Abstract: Several modular circuits are releasably connected to one another and with the plug which supplies AC power to a computer or the like. A first circuit rectifies the AC voltage and applies a DC voltage to one side of a motion sensing circuit, the opposite side of which is connected to a third circuit containing a portable, DC power supply. The third circuit include an electrically operated alarm, and an electronic valve, which normally maintains the alarm deenergized. The motion sensing circuit contains a capacitor in parallel with a mercury switch which, when subjected to sudden movement, closes and switches the electronic valve to a conducting mode, thereby energizing the alarm, provided the equipment has been unplugged so that the AC power has been removed from the first circuit. The capacitor delays the deenergization of the alarm after the mercury switch recloses; and the DC voltage provided by the AC power supply prolongs the life of the portable DC power supply (battery).