bool isToneOn = false;
int frequency = 3000;

void setup() {
//here is our PIR sensor
pinMode(2, INPUT);
//here is our buzzer
pinMode(3, OUTPUT);

}

void loop() {
//when PIR sensor gives us HIGH it means that it detects movement
if(digitalRead(2) == HIGH){
//we will turn on alarm for 15 seconds
//we are using tone() so we can control frequency of our beep sound
//to turn tone off we have to use noTone()
//if you want to change frequency of tone you can do it in the variable
//on the top of the code
for(int a = 0; a < 30; a++){
if(isToneOn){
noTone(3);
isToneOn = false;
}else{
//3 means our pin where buzzer is connected
tone(3, frequency);
//we have to change this variable to true, we have to know
//when to turn buzzer on and when to turn it on
isToneOn = true;
}
//delay 0.5 second, you can change this value so it will
//beep slower or faster
delay(500);
}
}
}