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!");
    analogWrite(BUZZER_PIN , HIGH); 
    delay(1000);
  }
  else
  if (motionStatePrevious == HIGH && motionStateCurrent == LOW) { 
    Serial.println("Motion stopped!");
    digitalWrite(BUZZER_PIN, LOW);  
  }
}