Hi,
I purchased and installed a 3rd party backup (sonic) sensor system in my car a few weeks ago but didn't like the beep from the buzzer that came with the system.
My plan was to snip the buzzer off of the audio output line and use that as an input to my Arduino uno and use that voltage to play a custom tone using the tone() command.
What I did was hookup the + line of the audio into A0 and the - ground line into the ground of the Arduino (the ground beside the 5v power). Doing an analogRead() on the A0 pin worked ok as long the Arduino was powered by the 5v USB line but when I tried to power the Arduino with the car's 12v battery I got completely different readings and seem to have damaged the sensor control box (using a multimeter I discovered that the control box was sending ~5v for a "positive" sensor reading, so I assume I somehow sent 12v the wrong direction into the control box which must have damaged it but I don't understand why).
Anyway, I have a new control box in the mail on the way but am still confused as to where the audio ground (that was going into the buzzer) from the control box should go. Does it need to be connected to another ground or can I just ignore it?