A Safe Replacement for Hazardous Photoresistors

Most city streetlights and many home exterior lights automatically turn on and off based on ambient light. Turning on at dusk and off at dawn saves money by reducing electricity consumption. These lights typically use a Photoresistor made from Cadmium Sulfide to detect the ambient light.

Cadmium Sulfide (CdS) is a hazardous substance; extremely dangerous to our health and environment. Photoresistors using CdS must be replaced with safe, silicon based Phototransistors like Vishay's new phototransistor, TEMT6000. It responds much like the human eye to visible light.

Figure 1 – Replacing S1 CdS with TEMT6000

Using the TEMT6000 in place of a photoresistor, in most cases, requires only a change in a resistor value. The schematic below shows just how easy it is to replace the photoresistor with Vishay's TEMT6000. In this application, the LED or light will turn on at dusk and off at dawn. The light-to-current transfer factor of Vishay's TEMT6000 is about 1/4 the 4 mm CdS photoresistor. For the same performance, replacing the photoresistor S1 CDS with TEMT6000 requires that R1 be replaced by a resistor with four times the value, ~8 kΩ.