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I'm trying to make an truck RPM reader that will output a signal once the RPM's exceed a set-point. I believe that I can easily code an Arduino to do this, but I'm having trouble with the circuit.

I have 2 approaches into what I'm trying to build, a simple RPM and a complex (not really complex).

Simple: have a potentiometer as a setpoint and simply read the alternator's frequency and when the reading gets over the mapped value on the pot the output LED will light up.

Complex: have a setpoint or maybe more than one setpoint that will light up 1 or more outputs, depending on the quantity of setpoints.

My problem:

I don't know the best way to actually shift down the signal to interface with the Arduino; in the simple approach a voltage divider might do the trick, but I'm not sure if I could trust the not-so-stable voltage output of the alternator. I'm looking for the cheapest way of doing this reliably. I'm researching what opto-coupler I could use since from my understanding the voltage is not always stable.

For the complex one, I'm not sure how to measure RPM since these trucks are diesel fueled and I don't have a spark-plug where I could sense the voltage on it to know revolutions. Does anyone have a suggestion on this approach?

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  • \$\begingroup\$ I would not try to use the alternator output to read engine speed. As you already stated, the output there is very erratic and not at all stable. And, unless you're working on a rather old vehicle the rectifier is internal to the alternator and you are getting DC power from the output. More information on the vehicle (or more specifically the engine) would be helpful here. Is this an older 100% mechanical diesel? I do not have nearly the electrical engineering knowledge or experience of most people here, but I have plenty of vehicle and engine experience. \$\endgroup\$ – LsD Aug 9 '17 at 11:22
  • \$\begingroup\$ I'm also thinking the alternator output is going to be too unstable and probably unsafe. Is there any way to get a photo sensor in front of the crank shaft pulley? An IR proximity sensor could theoretically also be used as a tachometer. They just output voltage proportional to reflected IR. Like with a hand held optical tach, use a small piece of reflective tape on the pulley. The IR sensor would output a stronger signal as it rotates past. Have the Arduino measure time between signal jumps or count bursts per X number of millis. Electrical isolation and complete accuracy. \$\endgroup\$ – Phil C Mar 9 '18 at 1:58

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