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I am using a voltage divider built with two resistors (1K and 330 Ohms) so that I can reduce a 13V output from an external device (in this case a burglar alarm) down to a level acceptable to a Raspberry Pi GPIO input.

When connected up, the GPIO input floats, even though its internal pull-down resistor is activated. Based on a line in the burglar alarm manual, I think that both the power source of the Pi and the burglar alarm need to share a common negative, but I don't know how to achieve this.

This is the circuit at the moment:

enter image description here

Thanks in advance.

  • 1
    Not Pi specific. Connect a Pi ground to the supply ground (marked 0V on your schematic). – joan Mar 20 '18 at 21:09
  • 1
    Electric circuits are so called because you NEED a complete circuit for current to flow. – Milliways Mar 20 '18 at 22:53

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