void setup() 

     {  // initialize digital pin LED_BUILTIN as an output.           	

         pinMode(LED_BUILTIN, OUTPUT);

      }

void loop() {  

     digitalWrite(LED_BUILTIN, HIGH);  // turn the LED on

            delay(1000);   // wait for a second 

     digitalWrite(LED_BUILTIN, LOW);   // turn the LED off

            delay(1000);   // wait for a second

}

DIAGRAM