Abstract: An alarm system includes an electronic central controller or control unit with an internal intrusion sensor. The controller is powered by a single nine volt battery. The system can be expanded by adding auxiliary sensors such as sound discriminators, glass breakage sensors and other low power battery operated sensors. The controller can perform all of the functions normally associated with large A.C. powered systems such as factory adjustable entry/exit and reset delays, entrance monitoring, and controlling of lights and message dialers. The system can be adapted for use in mobile environments.