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this page describes the process I went through in implementing my own tachometer.


filtering the signal

the hunt for reasonable rc values

the signal as it comes off of the negative terminal of your ignition coil is regular, but is not suitable as a trigger. it needs to be cleaned up a little first. through a process of trial and error, I found resistance and capacitance values for a RC filter which clean up the signal quite nicely.

first, here is the raw, unfiltered signal.


I randomly chose to start with 1 microfarad and 1 kilo ohm as initial values, and then started hunting around from that point. none of these produced usable results.

next I tried 1uf and 100 ohm, which produced a result I wasn't expecting, so I took a closer look at it, even though it wasn't usable.

the next values were closer to what I wanted, but still not quite there yet.


next, I tried 100uf and 100 ohm. jackpot! this is exactly what I was looking for. I took several different readings of this signal.


just to be thorough, I decided to try a few more values. they didn't yield anything better than what I already had.

half-wave rectification

we are only interested in the bottom half of that signal, so we use a diode to create a half-wave rectifier. this should work well as a trigger signal.


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