void setup()
 {
  // 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 (making the voltage level HIGH)
  
delay(1000);              // wait for a second
  
digitalWrite(13, LOW);    // turn the LED off (making the voltage level LOW)
  
delay(1000);              // wait for a second

}