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. The on/off switch will prevent the unit from accidentally sounding. It is great for home or travel and has 3 sensitivity modes: high, medium, low. Covered under a year warranty. Dimensions: 5 1/2" x 2" x 1 1/2". Requires one 9 volt battery (not included).