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Topic: LED L (Read 4802 times) previous topic - next topic


You have 0.01uF. Looking for 0.1uF.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


OK, I got that fixed. I will see if I can figure out how to do this without blowing up the universe or sending myself back in time.


I found some YouTube video that appears to claim it's a problem with Arduino's design ... I don't know if that's true.

It is in fact, not a problem at all.  It is a consequence of an absolutely deliberate design.  An Arduino pin that is not connected or defined as an output or as "INPUT_PULLUP" is by definition, floating - that is what it is supposed to to; that it is a very high impedance (hundreds of megohms) is what makes it possible to measure capacitance and function as a proximity sensor.

What the op-amp is doing is accurately reflecting the state of pin 13.  Nothing more, nothing less.  There is no need to make any modification whatsoever.  In actual fact, it serves as a reminder that it is actually a good idea to define unused ports in all sketches (used for serious purposes) as "INPUT_PULLUP" because floating inputs may cause "sneaker" currents in the input logic, mostly harmless unless you are wishing to minimise power consumption, especially on "sleep".  If however you wish the "L" LED not to be lit and you are not using it otherwise in the sketch, by all means set it as an output and LOW.

But the video seems to work. The video claims you're supposed to glue the 13 pin to the ground pin.

This is truly sad but - this is the Internet after all.  The video is bogus - insofar as it talks of this behaviour as a "problem" when it simply is not.  The worst is yet to come however.

Now what is shown, is applying some liquid PVC glue to the two adjacent pins.  PVC glue of course uses water as a solvent and is ever so slightly conductive.  As it dries, the water evaporates and it ceases being conductive.  Do you see?


Thanks for all the help.

It doesn't really matter to me that the LED was lit up. I just never noticed it before. I was playing around trying to get some servos working and I know that once or twice I hooked wires up in the wrong place.

So I thought maybe I damaged the Arduino and that's why the light was staying on. But now that I know it was designed to be that way, I'm not worried about it being broken.

Someone should give the guy that made the YouTube video a link to this thread. Because he states in the video that it's "unsolved issue on the Arduino forums".


Feb 28, 2014, 06:31 pm Last Edit: Feb 28, 2014, 06:33 pm by narenravi Reason: 1
When i searched for it , it was unsolved, and i knew what was the reason for that. It was just kind of funny solution, i dint claim it as solution. And FYI : i also mentioned , you can add a resistor across its pin to prevent the floating condition in the same video, But you guys got glued to the GLUE part ...


You can solve this with a piece of duct tape over the LED    ;)
No technical PMs.
The last thing you did is where you should start looking.


... i dint claim it as solution....

You named the title: "Arduino UNO pin13 LED always on_Solved"

Which means the solution, also you said the word "solution" at least 5 times in the video.

In any case, at least we know why it really happens now.

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