I am building a device consisting of a microcontroller (an Arduino mini) attached to various output devices, which will be triggered using a wireless doorbell receiver. The receiver is a single board device with its own integrated (and ungrounded) AC power supply. In order to tell if the doorbell has rung or not, I need to use the microcontroller to measure some signal from the receiver (possibly just the output going to the speaker), and ideally generate a logic-level signal as an interrupt to wake up the microcontroller when the doorbell rings. My current problem is that I don't know how to measure that voltage unless I have a common ground between the microcontroller and the wireless receiver.
I am considering that it might be feasible to identify the ground used on the board, and to hook that up to the ground coming from the separate power supply which I'll be using for the microcontroller.
My questions are:
Is this a stupid thing to try doing?
If not, what's the best approach? I have an oscilloscope.... Can I just use one of the speaker wires as the common ground?
If it is a stupid thing for me to try, is there a less-stupid alternative?
If the above hasn't already made it abundantly clear, I am a relative beginner at working with electronics. I'll read and study anything I can find, but I haven't been able to find anything yet which speaks to this question in a way that I can identify.