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 stopped!");
  digitalWrite(BUZZER_PIN, LOW) ;
 }
}