an apparatus and method is disclosed for controlling a load capable of functioning for a predetermined purpose in response to a predetermined amount of electrical power . the load can be operated from either an ac or dc source using a two - wire connection . the predetermined amount of electrical power is applied to the load if a sensor senses a triggering event . during the time period when a triggering event is sensed , power is cycled on and off to the load in order to enable continuous monitoring of the trigger event . during most of the time period , the sensor receives power to maintain its operations from a storage circuit . if the charge in a capacitor decreases to a predetermined value , the predetermined amount of electrical power is removed from the load momentarily to allow replenishment of the capacitor charge .