Troubleshooting notes


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!

enter image description here

  • I don’t know where you got this but it is unlikely to work – Milliways 6 hours ago
  • @Milliways I made it my self. So it is not possible to control a GPIO using a transistor switch? – Rikku 6 hours ago
  • I don’t know what you are actually trying to accomplish. Are you trying to use GPIO to turn LED on? – Milliways 6 hours ago
  • No. What I am trying to achieve is.. I would like to use the NPN transistor as switch for turning on GPIO Pin – Rikku 6 hours ago
  • And what is supposed to turn the LED on? – Milliways 6 hours ago



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! 🙂

Now I am making another step wise refinement to the circuit, to make it easier to do the following Agile oriented TDD (Test Driven Development)CIT (Continuous Integration and Testing):

  1. Continuous/Incremental Integration/Testing
  2. Structured/Functional/Loosely Coupled/Pair/Swapping oriented programming and development
  3. DevOp oriented Rapid Prototyping Documentation

converter 2

The time has come to do a selfie walktrough, followed by a test plan with expected resultsBEFOREtesting really begins.

/ to continue, …


NPN Transistor Tutorial – Electronics Tutorials

Reading suggestion – You don’t need to understand too much before you design something. I often design circuits without understanding what it does! 🙂 And as the mad engineer Heaviside says:

Am I to refuse to eat because I do not fully understand the mechanism of digestion? – Oliver Heaviside

Newbie Troubleshooting/System Development Suggestion – tlfong01

Speak Softly, Love – Andy Williams

  • used the wrong term.. that what I want to do.. to make GPIO as input.. – Rikku 2 hours ago
  • I followed your digram above but it seems that I am not getting the expected output. – Rikku 2 hours ago
  • Ah, perhaps you use the wrong resistors – not any resistor will do! Is the left most box working? – tlfong01 2 hours ago   
  • 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 2 hours ago
  • I am confused. Perhaps you can update your question with a schematic. – tlfong01 1 hour ago   

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