Abstract: In the disclosed apparatus, a photo diode has an electrical impedance representative of the ambient light intensity. An amplifier connects to this photo diode and converts the impedance to an analog signal whose magnitude indicates the ambient light intensity. The analog signal is compared via a first comparator with one reference voltage corresponding to the magnitude of analog signal that is generated by cloudy weather, and via another comparator to a reference voltage corresponding to the magnitude of signal that is generated at dusk. The first comparator operates to energize all window light in cloudy weather; and the second comparator operates to energize parking lot lights and electric signs at dusk. The second comparator also activates a timer which, after a manually selectable delay, de-energizes a uniformly distributed portion of all interior overhead lighting.