Tachometer
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this page describes the process I went through in implementing my own tachometer.
Contents |
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.
- 1uf & 1kohm (2v, 5ms, 10x)
- 1uf & 10kohm (2v, 5ms, 10x)
- 1uf & 100kohm (2v, 5ms, 10x)
- 1uf & 1Mohm (2v, 5ms, 10x)
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.
success
next, I tried 100uf and 100 ohm. jackpot! this is exactly what I was looking for. I took several different readings of this signal.
- 100uf & 100ohm (2v, 5ms, 10x)
- 100uf & 100ohm (1v, 5ms, 10x)
- 100uf & 100ohm (1v, 2ms, 10x)
- 100uf & 100ohm (1v, 2ms, 10x)
just to be thorough, I decided to try a few more values. they didn't yield anything better than what I already had.
- 100uf & 1kohm (1v, 5ms, 10x)
- 100uf & 47ohm (1v, 5ms, 10x)
- 100uf & 47ohm (1v, 2ms, 10x)
- 100uf & 470ohm (1v, 5ms, 10x)
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.
