#define LED_PIN 2

void setup() {
  Serial.begin(115200);
  pinMode(LED_PIN, OUTPUT);
  Serial.println("Type ON or OFF");
}

void loop() {
  if (Serial.available()) {
    String cmd = Serial.readStringUntil('\n');
    cmd.trim();

    if (cmd == "ON") {
      digitalWrite(LED_PIN, HIGH);
      Serial.println("LED ON");
    } 
    else if (cmd == "OFF") {
      digitalWrite(LED_PIN, LOW);
      Serial.println("LED OFF");
    }
  }
}