int sensor = 2;            //the pin that the sensor is attached to
int state = LOW;           //by dafault,no motion detected 
int val = 0;               //variable to store the sensor status (value)

void setup() {
  pinMode(sensor, INPUT);  //initialize sensor as an input
  Serial.begin(9600);      //initialize serial
}

void loop() {
  val = digitalRead(sensor);  //read sensor value
  if(val == HIGH) {           //check if the sensor is HIGH

    if(state == LOW) {
      Serial.println("motion detected");
      state = HIGH;       //update variable state to HIGH
    }
  }
  else {
      if(state == HIGH){
        Serial.println("motion stopped");
        state = LOW;        //update variable state to LOW
    }
  }
}