Abstract: A control circuit for a light projection system for energizing a spare lamp when the circuit detects failure of a main lamp. A pair of input lines apply an alternating-current voltage source to the circuit, to the main lamp and to a spare lamp which is substantially in parallel connection with the main lamp. A switch is in series connection with the main and spare lamp for selectively energizing one of the lamps. Current sensing means, generally a resistor, in series connection with the switch and the lamps, senses the current through the main lamp thereby dropping a small portion of the voltage source. A pair of opposite polarity voltage references are established by rectifying the voltage dropped across the current sensing resistor and by rectifying the voltage across the input lines of the voltage source.