Not too hard — there is 0.2ohm current sensing resistor combined with an op-amp based precision rectifier. The signal can then be fed into the ADC (on OpenSprinkler, the microcontroller itself has ADC pins, whereas RPi doesn’t have built-in ADC but OSPi has a PCF8591 ADC chip on board).
thanks Ray. I have been able to build my own OSPi but I am not that technical yet, so any way to tell me exactly where these parts would go on the board? Also part numbers and where to get them from? Thank you again Ray.