void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  while (Serial.available() == 0);
  String i = Serial.readStringUntil('\n');
  //Baca character tanpa enter
  
    if (i == "ON"){
      digitalWrite(LED_BUILTIN, HIGH);
      Serial.println("LED ON");
    }
    else if (i == "OFF"){
      digitalWrite(LED_BUILTIN, LOW);
      Serial.println("LED OFF");
    }
    else {
      Serial.println("Invalid, please use ON or OFF.");
    }
}