I have an external board which ends in a simple relay.

I connected one wire to one GPIO pin and the other to GND.

I need to check status change of that pin. I'm working in python.


  • Could you draw a diagram. I'm not sure what is going on. This probably isn't the right site for the question as it has almost nothing to do with the Raspberry Pi.
    – joan
    Nov 20, 2016 at 19:05
  • The external board is an electrified fence controller. It has a normal open or normal close relay that gives tension when an alarm is happening. I need to read that from my gpio.
    – leandronn
    Nov 20, 2016 at 19:09

1 Answer 1


Hoping that I understood the description correctly this is a circuit that should do the trick. Note the additional resistor (pull-up) compared to the way that is explained in the question. Without it the following happens: relays is open - Pi's input is "floating"; relays is closed - Pi's input is low. Which is why you are not detecting a proper change of state when the relays toggles. With the pull-up resistor a defined voltage level is applied to the input pin.

See also: https://www.raspberrypi.org/learning/physical-computing-guide/pull_up_down/ (note that they propose another series resistor to limit the current in the case the GPIO pin is set to output).


simulate this circuit – Schematic created using CircuitLab

  • You understood perfectly. I'll test it tomorrow and let you know. Thanks.
    – leandronn
    Nov 20, 2016 at 20:05

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