The general purpose photoconductive cell is also known as LDR – light
dependent resistor. It is a type of semiconductor and its conductivity
changes with proportional change in the intensity of light. There are two
common types of materials used to manufacture the photoconductive cells.
They are Cadmium Sulphide (CdS) and Cadmium Selenide (CdSe). Extrinsic
devices have impurities added, which have a ground state energy closer to
the conduction band - since the electrons don't have as far to jump, lower
energy photons (i.e. longer wavelengths and lower frequencies) are
sufficient to trigger the device. Two of its earliest applications were as part
of smoke and fire detection systems and camera light meters. The structure
is covered with glass sheet to protect it from moisture and dust and allows
only light to fall on it.
To study the variations, in current flowing in a
circuit containing a LDR, because of a variation:-
(a) In the power of the incandescent lamp, used
to ‘illuminate’ the LDR. (Keeping all the lamps at a
(b) In the distance of a incandescent lamp, (of
fixed power), used to ‘illuminate’ the LDR.
LDR and its characteristics When light is incident on it, a photon is absorbed
and thereby it excites an electron from valence band into conduction band.
Due to such new electrons coming up in conduction band area, the electrical
resistance of the device decreases. Thus the LDR or photo-conductive
transducer has the resistance which is the inverse function of radiation
λ0 = threshold wavelength, in meters e = charge on one electron, in Coulombs
Ew = work function of the metal used, in Ev
Here we must note that any radiation with wavelength greater than the value
obtained in above equation CANNOT PRODUCE any change in the resistance of
this device. The band gap energy of Cadmium Sulphide is 2.42eV and for
Cadmium Selenide it is1.74eV. Due to such large energy gaps, both the
materials have extremely high resistivity at room temperature.
• Light Dependent Resistor (LDR)
• Connecting Wires
• Source of different power rating (bulbs)
• Bulb Holder
• Metre scale
• Multi Meter
• Choose a specific position for the source and mount it using a holder, make
sure it is stable.
• Select the bulb with the lowest power rating and connect it to the holder
as shown in the figure.
• Connect the LDR, battery(6V) and the multimeter in series.
• Set the multimeter to ohm section and select suitable range and measure
the resistance with a bulb on.
• Similarly switch to current section and move to micro ampere in the
multimeter. This gives the value of the current.
• Repeat these steps with different power sources at different distances and
note down observations
• Lead sulfide (PbS) and indiumantimonide (InSb) LDRs are used for the mid infrared spectral region. GeCu photoconductors areamong the best farinfrared detectors available,
and are used for infrared astronomy and infrared spectroscopy.
· Camera Exposure Control
· Auto Slide Focus – dual cell
· Photocopy Machines – density of toner
· Colorimetric Test Equipment
· Electronic Scales – dual cell
· Automatic Gain Control – modulated lightsource
· Automated Rear ViewMirror
Automatic Headlight Dimmer
· Night Light Control
· Oil Burner Flame Out
· Street Light Control
• The LDR resistance decreases with increase in intensity of light and hence there is
an increase in the flow of current.
• There is an increase in the current as the distance from the source decreases.
• The intensity decreases as the distance from the source increases
• The error lies within the experimental limit.
• NCERT physics class XII
• Art of Electronics by paul worowitz