#define YELLOW D7
// defined the external LED connected to D7 pin

// the setup function runs once when you press reset or power the board
void setup() {
  
  pinMode(YELLOW, OUTPUT);
  
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(YELLOW, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(500);
  digitalWrite(YELLOW, LOW);   // turn the LED off by making the voltage LOW
  delay(500); 

  digitalWrite(YELLOW, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(500);
  digitalWrite(YELLOW, LOW);   // turn the LED off by making the voltage LOW
  delay(500); 
 
}