void setup() {
  for(int i=21; i<=23;i++){
    pinMode (i,OUTPUT);

  }
 
}

void loop() {
  //1红灯亮
  digitalWrite(23, HIGH);
  digitalWrite(22,LOW );
  digitalWrite(21,LOW);
  delay(5000);
  //2黄灯亮和红灯亮
  digitalWrite(23, HIGH);
  digitalWrite(22, HIGH);
  digitalWrite(21,LOW);
  delay(2000);
  //3绿灯亮
  digitalWrite(23, LOW);
  digitalWrite(22,LOW );
  digitalWrite(21,HIGH);
  delay(5000);
  //4黄灯亮
  digitalWrite(23, LOW);
  digitalWrite(22,HIGH );
  digitalWrite(21,LOW);
  delay(5000);
}