Arduino Forum

Using Arduino => Installation & Troubleshooting => Topic started by: ROGFalcom on Jan 01, 2016, 11:17 am

Title: "L" LED on Arduino UNO
Post by: ROGFalcom on Jan 01, 2016, 11:17 am
Hi,

I've got a little problem, when I connect my UNO to PC, the "L" LED doesn't turn ON, it is OFF. When I disconnect GND wire it turns ON, it never did before. Can someone explain me that ?

Title: Re: "L" LED on Arduino UNO
Post by: weedpharma on Jan 02, 2016, 01:52 am
Give a picture showing clearly what is connected and which wire you are removing.

Weedpharma
Title: Re: "L" LED on Arduino UNO
Post by: jurs on Jan 04, 2016, 01:16 am
I've got a little problem, when I connect my UNO to PC, the "L" LED doesn't turn ON, it is OFF. When I disconnect GND wire it turns ON, it never did before. Can someone explain me that ?
Is it an "UNO R3"?

With UNO R3 and pin-13 left INPUT and "not connected", the pin state is "floating" and the L-LED is either off, on, or even may blink from time to time. This is caused due to the R3 design and the LED driven by an OpAmp.

In older board designs, like first Uno, Uno R2 and Duemilanove, the LED should be off when pin-13 is left floating as INPUT.
In newer R3 design, the state of the L-LED is not determined as long as pin-13 is INPUT and not connected to anything.
Title: Re: "L" LED on Arduino UNO
Post by: justintime on Jan 04, 2016, 10:58 pm
Thanks for explaining that Jurs...  I had the same question yellow flashing "L" LED goes on and CONTINUES to flash, (when I plug my Arduino UNO (R-3) into computer via USB).

..weedpharma told me it wasn't a problem....that it should blink every second.(thank you Pharma).. but mine blinks far greater than once per second.. maybe 4 or 5 times per second.

So apparently it will continue to blink when I am operating a circuit unless I connect the pin-13 ??
Thanks
Title: Re: "L" LED on Arduino UNO
Post by: jurs on Jan 04, 2016, 11:13 pm
So apparently it will continue to blink when I am operating a circuit unless I connect the pin-13 ??
You don't need to connect pin-13 to anything.
To avoild blinking of the L-LED on R3 boards you also can set the pin by software to HIGH or LOW.

Try in the setup() function:
Code: [Select]
pinMode(13, INPUT_PULLUP); // L-LED should be "always ON"
or
Code: [Select]
pinMode(13, OUTPUT); // L-LED should be "always OFF"
(if the board is of "R3" design and nothing is connected to pin-13)

So you can fix the blinking just by software control.
Title: Re: "L" LED on Arduino UNO
Post by: justintime on Jan 04, 2016, 11:21 pm
Wow; thanks Jurs...very helpful...
Title: Re: "L" LED on Arduino UNO
Post by: a_SARAK on Dec 15, 2018, 03:32 pm
I have a problem with my new arduino mega board. Just testing "blink". Though pin 13 goes high - low as expected the L led is always off. Any suggestions?