I am just starting out with an Arduino Diecimila and am playing around with LEDs, and am confused by why some circuits result in an LED not lighting up.

Successful Scenarios:

  1. Yellow LED from pin13 to 16B on the breadboard, Green LED from 16A to GND. This works both LEDS light up.

  2. Yellow LED from pin13 to 16A on the breadboard. Wire from 16E to 14E. Green LED from 14A to GND. This works and both LEDs light up.

Failed Scenarios:

  1. Yellow LED from pin13 to 16A. Green LED from 16E to 14E. Yellow LED from 14A to GND. No LEDs light up. I would assume that all the LEDS are in series, and all 3 should light up, I assumed this would be exactly the same as my second successful scenario where the LED is acting the same as the wire.

  2. Yellow LED from pin13 to 16A. Wire from 16E to 14E. Yellow LED from 15A to GND. Green LED from 16B to 14B. Here I have both Yellow LEDS lit, but the Green LED is not lit. I was expecting that the Green LED would have been in parallel and should have lit up.

In both failed scenarios I have swapped around the LEDs and proved that all the LEDs function.

There must be some basic concept that I am not understanding with the circuits, any ideas?

  • 6
    How do you expect us to figure out the problem if you don't supply a schematic you're using and your code?
    – derstrom8
    Jun 20, 2014 at 0:58
  • 1
    Photograph of breadboard would also help. Value and placement of resistors. If you are not using resistors, I'm worried for the longevity of your LEDs and Arduino.
    – James Cameron
    Jun 20, 2014 at 1:24
  • 1
    Read up on LEDs here. There is a lot to consider even for driving these simple devices. Namely, forward voltage and active-mode current are very important. Driving LEDs without a series (current limiting) resistor will surely end your experimentation sooner than later. Either you will burn out the LED or you will destroy one of your Arduino's output pins.
    – sherrellbc
    Jun 20, 2014 at 5:17
  • Of course we assume you know which way round you are connecting the LED's (Hint ... they are not light bulbs that don't care)
    – Spoon
    Jun 20, 2014 at 9:33

1 Answer 1


Scenario 1 fails because yellow and green LEDs have forward voltages of about 2V each, so when putting 3 in series, you need a forward voltage to light them that exceeds the 5V available.

Scenario 2 fails because the wire you have in parallel with the LED provides a path with a very low voltage drop (close to 0V), so the path through the LED with a ~2V voltage drop is not taken (and if you remove the wire, the same reasoning as in scenario 1 applies, so nothing would work).

BTW, you should never run LEDs without a series resistor, as your experiments will surely demonstrate to you soon ;-)

  • s/a series resistor/some form of current limiting, which may be but is not limited to a simple series resistor/ </pedant> Jun 20, 2014 at 1:28
  • 4
    @IgnacioVazquez-Abrams, you forgot the opening <pedant> tag, but otherwise, you're, of course, right. Jun 20, 2014 at 1:29

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