L led turns on automatically when I open serial monitor

Using Arduino Installation & Troubleshooting
#1

I am a beginner of Arduino. Recently, I found that when I opened the serial monitor, the L led lighted on by itself. My code had nothing to do with L led (although it was executed correctly). I am confused.

Can anyone give an explanation? What happened when I open the "Serial Monitor" Dialog? I know my code will be executed from the very beginning, but why L led is on? I am using Arduino MEGA 2560.

Thank you in advance.

#2

Perhaps your code is doing it and you don't realize it. If you show your code we can check.

#3

Here is my code. The "Dimmer" Demo in tutorial.

const int led = 9;      // the pin that the LED is attached to

void setup()
{
  // initialize the serial communication:
  Serial.begin(9600);
  // initialize the ledPin as an output:
  pinMode(led, OUTPUT);
  Serial.println("I am ready");
}

void loop() {
  byte brightness;

  // check if data has been sent from the computer:
  if (Serial.available()) {
    // read the most recent byte (which will be from 0 to 255):
    brightness = Serial.parseInt();
    brightness = constrain(brightness,0,255);
    Serial.print("Set the led to ");
    Serial.println(brightness);
    // set the brightness of the LED:
    analogWrite(led, brightness);
  }
}
#4

Looks like nothing in your code hits Pin 13 (the 'L' LED). I'd guess it has something to do with the bootloader which runs whenever you re-start the Serial Monitor. If it's a problem you can turn Pin 13 off in setup().

#5

The R3 boards use an op-amp to buffer the LED from the pin. Occasionally noise will cause the LED to turn on unintentionally.

#6

Yes, and the user can test for this by either wiring a say 10k ohm resistor from pin 13 to ground or adding the following to his start up function

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

If either of those methods keeps the L led off then he/she will know that the cause is the floating input pin condition to the op-amp driving the L LED.

Lefty

#7

retrolefty:
pinMode(13,INPUT);
digitalWrite(13, LOW);

That's the state the pin is left in: INPUT with pull-up disabled. So that won't help.

The only thing that will help is the 10k resistor to ground.

#8

Your right I had a brain fart. I meant to post:

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

Lefty

#9

Thank you, retrolefty.

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

Works perfectly to turn off the L led.
However, wired pin 13 to ground via a 10k resistor don't have any effect.
I measure the voltage between pin 13 and ground and I found that, before I opened the "Serial Monitor", it was almost 0, but when the dialog was opened, the voltage jumped to 5V immediately. I guess it is caused by my boot loader. It is some problem in initialization. However, press the "Reset" button in my board will turn off the L led. It seems contradictory.

#10