Brinno KNS100 Knocking Sensor for PHV1330 Hidden Front Door Camera For use with the Brinno PHV133012 Digital PeepHole Viewer Fits into a slot on the back of the Digital PeepHole Viewer Takes pictures of anyone knocking at your door by sensing the vibration produced by knocking Provides added security by allowing the Digital PeepHole Viewer to record photos of who's at your door even when you're not home No batteries needed