enter image description hereI have a red LED that I just want to light up using the GPIO pins on the Raspberry Pi. I got it to light up using the 5v pin. I tried it on a GPIO pin with a simple python script and it failed to light. I tried to simply connect it to a 3.3v pin. The LED would light for maybe one second then turn off. I have tried a variety of different resistors and breadboard setups, but nothing seems to work. Is there a problem with my pi or something else?

I have a 330ohm resistor in this picture but I have used others.


I just recently replaced the cobbler and I am still having this problem. I have tested the voltages and straight from the pi they are perfect, 5v through the cobbler is perfect, but the 3.3v only reads less than 1 volt when tested through the cobbler.


I just tried using two jumper wires touching the pins rather than the cobbler and the LED still didn't light up. The voltage reading straight from the pins are perfect though

  • How are you powering the Pi? Can we see a picture of your wiring, and can you show us the code? Jun 21, 2014 at 18:43
  • I am powering my pi with a micro-usb phone charger plugged into a wall outlet. I don't have any code because I am not using the GPIO pins yet.
    – LukeK
    Jun 21, 2014 at 19:20
  • Can you measure the voltage between the 5 volt and ground pins on the Pi cobbler? Jun 21, 2014 at 19:26
  • I just measured it. Going from 5 volt to ground measures about 4.32 volts. Going from 3.3 volt to ground measures below 1 volt. Is this the problem?
    – LukeK
    Jun 21, 2014 at 23:16
  • You should be seeing something close to 3.3 volts between the 3.3v and ground. One volt isn't enough to overcome the forward bias of the LED. Did you solder the pi cobbler yourself? If so double check your soldering. Jun 22, 2014 at 0:11

2 Answers 2


Try checking that your ribbon cable is connected the right way round. I put my on backwards the first time. Pin 1 on the RPI connector is just above the "P1" silk screen on the board. You should see a little white square on the board by pin 1. On the back of the board, pin 1 has a square pad. The plastic cable connector should have a little triangle impressed in the plastic marking pin 1 of the connect. Pin 1 of the cable connector should exactly match pin 1 on the RPI connector. Check both connectors on the cable. When installed properly, one of the cable connectors will hang off the board and the other will lay across the board. Use the connector with the cable lay that you prefer or best fits your case if you are using one. Then use your ohm meter to test that pin 1 of the RPI goes to pin 1 of the Cobbler.

  • I bought the cobbler from here link. In the picture it shows the white cord on pin 25 though. I've tried it both ways and still nothing.
    – LukeK
    Jun 29, 2014 at 19:58
  • I like to use the white stripe on pin 1. The cobbler is keyed so the connector only goes in one way but the RPi isn't keyed so it can be plugged in backwards. If you have an ohm meter, check that you have low resistance between pin one on the cobbler and pin one on the RPi the cable is in place. Then ohm out all the pins. This verifies that the cable connectors are collect and that the wire in the cable all make contact. The resistance should be around 0.2 ohms (about the same as when hold the test lead clips together. Jun 29, 2014 at 20:16
  • I'm not sure that I understand what you are telling me to do. How do you ohm out all the pins?
    – LukeK
    Jun 29, 2014 at 23:01
  • I talked to the seller, and he informed me that the picture on the page was wrong. It works when i turn the ribbon cable around.
    – LukeK
    Jul 22, 2014 at 12:41

The 5V pin isn't fused, and so people recommend not using it. 3v3 with a 330 ohm resistor should light up your led ok, just a bit dull. I think that your issue is with wiring:

  1. Is your led wired up with the flat side and short leg connected to ground? I dont think that it is in the pic.

  2. Breadboards are only connected along the columns, not the rows. Is everything connected? Again, not sure if it is in the pic.

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