user23082: I am using a Arduino Uno 3 for line follower robot. However when I power the Arduino Uno, the L led is turned on and is never turned off. After a bit of searching I found out that the L LED is connected to pin 13. So I set the pin 13 to low but still the led is on. Is there a problem with the arduino? Or there's something that I am unaware about? (Amongst a lot of other things that I don't know.)

user69392: I guess the arduino is burnt cus it happens with me in the same way. The usb port of my PC is writing device not recognize, and the atmega328p on the arduino temperature is getting high. To save my time I bought another new one I think that should help.

user23083: Its possible its the Power LED you are looking at - that will always be on.
Simplest test is to load up the Blink Example that's included in the Arduino IDE - and the LED should blink once a second. That will show you if your Pin13 LED is working ok.

user57046: Now first check IC on your arduino UNO. if it is atmega328p, set the board to arduino nano in the software and set the processor th atmega328p. that makes it work.  :)

user7508: This is a “feature” introduced in the revision 3 of the Uno board. Up to
Rev 2, the “L” LED was connected to pin 13, through a 1 kΩ
resistor. On Rev 3, the same LED+resistor combination is driven by pin
13 through an op-amp. Thus, when pin 13 is in high impedance mode (i.e.
INPUT mode, the default mode when the program starts), the input of
the op-amp is floating. The voltage at this input (and thus at the
output) is not controlled and depends only on the parasitic conductances
and on leakage currents.
If you want to turn the LED off, you not only need to write LOW to it,
you also (and most importantly) should set the pin to OUTPUT.

23082: <p>I am using a Arduino Uno 3 for line follower robot. However when I power the Arduino Uno, the L led is turned on and is never turned off. After a bit of searching I found out that the L LED is connected to pin 13. So I set the pin 13 to low but still the led is on. Is there a problem with the arduino? Or there's something that I am unaware about? (Amongst a lot of other things that I don't know.)</p>
 69392: <p>I guess the arduino is burnt cus it happens with me in the same way. The usb port of my PC is writing device not recognize, and the atmega328p on the arduino temperature is getting high. To save my time I bought another new one I think that should help.</p>
 23083: <p>Its possible its the Power LED you are looking at - that will always be on.</p>

<p>Simplest test is to load up the Blink Example that's included in the Arduino IDE - and the LED should blink once a second. That will show you if your Pin13 LED is working ok.</p>
 57046: <p>Now first check IC on your arduino UNO. if it is atmega328p, set the board to arduino nano in the software and set the processor th atmega328p. that makes it work.  :)</p>
 7508: <p>This is a “feature” introduced in the revision 3 of the Uno board. Up to
Rev 2, the “L” LED was connected to pin 13, through a 1&nbsp;kΩ
resistor. On Rev 3, the same LED+resistor combination is driven by pin
13 through an op-amp. Thus, when pin 13 is in high impedance mode (i.e.
<code>INPUT</code> mode, the default mode when the program starts), the input of
the op-amp is floating. The voltage at this input (and thus at the
output) is not controlled and depends only on the parasitic conductances
and on leakage currents.</p>

<p>If you want to turn the LED off, you not only need to write <code>LOW</code> to it,
you also (and most importantly) should set the pin to <code>OUTPUT</code>.</p>
