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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.

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  • \$\begingroup\$ What about using bidirectional optocoupler? \$\endgroup\$ – venny Aug 24 '14 at 14:06
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More than likely, the voltage fed to the speaker is large enough to drive the photodiode in an opto- coupler. You can check this by using a standard low power LED and a resistor in series. You could also look at the signal across the speaker with your oscilloscope but there is a danger that the chassis of the doorbell receiver is "live". Best way to check this is with a multimeter on AC between local ground on your house wiring and one of the speaker terminals. If the chassis is live you'll certainly measure your local AC supply voltage and so don't use the o-scope.

Bottom line - be careful and wire the LED+resistor onto the speaker with power off.

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  • \$\begingroup\$ Thanks! I hooked up an LED+resistor as you suggested and as you suspected it did in fact light up, so I will try an optocoupler to generate the interrupt. \$\endgroup\$ – Jeremy Aug 26 '14 at 12:36

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