I am trying to connect my apartment door bell system to my raspberry pi; I am not too good with the wiring, and hope someone can help.

What I have:

  • 3 wires from the door bell system: red wire, black wire, white wire.
  • 1 digital multimeter
  • 1 AC/DC to DC Power Supply Converter Step Down

The AC/DC to DC Power Supply has the following specs listed:

  • Input: DC 0V ~ 30V or AC 0V ~ 20V
  • Output: DC 1.25V ~ 28V (continuously adjustable)
  • Output current: 2A Max.

What I am trying to do: connect the wires from door bell system to the power supply, then output a 3.3v DC logic input to the raspberry pi.

What I need: the transformer needs a ground and a live wire, I need to find out from that three colored wires from the apt door bell system

what's my problem: my multimeter showed me some frustrated digits, I'm out of clues to define the correct wires, and here I am...

My multimeter result with the button not pushed:

black and red: 10v AC; 
white and red: 12v AC; 
black and white: 1.5v AC;

If I press the door bell button outside

black and red = 0v AC;
white and red = 12v AC;
white and black = 0v AC;

So how do I connect wires to the power supply?

  • \$\begingroup\$ the transformer is designed for 2v-30v AC to 2v-28v DC \$\endgroup\$ – Joe Lu Dec 19 '15 at 4:22
  • \$\begingroup\$ there's no such thing as an AC to DC transformer. \$\endgroup\$ – jp314 Dec 19 '15 at 4:31
  • \$\begingroup\$ don't know if i'm allowed, this is what i've brought, now my quest is to find proper wire to connect to this unit as AC input.. amazon.com/gp/product/… \$\endgroup\$ – Joe Lu Dec 19 '15 at 4:46
  • \$\begingroup\$ hi Ecnerwal, yes, I'm trying to get a logic input when the button is pushed, the rasPi is powered from its usb port, not from the door bell wiring \$\endgroup\$ – Joe Lu Dec 19 '15 at 5:03

Seems like a standard doorbell, though your "pressed button" status seems like a probable measurement error.

For detecting the doorbell button, connect to black and white - if that does not work, connect to black and red. The reason I'm pretty sure you have a measurement error is that if black and red are 0V and red and white are 12V, black and white should be 12V, not 0V.


simulate this circuit – Schematic created using CircuitLab

Or, if black and white are 0V, and white and red are 12V then black and red should be 12V - Same circuit as above, swap labels for white and red.

So one of your measurements is probably not what you've said.

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  • \$\begingroup\$ thank you so much Ecnerwal, I will double check the measurement tomorrow again, and come back with you for the confirm figures. my apt is a very old one, I moved in last month and found out the door bell (or should be called intercom) was removed, all I found were three wires staying on the wall. The measurements sounds incorrect and that is exactly why I'm asking for help here. oh yes, I will stop using my figure, thank you again, lol \$\endgroup\$ – Joe Lu Dec 19 '15 at 5:26
  • \$\begingroup\$ Hi Ecnerwal, after spenting 2 days researching and trying, finally I figured out the odd behavior of these wires, in fact, they may all be live wires, connect white+red to form a circuit to open the door, the black itself as the logic of the push button. anyway, thank you so much for helping, wish you have a wonderful holiday. so i just need to wire the black one to the transformer, and maybe add a ground wire manually.. will try it later. \$\endgroup\$ – Joe Lu Dec 20 '15 at 18:23

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