Differential illumination sensitive switching circuit

A differential illumination sensitive switching system for selectively actuating at least one security light mounted on the exterior of a building to provide illumination adjacent a vehicular approach to the building in response to approaching vehicle headlights or the like, the system comprising:

• a reference light sensitive solid state device which is biased to vary in conductivity with variations in brightness of illumination, the reference light sensitive device being mounted on the exterior of the building such that it senses ambient illumination without sensing significant illumination from approaching headlights, the reference light sensitive solid state device being operatively connected between a source of high electrical potential and a bridge junction such that a level of electrical potential provided to the bridge junction is increased in proportion to the sensed brightness of the ambient illumination;
• a first resistive means operatively connected in parallel with the reference light sensitive device between the high potential source and the bridge junction to conduct the high electrical potential to the bridge junction even in darkness when the reference light sensitive device is non-conductive;
• a headlight sensing light sensitive solid state device which is biased to vary in conductivity with variations in brightness of sensed illumination, the headlight sensing light sensitive device being mounted on the exterior of the building such that it receives both ambient exterior illumination and illumination from the headlights of approaching vehicles, the headlight sensing light sensitive device being operatively connected between the bridge junction and the low potential source such that the electrical potential level of the bridge junction is reduced in proporation to the combined brightness of sensed ambient illumination and approaching headlights;
• the bridge junction assuming a higher potential level when both the reference and headlight sensing light sensitive devices are receiving substantially the same level of ambient illumination and assuming a lower potential level when the headlight sensing light sensitive device is more brightly illuminated than the reference light sensitive device, such that the bridge junction assumes the higher potential level over a wide range of light and dark ambient illumination levels and assumes the lower potential level when the sensors are illuminated at different levels regardless of ambient illumination level;
• a timing means for gating a controlled switch conductive for a selected duration in response to the bridge junction assuming the lower potential, the timing means being operatively connected with the bridge junction and the controlled switch;
• the security light being connected with the controlled switch such that the security light is turned on while the controlled switch is gated conductive by the timing means, whereby in response to the headlights of a vehicle approaching the building in any level of ambient illumination, the security light is turned on just as if the building were occupied by a person watching for approaching vehicles to provide illumination for the selected duration adjacent the approaching vehicle for the security and safety of a vehicle component disembarking therefrom.