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);
 }
}