I have connected my Raspberry Pi to the controller of an external device (an espresso machine) with a couple of solder connections to the power button (so the RasPi can turn it on and off via a software timer). This works great. However, I can't sense whether the machine is actually on or off already.
The same controller has LEDs that indicate whether it is on or off. Is there a circuit I can connect to the LED terminals to determine whether or not the LED is on or off? If I put a simple transistor switch in parallel with the LED, will I be able to tap a small enough current to turn on the switch and still light the LED? I'd rather not try to desolder the LED for something inline.