1. For combination with existing A.C. electrical wiring of a building, which wiring includes a wall switch box that has an A.C. power lead that is connected with a source of electrical power and a controlled lead that is connected with wiring that runs from the switch box through walls of the building to a light fixture further wiring in the walls of the building connects the light fixture with ground; a differential illumination switching circuit comprising: a transformer having a primary winding connecting across the power and controlled leads; a triac connected in parallel with the primary winding across the power and control leads, whereby the light fixture is connected between the parallel connected triac and primary winding and ground; a D.C. power supply circuit connected with a secondary winding of the transformer for providing D.C. electric power, the D.C. power supply circuit having a high potential output and a low potential output; a first light sensitive solid state device operatively connected between the high potential output and a bridge junction; a first resistor operatively connected in parallel with the first light sensitive device between the high potential output and the bridge junction; a second light sensitive solid state device operatively connected between the bridge junction and the low potential output; a timing means for producing a timer output signal of a selected duration on a timer output in response to receiving a low potential signal on a trigger input, the trigger input being operatively connected with the bridge junction; an electrical isolation means operatively connected with the timing means to be gated to a conductive state by the timer output signal, the isolation means being operatively connected between the power lead and a gate of the triac, such that the triac oscillates between conductive and non-conductive states with each cycle of A.C. power, whereby (i) during one portion of each A.C. cycle, the triac shorts the primary winding and conducts A.C. power to the light fixture and (ii) during another portion of each A.C. cycle, the triac is non-conductive permitting the A.C. power to flow through the primary winding to provide electric power to the D.C. power supply circuit, the first and second light sensitive solid state devices, and the timing means.