Page 1009 - Basic Electrical Engineering

called light dependent resistors (LDRs). Widely used optoelectronic devices
are discussed in this section.

Figure 14.46 (a) Symbol of an LDR; (b) constructional details of an LDR

14.13.1 Light-dependent Resistor

An LDR is a semiconductor device whose resistance varies with the amount

of light falling on it. Light energy falling on its surface provides sufficient
energy to the valence electrons to break away from their atoms. Thus, these

charge carriers, i.e., free electrons and holes make the material more
conductive. A material becoming more conductive means its resistance is

getting reduced. That is why these resistors are also called photoresistors, i.e.,
light-dependent resistors.

Cadmium sulfide and cadmium selenide are used in making LDRs. The
material in the form of a long strip is arranged in a zigzag form on a base. A
glass or plastic cover is provided for protection. The symbol and the

constructional arrangement of an LDR have been shown in Fig. 14.46.
The response time of these photo-sensitive material is quite small, of the

order of few milliseconds. The variation of resistance with falling light on
these materials is quite high. For example, in the absence of light, i.e., in

darkness, the resistance could be 100 kΩ, while in bright sunlight while the
resistance may change to 10 to 20 kΩ.

The principle of operation of a street light circuit using an LDR has been
illustrated in Fig. 14.47. In sunlight, the streat lights should be off. Under