Here’s a cool AVR based RPM meter for cars, Bitman writes -

I got an idea of making a digital RPM-meter for my car. First I made a filter and used to test my code with a dev-board.

Pulse comes straight for cars rpm-meter.

CAR: Honda Civic 1988 Dual Carburetor
MCU: Mega32

The layout is a bit messy, but I think the pcb will tell the rest.

The filters main part is 7414 cause it has a Schmitt Trigger to make almost any kind of pulse to square wave.

Code uses interrupt to count the rising edges of the pulse in a time unit, which is around one second.

:: AVR Freaks – Link.