I've got dirty power from the primary side of a small engine coil. I attached a 5v led with built in resistor to the primary side of the coil and ground. The led lights up and blinks with the pulse of the ignition.
On the scope I get a very quick 200 - 400 volt spike every time the coil fires. I'm new to using a scope and I'm very skeptical of my scope data. My next move was to use an opto isolator so Arduino can count the spikes. Works fine. The current goes from the primary coil wire to the + input side of the opto. The - input side of the opto is grounded back to the motor.
After the current runs through the opto isolator, can I run it into a capacitor and then a voltage regulator to power Arduino? If so what size capacitor should I use and how should that circuit be constructed?