const int MOTION_SENSOR_PIN = 7;
const int BUZZER_PIN = 3;
int MotionStateCurrent = LOW;
int MotionStatePrevious = LOW;

void setup () {
  Serial.begin (9600);
  pinMode (MOTION_SENSOR_PIN, INPUT);
  pinMode (BUZZER_PIN, OUTPUT);
}

void loop () {
MotionStatePrevious = MotionStateCurrent;
MotionStateCurrent = digitalRead (MOTION_SENSOR_PIN);

if (MotionStatePrevious == LOW && MotionStateCurrent == HIGH){
Serial.println("Motion detected!");
digitalWrite (BUZZER_PIN, HIGH) ; // turn on
} 
else
if (MotionStatePrevious == HIGH && MotionStateCurrent == LOW){
Serial.println("Motion detected!");
digitalWrite (BUZZER_PIN, LOW) ;
}
}