I am having some issues with a simple circuit. I would expect this circuit to light up this LED constantly when the Arduino is plugged in. I have the Blink tutorial sketch loaded, but I don't think that should matter with the pins I am using.

This is what I think I have going on: Arduino 5V Power --> BB power --> 100 Ohm resistor --> BB line 25 --> this LED --> BB line 30 --> BB Ground --> Arduino Ground

Please check out this picture for confirmation:

Arduino Question

Thanks in advance for any help that you can provide!

  • How is the Arduino powered?
    – jfpoilpret
    Jun 22, 2014 at 21:50
  • Powered by USB.
    – Rip Leeb
    Jun 22, 2014 at 21:50
  • If the Arduino is powered, then the LED should either always be lit since it is always between 5V and GND, or never if it was mounted reverse (that is not visible in the picture you posted). It won't ever blink if you wire it this way!
    – jfpoilpret
    Jun 22, 2014 at 21:52
  • The long leg of the LED is on the positive side - the left. Your first comment made me go back to the Uno spec and I read: "The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable." So maybe that is my issue since USB is only 5V.
    – Rip Leeb
    Jun 22, 2014 at 21:54
  • No, USB is fine for Arduino. The problem is that you did not connect your LED to a digital output of Arduino. Please take a careful look at the circuit in the blink tutorial you referred to.
    – jfpoilpret
    Jun 22, 2014 at 21:59

1 Answer 1


Figured it out - stupid mistake as expected. The power and ground lines on my bread board don't go all the way across. They break right in the middle, as the lines show. I moved my ground connection to the left a few slots and it worked. Thanks for working through it with me jfpoilpret!

  • 3
    How couldn't I figure that one first? I have been bitten by the same stupid mistake on a few occasions already :-) That's why I have put staples to connect halves of power lines of all my breadboards now! Staples work great for that.
    – jfpoilpret
    Jun 22, 2014 at 22:06
  • Don't feel bad, we've all made this mistake more than once. I like the staples idea!
    – Tom
    Jun 23, 2014 at 8:37
  • What I usually do is just place jumpers to connect the two sides of the power/ground rails. This way they're connected by default and if I need to separate them, I can just remove that wire.
    – sachleen
    Jun 23, 2014 at 16:50

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