Morgan, thanks for the explaining the schmitt trigger in the detail you went into-- that made a lot more sense to me, and I understand it better than the videos I had watched. I guess mainly because tying one into an Arduino brings the complications you mentioned. When you mentioned the schmitt trigger in post #9 of this thread, I thought you were suggesting I look at circuits including a schmitt trigger to accomplish what I needed.
I spent a good several hours yesterday and today researching a little on how our ignition works on this particular engine (GM LS2, but same as the infamous LS1 and the other variants [LS3, LS6, LS7]). Apparently it uses a ground pulse to each coil to fire that cylinder (it has a separate, dedicated coil for each of the 8 cylinders, just as an FYI). Short of learning that, threads on other forums about this ignition system seem to dead-end. I read someone mention that the ground will be what gets voltage spikes, which I don't understand, and can't confirm nor deny if that's true from further digging I've tried doing. I did confirm that each coil is pulsed twice per revolution, so that helps.
So I went into digging up how optocouplers/optoisolators work, which user larryd suggested as the very first reply to this thread. At my first glance at that component after reading what larryd posted, it didn't appear that that was what I needed, but after re-visiting it and learning more about them, from what I can tell, that seems like exactly what I need for this project, but the ground pulse of the ignition system getting + voltage spikes is what confuses me as to how it would work, again, if the ground pulse does in fact see voltage spikes.
Obviously if it was just a ground pulse with no voltage spikes, I assume I could hook the "input" side of the optocoupler to constant (when ignition is on), positive voltage, then hook the ground side to the pulse wire going to one of the ignition coils... I just don't know if the LED and/or light sensor can display/read at the speed of 233 times per second, if the engine were at 7,000 RPM, pulsing the coil twice per revolution. Am I even at all on the right track here with the optocoupler theory?? If so, will it support that type of frequency??
I've search various keywords, such as "rpm coil arduino", "engine rpm arduino", "engine rpm optocoupler", "engine rpm optoisolator", multiple variants of those keywords, and a bunch of other keywords. What I see is ways to do it using a capacitive pickup, or an inductive pickup. I've seen people asking how to read from the ground pulse going to an ignition coil, even here on this forum, but pretty much all of the threads end without a solution. Being that I'm not really sure what direction I need to go, I'm also having trouble finding anything about PCB layouts for the components I need. If I know I'm looking for using an optocoupler then perhaps I can narrow my search terms down some, but so far I have not seen any particular way to lay out the circuit to prevent voltage from jumping around.... all I have found is people placing the optocoupler in random places on their solderless breadboards.
If I'm wrong about using an optocoupler then I'm really at a loss here after all of the reading and videos I've been going through. Lol at that point it would have to be time for me to throw in the towel, because I'm not electronically-savvy enough to find what I need for this unfortunately.