I am trying to make this circuit

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I have only attached two pins. By default shouldn't both pins be ON? Why only GPIO 17 is ON (shows light)? enter image description here

I notice that once I connect all the wires, the GPIO points lit up but SDA, SCL and CE don't. Why?

enter image description here

  • 1
    The default values of the different pins vary. You can for instance use gpio readall to find the current values. Also, try to avoid using sda and scl for gpio if possible, they have comparability weak pull-ups attached.
    – PMF
    Sep 12, 2021 at 20:36
  • You are using a negative breadboard rail for a positive voltage source instead of as ground. You should use a breadboard in a manner consistent with its labeling (which will help both you and us troubleshoot). Sep 13, 2021 at 1:08
  • @joan Sorry, I meant to say that. Weak as in "low resistance", but used bad terminology.
    – PMF
    Sep 13, 2021 at 6:11

1 Answer 1


This appears to be a minor variant of your previous question.

Unless YOU have configured them all pins are INPUTS; the labels on breakout are just common usage. Pins 3, 5 have 1.8k pull-ups.

GPIOreadall displays the ACTUAL pin configuration.

None of this would be the likely cause of your problem, although putting 3.3V on both ends of the LED is unlikely to help.

If you are going to use pins 3, 5 you should connect the load to Gnd as this is the way they are designed to be used.

You have provided no details but most of these bar displays have common cathode or anode so can not be used the way you suggest. You will need to determine WHAT you actually have.

INCIDENTALLY if you continue to misuse the breakout panel this way you are likely to have problems. Use the power rails consistently.

PS you will be sending ~9mA through each LED. If all 10 LED illuminate this is > 90mA. The Foundation now suggests 50mA total.
"The GPIO pins can draw 50mA safely (note that that means 50mA distributed across all the pins: an individual GPIO pin can only safely draw 16mA)"

  • Thanks @Milliways. I'll be honest, some things are not clear to me but I'll read about them. One things which didn't make sense to me is why does it matter if I connect positive rail to ground or negative to power? In the end, isn't a breadboard a passive device with metal connections underneath? Sep 14, 2021 at 17:53
  • @ManuChadha Electrically it makes no difference, but then the labels and wire colours are totally unnecessary to the electrons. To a human being it makes considerable difference. After 60 years playing with circuits I find it better to design for humans. Using red wires for Gnd has probably destroyed many devices. As for your other confusions this could easily be resolved if you include detail of the bar display you are using.
    – Milliways
    Sep 15, 2021 at 8:49

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