#define GREEN 2
#define YELLOW 4
#define RED 5

void setup () {
  pinMode(GREEN, OUTPUT);
  pinMode(RED, OUTPUT);
  pinMode(YELLOW, OUTPUT);
}

void loop () {
  digitalWrite(GREEN, LOW);
  digitalWrite(YELLOW, LOW);
  digitalWrite(RED, HIGH);
  delay(500);

  digitalWrite(GREEN, LOW);
  digitalWrite(YELLOW, HIGH);
  digitalWrite(RED, LOW);
  delay(500);

  digitalWrite(GREEN, HIGH);
  digitalWrite(YELLOW, LOW);
  digitalWrite(RED, LOW);
  delay(500);
}