I recently picked up an Uno clone and noticed that if pin 12 is set to an output and I run the Blink sketch on it, the built in led (supposedly connected to pin 13) blinks. It's not as bright as it is when I run it on pin 13, but nonetheless it blinks. If I run it on any other pin, nothing happens, which is the behavior I expected with pin 12.

Is this a common issue with arduino boards, the microcontrollers, etc. or did I get a bad board?

Overall the board visibly appears to have solid construction and appears as the one on the site that I linked to.

Lastly as a side note, I'm currently living in India and won't have a problem going back to their factory for a replacement, if it is a bad board. They include a 6 month warranty.

int ledPin = 12;
int delayPeriod = 100;

void setup()
  pinMode(ledPin, OUTPUT);

void loop()
  digitalWrite(ledPin, HIGH);
  delayPeriod = delayPeriod + 100;
  • Is there a schematic available anywhere? This behavior is not expected, and certainly not characteristic of an Arduino. Commented Aug 1, 2015 at 18:19
  • Good question. I'm not sure. I've just emailed the company asking them if a schematic is available. I've also let them know about this unexpected behavior. I'm curious if this has happened to anyone else.
    – Paul Masek
    Commented Aug 1, 2015 at 18:45
  • After some digging I came up with this pdf on their website. The second to last page, pg.121, may be it.
    – Paul Masek
    Commented Aug 1, 2015 at 19:01
  • Those are the reference schematics, for the 2009 and Uno r1. Commented Aug 1, 2015 at 19:07
  • 1
    Try measuring the resistance between the two pins using a multimeter, if you have one. Could be some flux residue or something between the solder joints on the underside, between pins 12 and 13. This residue is slightly conductive.
    – Gerben
    Commented Aug 1, 2015 at 19:48

1 Answer 1


See Have I bricked my Arduino Uno? Problems with uploading to board

As I state on that page, if pin 13 is set to input the LED may turn on due to being driven by an op-amp. A nearby pin turning on may be enough to cause that effect. Try adding:

 pinMode(13, OUTPUT);
 digitalWrite (13, LOW);

See if that changes the behaviour.

  • Yes, adding that stopped the blinking. So what should I do next?
    – Paul Masek
    Commented Aug 2, 2015 at 14:17
  • According to this answer. The onboard led might turn on if you leave the pin floating and in input mode. So you need to leave it in output mode, or make sure it's not floating. A pull-down or pull-up resister may help if you need it in input mode for some reason.
    – Daniel
    Commented Aug 2, 2015 at 15:25
  • Thanks all for the help. You've given me some insight into the behavior I witnessed as well as more knowledge about this device. Also, Nick, you're page that you linked to is an excellent resource, that I'm sure I'll refer to again in the future.
    – Paul Masek
    Commented Aug 3, 2015 at 6:30

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