LDR-Light Dependent Resistors

Electronic optosensors are devices that al­
ter their electrical characteristics, in the
presence of visible or invisible light. The
best known devices of these types are the
light dependent resistor (LDR), the
photodiode and the phototransistor.

How to make/construction of an LDR

Light dependent resistors (LDRs)
are made from cadmium sulphide
containing no or very few free electrons
when not illuminated. Its resistance is
then quite high. When it absorbs light,
electrons are liberated and the
conductivity of the material
increases. Cadmium sulphide (CdS) is,
therefore, a photoconductor. The
approximate relationship between the
resistance and illumination is R = AE~ a
where E is illumination in lux, R is resist­
ance in ohms, A and a are constants. The
value of a depends on the cadium
sulphide used and on the manufacturing
process. Values around 0.7 to 0.9 are
quite common.

Basic structure of an LDR-Light dependent


Device consists of a pair of metal film
contacts separated by a snake-like track
of cadmium sulphide film, designed to
provide the maximum possible contact
area with the two metal films. The
structure is housed in a clear plastic or
resin case, to provide free acess to
external light. Practical LDRs are
available in a variety of sizes and
package styles, the most popular size
having a face diameter of roughly 10

LDRs are sensitive, inexpensive, and
readily available devices. They have
good power and voltage handling
capabilities, similar to those of a
conventional resistor. Their only signifi­
cant defect is that they are fairly low
acting, taking tens or hundreds of milli-
seconds to respond to sudden changes in
light level. Useful practical applications
of LDR include light and dark-activated
switches and alarms, light beam alarms
and reflective smoke alarms etc.

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