int minute=3;
 int sekunde=10;

void setup() {

 
  // put your setup code here, to run once:
Serial.begin(9600);
Serial.println("Eieruhr");
 
  // Pins Festlegen 13 LED, 7 Taster
 pinMode(13, OUTPUT);
 pinMode(7, INPUT_PULLUP);
}

void loop() {
  // put your main code here, to run repeatedly:
 digitalWrite(13, LOW);
 
  if (minute>=0 && sekunde<=59 && sekunde>=0)
  {
  
      if(sekunde>=10)
        {
          Serial.print("0");
          Serial.print(minute);
          Serial.print(":");
          Serial.println(sekunde);
        sekunde--;
        delay(1000);
        }
     else if(sekunde<=10)
        {
          Serial.print("0");
          Serial.print(minute);
          Serial.print(":");
          Serial.print("0");
          Serial.println(sekunde);
          sekunde--;
          delay(1000);
        }
  }

  else if (sekunde<0)
    {
    minute--;
    sekunde=59;
    }

  else if(sekunde==0 && minute==0)
    {
      Serial.print("0");
      Serial.print(minute);
      Serial.print(":");
      Serial.print("0");
      Serial.println(sekunde);
      minute--;
    }

  else if(minute<0)
    {
    Serial.println("Der Timer ist abgelaufen!");
    digitalWrite(13, HIGH);
    delay(1000);
    }
}