I am setting up an IR receiver/transmitter (with an Arduino Uno) that turns certain AV components on and switches inputs, and now it is controlled by buttons, but I'd like it to have knowledge of the on/off state of certain hardware (apple tv and cable box) so it could turn to the correct input when a certain device comes on. There may be a better way here, like watching the power draw on the wall plug (although the apple tv is always on, in standby mode), or something else, but the one thing both devices have in common is their power status LEDs. Both boxes are very easily disassembled, so I'd like to tap into their LEDs and have the arduino sense the voltage drop on an input (not sure if I need to use Analog or Digital in though). There is probably a simple answer to this, but I have more of a CS background than EE and I am confused by the different grounds in this situation, so I'm not sure how to hook this up in a way that doesn't mess with the other components negatively. Let me know what you think.