngnut:
I have the appropriate circuit to convert the 12v to 5v to read on the digital input.

Hi.

What is that appropriate circuit ?

Looking at the picture on your scope, i see a LOW level tach pulse (of about 1.2 divisions length), then some 1.5 division of a noisy level, then 2 pulses, followed by 2 divisions of a nice, straight line, which is about 0.2 division higher than the noisy line.
As this signal comes from your distributor, it is very likely that you're picking up some firing pulses.
How are you connecting to the distributor ?
Are you powering the Arduino from the car battery ?

The spikes are quite short (hence that name).
You could use a combination of a capacitor and a resistor to filter these short pulses out.
The flanks on the tach pulses will become less steep because of this, but the pulses will still be long enough to register.

Try a suitable resistor and a 4v7 or 5v1 zener diode.
Use two or three resistor in series to cope with the voltage.
Leo..