void setup() {
  // pins are all INPUTs at start-up
  pinMode(13, OUTPUT);     // LED pin is an OUTPUT
}

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(13, HIGH);  // LED on
  delay(500);              // for half a second
  digitalWrite(13, LOW);   // LED off
  delay(500);              // for half a second
}