I'm looking to see if there's a solution without using a microcontroller to find the engine RPM (perhaps spark frequency is a better term?) using only the magneto's coil grounding wire. This wire kills the engine when you touch it to the block. I believe this wire carries an AC current (correct me if I'm wrong) and a high voltage, so I was not sure how I could go about designing a simple circuit that won't be killed by these two factors.
I basically want to turn a transistor off once a certain frequency has been achieved. The transistor is normally on until this frequency has been reached.
I've seen engine hour meters and tachometers that clip onto or wrap around the spark plug wire, but that isn't what I'm trying to do in this case. I'm pretty sure there is a frequency on the grounding wire too, but correct me if I'm wrong.
I'd appreciate any help offered.
Here's a crude picture of the magneto setup:
Best Answer
A simpler approach is to make a crude optoisolator using a phototransistor and a neon tube. enclose both inside a plastic camera film canister to shield from external light sources. 3 or 4 turns around the high voltage lead going to one lead of the neon bulb, the other goes neon bulb lead is grounded. Cheap, easy, not very time consuming and gives your tach circuit a lot of high voltage isolation.