At only 2 1/2 inches long this economical alarm inconspicuously protects your home from intruders. This unit uses a magnetic sensor to detect the opening of a protected door or window. A 90 dB alarm will then alert you to an attempted entry and scare off potential intruders. The owner may open the window or other protected object, without setting off the alarm by switching the on/off switch to the off position. May also be used within the home to alert parents to children opening a gun or medicine cabinet or used to protect exterior doors, particularly those hard to protect sliding glass doors. Commercial uses include showcases and cash registers. Batteries included. NOTE: this is ONLY SOLD in a package of 4 devices.