Infra-red light switch

INFRA-RED LIGHT SWITCH Anyone who has ever fried to switch the light on when entering a dark room with both hands full will appreciate this light switch. It operates automatically upon being triggered by your body heat The idea behind this infra-red con- trolled light switch is quite simple. The body heat - which occupies the electromagnetic spectrum between light and radio waves, i.e. 0.74 to 300 pm - is picked up by a sensitive Fresnel lens. This lens, which has at its focus a double differential pyroelectric sensor, IR, , is largely unaffected by other electrical radi- ation. The area served by the light switch is divided into a number of zones as illustrated in Fig. 1. When someone moves from one zone into another, there is a change of tem- perature, which is collected by the lens as a variation in electromagnetic energy. The sensor reacts to this change by generating a small elec- tric signal. That signal is processed and used to operate the light switch. Instead of a Fre...