3. • light dependent resistors are electronic components that
are used to detect light & change the operation of a circuit
dependent upon the light levels.
• It can be described by a variety of names from light
dependent resistor, LDR, photoresistor, or even photo cell,
photocell or photoconductor.
• It is a device whose resistivity is a function of the incident
electromagnetic radiation. Hence, they are light-sensitive
5. Working Principle of LDR
It works on the principle of
photoconductivity whenever the light
on its photoconductive material falls it
absorbs its energy and the electrons
of that photoconductive material that
is in the valence band get excited and
go to the conduction band and thus
increases the conductivity as per the
increase in light intensity.
• This type of photoresistor is
made with pure semiconductors
without any doping. These are
made of pure semiconductor
materials such as silicon or
germanium. When the incident
light having an adequate amount
of energy falls on this then
electrons gain that energy and
got excited and few of them go
to the conduction band.
• This type of photoresistor uses
the doped semiconductor; this
means some impurities are
mixed with the semiconductor to
make this photoresistor, which
are called dopants.
• These dopants create new
energy bands above the valence
band which is filled with
electrons. Hence this reduces
the bandgap and less energy is
required in exciting them.
9. Construction of ldr
• The LDR is constructed by placing a thin zigzag
shaped strip of photosensitive device upon the
• The main component for the construction of LDR is
cadmium sulphide (CdS), which is used as the
photoconductor and contains no or very few
electrons when not illuminated.
• It is designed in such a way as to provide maximum
possible contact area with the two metal films. The
structure is housed in a clear plastic or resin case, to
provide free access to external light.
10. Advantages of LDR
• Low cost.
• Ease of manufacture.
• Ease of use.
Disadvantages of LDR
• High response time.
• Gives inaccurate result if working temperature changes.
• Alarm clocks
• Burglar alarm circuits
• Light intensity meters
• Street lights
• Automatic shutter control of the
• Brightness control of TV screens
and mobile screens.