Homemade 555 Tachometer

Hi everyone, I found this schematic online and I'm curious how I can adapt it to my motorcycle.

My bike is a 1 cylinder, 4-stroke engine that fires the plug on every stroke (not every other like normal). As I understand it the circuit outputs 0.01 ua for every 1 RPM. 50ua = 5000 RPM. In my case I would need a 100 ua meter to read 5000 RPM since my plug fires twice as much as most engines. The circuit is designed to read a plug that fires every other stroke. My bike has a "wasted spark" ignition system. There also are no points. It is a CDI ignition system. Can I wrap a few turns of wire around the spark plug as input instead of connecting to the non-existent points? Does any of this look out of place or could it be done better?

Kris01 said:

In my case I would need a 100 ua meter to read 5000 RPM since my plug fires twice as much as most engines.

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The schematic shows a potentiometer in series with the meter. By running the engine at some known rate, and then adjusting meter travel to match, that ought to be sufficient to calibrate your tachometer.

Can I wrap a few turns of wire around the spark plug as input

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It's chancey although it's possible. I have clipped aluminum foil around spark plug wires to grab ignition firing signals for my oscilloscope. You need sufficient voltage that can pull pin 2 low, to start a timing cycle. To achieve this you may need to rework a few things, alter the schematic, adjust some component values.

Have you seen whether there is a magnet spinning around the edge of a metal disk, which flies past a coil, triggering an ignition spark? I saw such a system in a lawn mower. That may be a good spot to detect a signal. I don't know much about motorcycle ignition systems, however. A more knowledgeable reader might have a better answer to your question.