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:
Thanks in advance.