A high-efficiency low-cost electronic controller circuit for powering incandescent lamps which provides constant RMS voltage output from a battery or other varying output DC supply. The controller prolongs lamp life by limiting the cold filament in-rush power thereby reducing filament thermal shock deterioration. The controller further prolongs lamp life by supplying rectified AC power which reduces lamp filament embrittlement. A supply voltage monitor indicates a low-battery condition by flashing the lamp. A battery tester enables determination of the battery voltage which is indicated by pulse-coded modulation of the incandescent lamp brightness. A transistor contained in the controller eliminates the usual high-current mechanical power switch.