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: