Led 13 always on


I've just bought an Arduino Uno r3 and from the moment I connected it, the LED on pin 13 was on. Is it natural?

Best regards, lightspot21

The LED is driven by an op amp configured as a comparator. If D13 is left as an Input, it's level can "float" and be seen as a high by the comparator, causing its output to go high and turn the L LED.

Thanks for the reply, but I meant that from the moment I connect the USB cable (no windows of Arduino IDE open) the light is still on, even if I have nothing on the flash memory.

What CrossRoads said is that the pin D13 may be floating. Tie it to GND a with 10k and see what happens.


If you bought a new Arduino UNO R3, then the chances are that it came preloaded with the Blink example. SO even if you haven't uploaded anything onto it, the LED will turn on. If you really wanted to make sure everything is alright, perhaps you should just upload your own sketch and see what happens.