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This is a reformulated question to make it shorter.

I would like to build a tachometer for an old moped. Since my moped has a magneto and points for ignition I must obtain the signal directly from the spark plug. I would like to obtain a clean pulse signal which I can then feed to a microcontroller.

I need ideas, if possible, very well explained, on how to develop such circuit. Inductive pickup, capacitive pickup, neon bulb - photodiode, anything will be ok, as long as reliable and universal (must fit on other similar engines). I am by no means an expert in electronics so I need practical, real circuits.

I have tried using a neon bulb (attached to the spark plug and chassis) + photodiode, but at higher RPMs the neon conducts so well that the current runs through the bulb instead through the spark plug. It also does not seem a reliable way.

Thank you in advance.

Blaž Umek
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    What operates the points? Can you attach a magnet to that and use a [Hall-effect sensor](https://www.electronics-tutorials.ws/electromagnetism/hall-effect.html) to get a signal? That has the advantage of minimising your exposure to high voltages. – Andrew Morton Jun 02 '18 at 18:52
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    I just got another idea: since the rotor is a permanent magnet, would it work to put a Hall sensor in the proximity of the rotor and then measure the CHANGE of the magnetic field? One rotation has two changes (possibly more) and hence you have the RPM. Can a Hall sensor be used this way? – Blaž Umek Jun 02 '18 at 22:58

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You shouldn't connect the neon directly to the spark plug. It should be sufficient to simply hold the bulb close to the spark plug wire.

Alternatively, use a pickup coil (current transformer).

Dave Tweed
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  • Holding the neon close to the spark plug wire does not work. Also, where should the neon's anode and cathode go? A pickup coil cannot be used - the device must take the signal from the spark plug wire. – Blaž Umek Jun 02 '18 at 18:23
  • @BlažUmek Presumably Dave's suggestion is to energize the neon lamp slightly below its striking voltage, so that the electric field of the high voltage ignition pulse ionizes (through wire insulation and glass) the gas within the lamp via capacitive coupling, causing it to conduct. This is similar in principle to how flash lamps work. – jms Jun 02 '18 at 18:55
  • One thing to try: ground one side of the neon, connect the other side to a wire that is taped parallel to the spark plug wire, but not making any electrical connection to it. This is sometimes called a "gimmick" capacitor. – Dave Tweed Jun 02 '18 at 19:55
  • @DaveTweed Thanks, I will try that. I have also tried a similar setup today ( https://kokoraskostas.blogspot.com/2013/12/arduino-inductive-spark-plug-sensor.html ), with a few turns of solid core wire around the spark plug lead but it could not drive a LED. Is this normal? – Blaž Umek Jun 02 '18 at 23:08
  • I would think so. The LED requires significantly more current than the neon bulb. – Dave Tweed Jun 02 '18 at 23:26