Going Green – A Light in the Dark

The European Union’s RoHS (Restriction of Hazardous Substances) Directive came into force a year ago. This directive bans new electrical and electronic equipment containing more than agreed levels of lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants. Companies have spent billions of dollars developing and qualifying alternative materials. Developing lead (Pb)-free products and processes requires a tremendous effort. In some cases, however, finding replacements for hazardous substances has been much easier. The replacement of cadmium-based ambient light sensors with the simple, inexpensive, and readily available silicon-based phototransistors is one such success story. Ambient light sensors are used to detect light in a manner similar to the human eye. They’re commonly found in industrial lighting, consumer electronics, and automotive applications. Such sensors are typically used to turn features on and off or adjust them as a convenience, in order to conserve battery power or to provide added safety. A familiar application is automatically turning on and off street lights and other outdoor lighting. Silicon-based phototransistors like Vishay’s TEMT6000X01 have replaced photoresistors in these applications mainly because they’re not made from hazardous substances like cadmium. In most cases, using a phototransistor (Fig. 1) in place of a photoresistor requires only a change in a resistor value. The figure shows how the photoresistor, S1-CDS, is replaced by the TEMT6000X01. The light-to-current transfer factor of the TEMT6000X01 is about 25% of the 4-mm photoresistor. For the same performance, the value of resistor R1 must be increased approximately four times to 8.2 kW. While it’s unclear if other countries like the United States, China, or India will enact such stringent restrictions as the European Union, it’s clear they’re moving in a that direction. Given that most products today are designed for global markets, and given how easily phototransistors can replace cadmium-sulfide photoresistors, design guidelines now call for a phototransistor in ambient light sensing applications.

Jim Toal is a Sr. Marketing Manager at Vishay Optoelectronics (www.vishay.com)