void setup () {
  // initialize digital pin LED_BUILTIN as an output
  pinMode (LED_BUILTIN, OUTPUT) ;
}

// the loop function runs over and over again forever
void loop () {
  digitalWrite (LED_BUILTIN, HIGH); // turn the LED on (HIGH is the voltage level) 
  delay (1000) ; // wait for a second
  digitalWrite(LED_BUILTIN, LOW); // turn the LED off by making the voltage low
  delay (1000) ; // wait for a second
}