// variables

int LED = 13;
int flash = 500;

void setup() 
{
    pinMode(LED, OUTPUT);    //Pin 13 controls onboard LED and sets it ready for output 
}

void loop() 
{
  digitalWrite (LED, HIGH); //Turns on onboard LED (green)

  delay(flash);               //Pauses by the value of flash in 1000th of seconds

  digitalWrite (LED, LOW);  // Turns off onboard LED (green)

  delay(flash);             // Pauses by the value of flash in 1000th of seconds
}