Tom M'Guinn

The speedometer is powered by 12 volts and ground from a wall wart. The same power supply is used to power the arduino. The simulated speedometer pulse is a 0-5v output directly from the arduino digital pin, and triggers the speedo just fine that way. The factory sensor doesn't have a voltage at all, and just pulses from ground to an open circuit (so the speedo probably has a pull up resistor inside anyway). If memory serves, the arduino digital pin sinks 40mA, which is proving to be plenty. I've been running the calibrator in my car for a year or so, and all is well, with no transistor or MOSFET, etc...