volatile int zaehler = 0;

#define ENCODER_CLK 2
#define ENCODER_DT  3
#define ENCODER_SW  4

void setup() {
  Serial.begin(9600);
  pinMode(ENCODER_CLK, INPUT);
  pinMode(ENCODER_DT, INPUT);
  pinMode(ENCODER_SW, INPUT);
  attachInterrupt(digitalPinToInterrupt(ENCODER_CLK), readEncoder, FALLING);
}

void loop() {
    Serial.println(zaehler);
    delay(500);
}

void readEncoder() {
  if (digitalRead(ENCODER_DT)) {
    zaehler++;
  } else {
    zaehler--;
  }
}