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Ultimately I would like to try to detect ac voltage on a 220V AC line using capacitive coupling. I don't want to have to hold the device, I just want it to sit there and monitor if the line is active or not.

To do this I think I need to tie the Raspberry Pi GND pin to be tied to the the AC GND of the house wiring.

It's been a while since I did any AC power work, but it seems like I should be fine to just tie the gnd pin from the gpio bus of the Pi to ac gnd via a wire to the center screw of an ac outlet (assume it is properly grounded) and then just make sure everything uses that common ground bus as a reference.

Same would apply if I wanted to power the sense circuit from a separate supply, say a second usb charger. Tie the "gnd" pin output of the usb charger to the same gnd bus as the PI and the ac main, then I could use the 5v as power for the sense circuit.

This seems pretty strait forward, but I wonder if I am missing anything?

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Yes connecting signal grounds to mains ground is generally fine. Pretty much every desktop PC does it.

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