I found an old circuit diagram online but have never been able to find it again. Here is what I am trying to do: The three-phase DC motor I am working with is a 4-pole motor. Therefore there is a direct relationship between the frequency between the motors phase-leads, and the current RPM of the motor. IE - 2 magnetic cycles per revolution What I need to do is take frequency measurements between the leads and convert that to a voltage that is representative of the motors RPM, which i can then display on a LCD voltage meter. I believe this to be a much cheaper method of obtaining a tachometer (plus it is an interesting project). Interesting side-note: Using wheel radius and gear ratio I could easily output MPH using op-amps, although it would be inaccurate while the car is gliding in neutral. If anyone has any ideas or links to a good circuit diagram it would be greatly appreciated. My background is in Physics, so my ability to design complex circuits like this is very limited without extensive research.