I am writing an Arduino program to read speaker inputs for a test station. I need only to know when a speaker lead is on or off. This will be done by using the fade and balance. Originally I was connecting the speaker leads to the analog inputs and could get a single line to work as a simple VU meter. Multiple lines gave issue. Since I don't need to know the actual level, just whether they are on or off I'd like the speaker lead to trigger a transistor connected to a digital I/O pin on the Arduino. Any thoughts or quick schematics on how to do this?