Abstract: At a protected site, intrusion sensors are connected to a radio transmitter which signals a remote receiver. In the transmitter, a code matrix is scanned in response to signals from a clock pulse generator to produce a coded pulse sequence. The receiver has a matching matrix arrangement to render it responsive only to the particular pulse sequence of its associated transmitter. When the system is not in use, the receiver is stored in a container arranged to automatically deactivate the system when the receiver is present and automatically activate the system when the receiver is removed.