#define LED_PIN 4

void setup() {
  Serial.begin(9600);

  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);
}

void loop() {
  if (Serial.available()) {
    char data = Serial.read();

    if (data == '1') {
      digitalWrite(LED_PIN, HIGH);
    }
    else if (data == '0') {
      digitalWrite(LED_PIN, LOW);
    }
  }
}