I have wired up a small project with a shift register (see below), but depending on how I put the 74HC595 IC (8-bit shift register with 3-state output register), the green and yellow LEDs of the Arduino stop shining.

What are these LEDs for? Do they tell something about a possible damage of the IC/short circuit in the setup?

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4 Answers 4


The green LED is marked with ON, and it indicates that the Arduino has power.

The yellow LED is marked L and is just connected to pin 13. So when you set pin 13 to HIGH the LED lights up. You can however still use pin 13 for other things.

When the green LED stops shining you most likely have a short somewhere. As the short will drain all power away, so the Arduino has no power left to light the LED(s).

  • 2
    Or if you draw too much power from the +5V, the thermal protection circuit on the voltage regulator will trip and the whole Arduino will shut down.
    – Duncan C
    Jul 13, 2014 at 1:12

Also be warned that the L LED (the one activated by pin 13) may light up if pin 13 is high-impedance (set to input), and is not connected to anything.

On my Uno, a trivial sketch like this lights up the L LED:

void setup ()
  }  // end of setup

void loop ()
  }  // end of loop

This is because the LED is not driven directly by pin 13, but by an op-amp (U5B) which -- if the pin is configured as an input -- may pick up enough stray voltage to activate and light up the LED.

This can be confusing because you might look at your board and think "huh? I didn't command that LED to come on".

  • 2
    What, an op-amp on the L LED? Uh, you are right. They added it in Rev3. It was not there on the Uno Rev2. Jun 29, 2015 at 7:47
  • I suspect that driving the LED directly was influencing the pin when it was being used for other things (like SPI). So inserting a high-impedance driver (the op-amp) minimized that. And you are right, it was a moderately new addition, but I didn't have the energy to find exactly which board had it, so I left that unsaid. ;)
    – Nick Gammon
    Jun 29, 2015 at 9:04
  • This is a new "feature" in Uno R3 boards. Feb 4, 2020 at 11:15

Also note that there is the TX and RX pins on Uno (orange/yellowish) are for the USB connection. When the LED labeled TX is lit up, the Arduino is transmitting data to the computer. When the RX is lit up, it is receiving data from the computer.


Just wanted to re-iterate that On the MEGA 2560 Revision 3 you can't change the L led pin 13 due to the op-amp like Nick Reported.

Spent a couple hours on this scratching my head only to find this out

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