const int redLED = 18;
const int yellowLED = 19;
const int greenLED = 21;
const int faultButton = 23;

void setup() {
  pinMode(redLED, OUTPUT);
  pinMode(yellowLED, OUTPUT);
  pinMode(greenLED, OUTPUT);

  pinMode(faultButton, INPUT_PULLUP);
}

void loop() {
  if (digitalRead(faultButton) == LOW) {
    // Simulated circuit fault
    digitalWrite(redLED, HIGH);
    digitalWrite(yellowLED, LOW);
    digitalWrite(greenLED, LOW);
  } else {
    // Normal operation
    digitalWrite(redLED, LOW);
    digitalWrite(yellowLED, HIGH);
    digitalWrite(greenLED, LOW);
    delay(500);

    digitalWrite(yellowLED, LOW);
    digitalWrite(greenLED, HIGH);
    delay(500);
  }
}