Hello guys I am currently working on a project. and I would like to turn on a GPIO pin when my LED on the circuit below turns on. I'm lost to where I should tie my input GPIO pin. please help. Thanks!

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  • I don’t know where you got this but it is unlikely to work
    – Milliways
    Commented Apr 23, 2019 at 3:59
  • @Milliways I made it my self. So it is not possible to control a GPIO using a transistor switch?
    – Rikku
    Commented Apr 23, 2019 at 4:04
  • I don’t know what you are actually trying to accomplish. Are you trying to use GPIO to turn LED on?
    – Milliways
    Commented Apr 23, 2019 at 4:06
  • No. What I am trying to achieve is.. I would like to use the NPN transistor as switch for turning on GPIO Pin
    – Rikku
    Commented Apr 23, 2019 at 4:15
  • And what is supposed to turn the LED on?
    – Milliways
    Commented Apr 23, 2019 at 4:18

1 Answer 1



How to use a NPN transistor to "turn on" a Rpi GPIO pin?


Ah, you cannot "turn on" a Rpi GPIO pin. She won't listen to you, but you can ask her to behave as input or output! :)

In geek English:

  1. A Rpi GPIO pin can be in "input mode" or "output mode".

  2. An "input" pin can input/read a signal which can be High or Low.

  3. An "output" pin can output/switch a LED, eg, High to switch on, Low to switch off, (or vice versa, depending on circuit).


I have drawn a circuit showing how to convert a loud (5V) signal to a soft (3V3) signal. The fair lady prefers a soft voice - loud voice can kill her! :)

converter 2

/ to continue, ...


NPN Transistor Tutorial - Electronics Tutorials

  • used the wrong term.. that what I want to do.. to make GPIO as input..
    – Rikku
    Commented Apr 23, 2019 at 7:32
  • I followed your digram above but it seems that I am not getting the expected output.
    – Rikku
    Commented Apr 23, 2019 at 7:34
  • Ah, perhaps you use the wrong resistors - not any resistor will do! Is the left most box working?
    – tlfong01
    Commented Apr 23, 2019 at 7:41
  • yep it worked fine.. I replace it with another base voltage as trigger.. this is still somehow connected to the pump I ask about in the other question.. What I did was I used the voltage across the pumps wires run it throug a voltage divider circuit with 3V output and connected it to the transistor base as trigger. what resistor values should I use.
    – Rikku
    Commented Apr 23, 2019 at 7:53
  • I am confused. Perhaps you can update your question with a schematic.
    – tlfong01
    Commented Apr 23, 2019 at 8:27

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