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Author Topic: RPM input circuit from coil  (Read 1104 times)
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I want to build a RPM counter for my car using an Arduino.

The rpm signal need to be picked up from the negative side of the ignition coil.

Now I know there are some nasty spikes being generated.

I need help with a circuit diagram and components to be able to input the rpm signal to the Arduino without damaging it.
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Montreal
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Here you are: http://www.sportdevices.com/rpm_readings/index.htm
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Thanx Magician,
I had a look at the link you posted.
Thay use an inductive pickup on the HT lead.

However I want to pick up the signal directlu from the negative side of the coil.

Any suggestions?
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Humboldt, CA
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Use an optocoupler rated at 500V or more.  The - side of the coil spends most of it's life seeing 12v, to fire the coil the - side is pulled to ground for between 0.5ms and 4ms and then sent high again.
For your purposes detecting when the - post goes low will give you a nice trigger.  Wire the trigger side of the optocoupler so the internal LED grounds to the coil - post and receives power from a 12v source (with resistor, if needed).
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