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.
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?