// initialize digital pin 13 as an output.

   pinMode(13, OUTPUT);

}

// the loop function runs over and over again forever

void loop() {

  digitalWrite(13, HIGH);    // turn the LED on (HIGH is the voltage level)

   delay(1000);              // wait for a second

  digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW

  delay(1000);              // wait for a second

}