const int LED_PIN = D0;

bool   ledState    = false;
int    brightness  = 255;
String inputBuffer = "";

void setup() {
  Serial.begin(9600);
  pinMode(LED_PIN, OUTPUT);
  analogWrite(LED_PIN, 0);
  delay(1000);
  sendState();
}

void loop() {
  while (Serial.available()) {
    char c = (char)Serial.read();
    if (c == '\n') {
      processCommand(inputBuffer);
      inputBuffer = "";
    } else if (c != '\r') {
      inputBuffer += c;
    }
  }
}

void processCommand(String cmd) {
  cmd.trim();
  if (cmd == "LED_ON") {
    ledState = true;
    analogWrite(LED_PIN, brightness);
  } else if (cmd == "LED_OFF") {
    ledState = false;
    analogWrite(LED_PIN, 0);
  }
  sendState();
}

void sendState() {
  if (ledState)
    Serial.println("STATE:ON:" + String(brightness));
  else
    Serial.println("STATE:OFF:0");
}