void setup() {
  pinMode(13, OUTPUT);
  
  Serial.begin(9600);
}

void loop() {
  if (Serial.available() > 0) {
    char chr = Serial.read();

    if (chr == 'H') {
      digitalWrite(13, HIGH);
    } else if (chr == 'L') {
      digitalWrite(13, LOW);
    }
  }
}