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I have a problem with the on-board LED staying lit up despite running a blink program.

I was running the blink program below, which writes HIGH/LOW to pin 13, but the light was solid. As soon as I connected the circuit between ground and pin 13 with the multi-meter, the LED started blinking.

Here's a video showing my setup. The video shows the "L" LED lit solid. The digital multi-meter (connected to pin 13 and GND) is switched from OFF to DCV, at which point the LED starts to blink. The wire connections are removed, and the LED is again lit up solid.

When the pin is set HIGH, the voltage reading is 5.07V. However, when the pin is set LOW, the voltage reads 0.61V whereas other pins read 100 times less, around 6mV.

void setup() {
  pinMode(13, OUTPUT);
}
void loop() {
  digitalWrite(13, HIGH);
  delay(2000);
  digitalWrite(13, LOW);
  delay(2000);
}

The question is why is this happening? Is my board bugged?

Reference schematic linked here. "arduino-mega2560_R3-sch.pdf"

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    \$\begingroup\$ You 'completed' (?) the circuit with a multimeter in what setting? \$\endgroup\$ – Wouter van Ooijen May 5 '18 at 20:28
  • \$\begingroup\$ This is actually pretty odd. I’m looking forward to seeing some responses. You may want to edit the question to state which Arduino you’re using, and clarify the “completed the circuit” bit. Also, I believe the LED is yellow (amber) on the Mega board, not green. \$\endgroup\$ – Blair Fonville May 5 '18 at 20:42
  • \$\begingroup\$ Any chance of getting a schematic? Also, I would ask such questions at Arduino.SE rather than here. \$\endgroup\$ – Dmitry Grigoryev May 5 '18 at 20:49
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    \$\begingroup\$ Oh ok. Then I stand corrected. That’s actually a big difference since the pin now feeds a voltage follower. \$\endgroup\$ – Blair Fonville May 5 '18 at 21:03
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    \$\begingroup\$ MCU = Microcontroller. I would suspect a bad PCB or solder joint to be much more likely than a bad MCU chip (which usually go short, not open). In any case, it appears to be a hardware problem so you may wish to return it if it is under warranty. \$\endgroup\$ – Spehro Pefhany May 5 '18 at 22:22
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Connect 10k PUl-up resistor(Similair like on imput) to the Pin 13 in some cases internal induction on some supplies may generate enough background voltage to keep LED on although internal capacitance may slow down blinking (dimming).

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Welcome to EE.SE, Tomas, but you seem to be guessing. If you look at the schematic for this Arduino you will see that the LED is not driven directly from the GPIO. There is a buffer. There is no "background voltage" - the Arduino has an on-board regulator. Internal capacitance does not slow down blinking. Blinking is controlled by the program. Keep studying. \$\endgroup\$ – Transistor May 9 '18 at 17:07

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