# Finding the correct wire

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?

• the transformer is designed for 2v-30v AC to 2v-28v DC – Joe Lu Dec 19 '15 at 4:22
• there's no such thing as an AC to DC transformer. – jp314 Dec 19 '15 at 4:31
• 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/… – Joe Lu Dec 19 '15 at 4:46
• 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 – Joe Lu Dec 19 '15 at 5:03