A  PIR (passive infrared sensor) sensor is an electronic sensor that measures Infrared (IR) light radiating from objects in its field of view.

pir_example.ino

    int LED = 13;
    int inputPin = 2;
    int PIR_STATE = LOW;
    int VALUE = 0;

    void setup() {
        pinMode( LED, OUTPUT );
        Serial.begin( 9600 );
    }

    void loop() {
        
        VALUE = digitalRead( inputPin );
        if ( VALUE == HIGH ) 
        {
            digitalWrite( LED, HIGH );
            if ( PIR_STATE == LOW ) 
            {
                // ————–we have just turned on
                Serial.println( “Motion has been detected!” );
                PIR_STATE = HIGH;
            }
        } 
        else 
        {
            digitalWrite( LED, LOW );
            if (PIR_STATE == HIGH) 
            {
                // ———we have just turned off
                Serial.println( “Motion has been stopped!” );
                PIR_STATE = LOW;
            }
        }
    }