const int red = 2;     //紅色LED 接 PIN 2
int yellow = 5;  //黃色LED 接 PIN 5
int green = 23;   //綠色LED 接 PIN 23
unsigned long previousMillis = 0;
long interval = 0;
void setup() {
  
  Serial.begin(115200);
  pinMode(red, OUTPUT);     //設定 PinMode 皆 OUTPUT, Arduino 輸出電位到 Pins 2, 5, 23
  pinMode(yellow, OUTPUT);
  pinMode(green, OUTPUT);
  
}

void loop() {
  unsigned long currentMillis = millis();
  // 紅燈亮30秒
  digitalWrite(yellow, LOW);
  digitalWrite(green, LOW);
  digitalWrite(red, HIGH);
  delay(30000);
  interval = 30000;
  // 黃燈亮10秒  
  currentMillis = millis();
  if (currentMillis - previousMillis >= interval){
    previousMillis = currentMillis; 
    digitalWrite(green, LOW);
    digitalWrite(red, LOW);
    digitalWrite(yellow, HIGH);
    delay(10000);
    interval = 10000;
  }
  // 綠燈亮20秒 
  currentMillis = millis();
  if (currentMillis - previousMillis >= interval){
    previousMillis = currentMillis; 
    digitalWrite(yellow, LOW);
    digitalWrite(green, HIGH);
    digitalWrite(red, LOW);
    delay(20000);
    previousMillis=0;
  }
}