LDR Photoresistor: Sintered Cadmium Sulphide Layer on Glass with Silver Electrodes (Thomsen, RCA 1956)

A light-dependent resistor: a layer of cadmium sulphide crystals that conducts in light and nearly insulates in the dark. Cadmium sulphide, a little cadmium chloride and a trace of copper are painted onto glass and fired at 600 °C. The chloride melts, the sulphide recrystallises into one interlocked layer, and the chloride boils away. Two silver paste electrodes painted on top make the cell. Source: Soren M. Thomsen, US 2,765,385 (RCA, 1956), Example 1 and Fig. 1. Unknown: the silver paste, the glass plate and the coating thickness, linked to unknown heads. Cadmium compounds are poisonous and the firing gives off cadmium chloride vapour. Picture: the patent's Fig. 1.