The circuit has a detector which signals whether the lamp is emitting light or not. An input voltage monitor sends a signal to a switching device to cut off power to the lamp when the input voltage falls below a first threshold at a time when the detector indicates that the lamp is emitting light. When the input voltage exceeds a second threshold the monitor sends a signal to switch the power to the lamp. When the detector signals that the lamp is off the first threshold is reduced wrt. the level for when the lamp is on and/or the second threshold is reduced wrt. the level for when the lamp is on.
