I'm just wrapping up a bizarre contract design job, most of which I can't talk about due to an NDA, but part of the functional spec which I am able to discuss was a circuit to reliably trigger a HV switch that interrupts the output of a power supply during the entire time the spark plug is firing.
There are numerous ways to skin this proverbial cat, but I opted use the signal on the wire that goes from the primary side of the ignition coil to the red on/off switch invariably found on the recoil starter housing. Since I had only a vague understanding of how magneto-based ignition systems worked, I did the sensible thing and scoped the waveforms on both the primary and secondary (capacitively-coupled) of the ignition on an actual engine (a 6.5hp "Chonda" - Chinese Honda clone - on my backup generator). It turns out the secondary signal was not at all usable for my purposes because there is a sharp negative spike of anywhere from 3kV to 10kV, followed by a prolonged (1ms or so) pedestal at the arc discharge voltage of the spark plug, which itself varies considerably over RPM, fuel/air ratio, etc. However, the primary signal looked eminently usable, except I needed to only pay attention to the negative-going part that was sandwiched in between two positive pulses, all of which occurred each revolution. While I probably could have pulled this off with a Schmitt-trigger gate and a few passives, it only cost a few more components to make a proper comparator with hysteresis so I went that route.
Attached are the schematic of the actual circuit, including the HV switch portion, as well as a screenshot from LTSpice and the simulation file that generated it. The PWL approximation of the ignition signal does a credible job as the actual circuit works just as it did in the simulation, something the me of 20+ year ago would have never believed... (trust me, SPICE really sucked back then).
Anyone is free to use this as they wish, so I guess that makes this open source. Enjoy!
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