PURPOSE: To surely detect the failure of a discharge lamp by detecting the discharge voltage of the discharge lamp for the low beam of the vehicle, and dispatching the signal of the abnormality when the discharge voltage is continuously higher than the first reference voltage on the high level side, or lower than the second reference voltage on the low level.
CONSTITUTION: Voltage detecting resistances 7a, 7b to detect the discharge voltage Vl of a discharge lamp 1 for the low beam are provided. When the detected discharge voltage Vl is higher than the first reference voltage Vref1 on the high level side, or the discharge voltage VL is lower than the second reference voltage Vref2 on the low level side, the respective positive outputs are generated from a first comparator 8 or a second comparator 9, and when this positive output lasts for the prescribed period of time, the positive output is generated from a timer circuit 11 through an OR circuit 10. A common switch 12a and an auxiliary lamp lighting switch 12c are tuned on by a lighting changeover device 12, and the changeover is made from the discharge lamp for the low beam to a back-up lamp lighting by the auxiliary lamp 14.
TOYAMA KOICHI
KATO KOICHI
