Simply place the alarm behind any door. If an intruder attempts to open the door, the wedge-shaped design will prevent it from opening and activate a 120 dB alarm. The alarm will scare off the would-be intruder and alert you to the attempted entry. An adjustable sensitivity switch prevents tampering. When traveling, just toss this lightweight unit in your luggage. The on/off switch will prevent the unit from accidentally sounding. Requires one 9 volt battery that is not included. Measurements are 5 1/2" x 2" x 1 1/2".