Abstract: A first phototransistor (20, 20') is mounted on the structure such as a home (12) or a garage (14) to sense the level of ambient illumination. A second phototransistor (22, 22') senses both the level of ambient illumination and illumination from the headlights of an approaching vehicle or the like. The phototransistors are arranged with a pair of resistors (54, 56) in a balanced bridge (50). A central bridge junction (52) assumes a higher potential when both phototransistors are illuminated the same, whether light or dark, and a lower potential when the second phototransistor is illuminated more brightly than the first. In response to the uneven illumination of the phototransistors, a timer (60) produces an output signal for a preselected duration. An optic coupler (90) gates a triac (92) conductive for the preselected duration of the timer.