int ledpin=5;
int buzzerpin=12;
/*
A sensor always gives signals in analog values .*/
int ldrpin=A0;//light dependent resistor
void setup() {
  Serial.begin(9600);
  pinMode(ledpin, OUTPUT);
  pinMode(buzzerpin, INPUT);
  pinMode(ldrpin, INPUT);

}

void loop() 
{
int ldrstatus=analogRead(A0);//An analog signal is a continuous signal 
//that changes continuously over time.
if(ldrstatus<400)
{
tone(buzzerpin,100);
digitalWrite(ledpin, HIGH);
delay(3000);

Serial.println("ALARM ACTIVATED");
}
else
{noTone(buzzerpin);
digitalWrite(ledpin,LOW);
Serial.println("ALARM DEACTIVATED");
}
}