int buzzer = 11;
 int sensor = 8;
 int led = 13;
 void setup()
 {
  pinMode(buzzer, OUTPUT);
  pinMode(sensor, INPUT);
  pinMode(led, OUTPUT);

  digitalWrite(led, LOW);
  digitalWrite(sensor, LOW);
  digitalWrite(led, LOW);
  while(millis()<13000)
  {
    digitalWrite(led, HIGH);
    delay(20);
    digitalWrite(led, LOW);
    delay(20);
  }
  digitalWrite(led, HIGH);
 }
 void loop()
 {
  if(digitalRead(sensor)==HIGH)
  {
    digitalWrite(buzzer, HIGH);
    delay(3000);
    digitalWrite(buzzer, LOW);
    while(digitalRead(sensor)==HIGH);
  }
 }