int zaehler = 0;
char c;

void setup() {
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
  Serial.begin(9600);
  Serial.println("Serial Monitor Start");
}

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

    if (c >= '0' && c <= '9') {
      zaehler = c - '0'; // Umwandlung von ASCII-Zahl (z. B. '5') in int 5
    } else if (c >= 'A' && c <= 'F') {
      zaehler = c - 'A' + 10; // 'A' bis 'F' für 10 bis 15
    } else if (c >= 'a' && c <= 'f') {
      zaehler = c - 'a' + 10;
    }

    zaehler = constrain(zaehler, 0, 15);

    digitalWrite(10, zaehler % 2);
    digitalWrite(11, (zaehler / 2) % 2);
    digitalWrite(12, (zaehler / 4) % 2);
    digitalWrite(13, (zaehler / 8) % 2);

    Serial.print("Empfangener Wert: ");
    Serial.println(zaehler);
  }
}