Be alerted when a door or window in your home is opened. This unit consists of two small magnetic contact bars and a transmitter. When the door or window is opened, the two bars lose magnetic contact, triggering the transmitter. The transmitter then sends a signal to any 318MHz Silent Call receiver (required - sold separately). Easy to install; adhesive strips are included. Operates on 2 AAA batteries. Range of up to 500 feet depending on receiver.