The Intermatic K4121C - Photo Control and relay provides dusk-to-dawn lighting control. The thermal type controls feature a cadmium sulfide photocell and sonic welded polycarbonate case and lens to seal out moisture. The design utilizes a dual temperature compensating bimetal and a metal film resistor for reliable long life operation over ambient temperature extremes. The thermal type photo controls provide a delay action which eliminates loads switching OFF due to car headlights, lightning, etc. Includes washer gasket and locking nut.