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);
  }
}