A controller for controlling a voltage applied to a load from a power source includes a square wave producing circuit for connection in the circuit with the battery and the lamp. The square wave producing circuit produces a variable duty cycle square wave for controlling application of power to the load in accordance with the duty cycle of the square wave. The square wave producing circuit has a control input for varying the duty cycle of the square wave in response to a voltage at the control input. A voltage varying circuit is connected to the power source for producing a selectively variable voltage that is fed to the control input of the square wave producing circuit for selectively adjusting the duty cycle of the square wave. A low voltage detection circuit is connected to the power source and coupled to the control input of the square wave producing circuit for automatically producing a voltage that decreases the duty cycle of the square wave when the battery voltage decreases to or below a selected level. The selected level may be set according to a value of at least one impedance within the low voltage detection circuit.