If you are already using or plan to purchase the ElkGuard Wireless Home Security System or the ElkGuard Wireless Home Security System with Cellular GSM, the ElkGuard Mini Door and Window Wireless Magnetic Contact Switch is an excellent accessory. It can be added to either wireless security system to sense when something such as a door or window is opened, and because each system can handle up to 23 additional devices, you can place multiple units throughout your home. This battery powered device communicates wirelessly via radio frequency (RF), so difficult in-wall wiring or hardwired connections aren't required. The switch is designed to send a wireless signal to the security system when the contact between the transmitter and corresponding magnetic sensor is broken or if the unit is being tampered with, it will also send a signal when the battery is running low. The compact design of this unit also makes it easy to place in a discrete location. This reliable switch will boost security on any ElkGuard wireless security system and allows you to monitor doors, windows, gun cases, jewelry boxes, or just about anything that opens and closes. This product requires the use of a 9-Volt Lithium Battery, included. A standard size ElkGuard Door and Window Wireless Magnetic Contact Switch is also available. It features an external switch input that includes a processor for a vibration sensor; it is approximately twice the size of this switch.The ElkGuard Mini Door and Window Wireless Magnetic Contact Switch is a protective device that is typically used on entry points, such as doors and windows, that are normally closed.