Electronic – arduino – Will this circuit manage to kill a 2-stroke engine


I'm currently building a project that involves a 2-stroke engine. I have tapped into the wires where you would normally place a kill switch in order to get power for some electronics. The voltage is 9VAC. The first half wave is used to supply my battery and logic circuitry with power. The second half wave, I've discovered, that if you draw more than a few mA it will mess with the ignition and stop the engine, so I didn't use it for power.
But then I found the need to be able to use my MCU (an ATtiny84A) running on the first half wave's power to short the other half wave so it can stop the engine if something goes wrong.

  • I can't use a relay: it draws too much power.
  • I can't use a MOSFET: the two waves have the opposite potential.
  • I can't use a latching relay: not in stock where I live (can't buy
    online due to shipping restrictions).
  • I can't use a reed relay because the ones I find are NO but I need it to be NC.
  • I can't use an SSR: only available for higher voltage range.

What do I do?
I came up with the idea of using an optocoupler with a MOSFET but I'm not sure if it's gonna work. Here's the schematic that I created but if you find a better solution let me know!
My schematic

Best Answer

I think the magneto peak voltage is definitely too high for D1/D2, probably the optocoupler and maybe the MOSFET (no part numbers being provided).

Try replacing D1 with a 1N4007 and and replace the optocoupler/MOSFET with an opto-MOS SSR.

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Edit (for general info): The objective is to replace this kind of "kill switch"

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A magneto is a permanent magnet generator that forms part of an internal combustion ignition system that requires no battery or electronics of any kind, just the magneto itself, contact points that open at the appropriate time to generate a spark, and an ignition coil. They are used on small engines and on many light aircraft (in a dual configuration) where it is very undesirable to have a failure that causes the engine to cease rotating.