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:

enter image description here

  • \$\begingroup\$ if you can tap into/sense the frequency of it's firing, you can use a frequency -> voltage converter op-amp style IC. Have you looked at this option yet? \$\endgroup\$ – KyranF Oct 23 '14 at 19:57
  • \$\begingroup\$ @KyranF yes, I posted a similar question using a hall effect sensor and the answerer said to go with an LM2917, but I've had no luck getting it to work, so I thought to just try to use the coil's grounding wire instead of a sensor (less parts). \$\endgroup\$ – klcjr89 Oct 23 '14 at 19:59
  • \$\begingroup\$ Can you describe more the actual 'signal' you are trying to measure? what does it look like? how does it behave? there may be easy ways to measure/use it.. \$\endgroup\$ – KyranF Oct 23 '14 at 20:04
  • \$\begingroup\$ @KyranF no I can't, I was hoping someone here would know a typical magneto output. \$\endgroup\$ – klcjr89 Oct 23 '14 at 20:07
  • \$\begingroup\$ Is it a HT mag, or an energy transfer mag (external coil)? \$\endgroup\$ – Majenko Oct 23 '14 at 20:18

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.


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