"LED_BUILTIN"

"LED_BUILTIN"

• Which indicates the builtin LED on the board
• Most Arduinos have an on-board LED which can be controlled. 
• On the UNO, MEGA and ZERO it is attached to digital pin 13, on MKR1000 on pin 6.

pinMode(LED_BUILTIN, OUTPUT);  // initialize digital pin LED_BUILTIN as an output.

digitalWrite(LED_BUILTIN, HIGH);   // turn the LED on (HIGH is the voltage level)

digitalWrite(LED_BUILTIN, LOW);    // turn the LED off by making the voltage LOW