L SMD diode lights all the time

Hello. L diode on Arduino PCB lights all the time. When I upload this program:

void setup() {                

void loop() {

L diode is lighting. When I put LED diode in pin 13 and GND L diode turning off. It is normal? Moreover my Arduino was detected in computer as "Unknown device". Could anyone help me and check this code on Arduino UNO R3?


The pin has a weak internal pullup, it might be enough to make the LED appear on, when it is not really.

Did you install the drivers for the Uno?

Once you can download a sketch, see if the brightness changes when you actually drive the LED:

void setup(){
pinMode (13, OUTPUT);
void loop(){
digitalWrite (13, HIGH);
delay (500);
digitalWrite (13, LOW);
delay 500);

Thanks for reply. Yes, I installed drivers for Arduino. When I drive LED in pin13 and GND (without resistor), L diode lights less. But when I connect it with 220? resistor, there ale no changes. Moreover, L brightness (without program) changes when I touch the pins 13 and GND with finger. I bought Arduino few days ago, should I send it back?

I just got an Uno R3 from NKCElectronics, haven't even opened the shipping package yet. I''ll see if mine does the same.

What are you using for power supply? USB port from PC? Wallwart into the barrel jack? Something else?

I’m using USB port.

Here is the problem with touching pin 13 and GND: http://www.youtube.com/watch?v=zuQKGZET6H8 (without program).

Did you solve the problem?

What problem? Seems to be normal operation. Mine does the same.

So yo say that every Arduino UNO behaves in the same way. Because if this is a normal operation, then our Arduino's are fine and all the others are faulty...

Put a little code in, the LED turns off. What's the big deal?

I think the bootloader blinks the LED. When it finishes it sets it to input, but may be still HIGH. The SMD LED is powerful enough to glow. It probably shouldn't do that, but you can always set pin 13 to LOW as the first line of your program it it troubles you.

The issue is solved by just a simple and funny solution,. check my video,. watch it till the end,.


We've established that the R3 Unos have an op-amp that drives the LED, and if the pin is floating the op-amp will probably turn the LED on. Your solution of a small resistance effectively brings that pin (13) low with a weak pull-down, and then the op-amp doesn't drive the LED.

well i guessed there is not enough resistance so added some glue,. :wink: :slight_smile: :slight_smile: