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

void loop() {
  int helligkeit1 = analogRead(A0);
  Serial.print(helligkeit1); Serial.print("     ");
  delay(500);
  int helligkeit2 = analogRead(A0);
  Serial.print(helligkeit2); Serial.print("     ");

  Serial.print(helligkeit1 - helligkeit2); Serial.print("     ");

  if ((helligkeit1 - helligkeit2) > 50  ||  (helligkeit2 - helligkeit1) > 50) {
    Serial.println("Alarm!!!");
    tone(9, 2000, 2000);
    delay(2000);
  }
  else  {
    Serial.println("alles ok");
  }
}