void setup(){
  pinMode(23, OUTPUT);
  pinMode(22, OUTPUT);
  pinMode(21, OUTPUT);
}

void loop ()
{
  digitalWrite(23, HIGH);
  delay(1000);
  digitalWrite(23, LOW);
  delay(1000);
  digitalWrite(22, HIGH);
  delay(1000);
  digitalWrite(22, LOW);
  delay(1000);
  digitalWrite(21, HIGH);
  delay(1000);
  digitalWrite(21, LOW);
  delay(1000);
}