A light dependent resistor also know as a LDR,
photo resistor, photoconductor or photocell, is a
resistor whose resistance increases or decreases
depending on the amount of light intensity.
The way an LDR works is that they are
made of many semi-conductive materials
with high resistance.
The reason they have a high resistance is
that are very few electrons that are free and
able to move because they are held in a
crystal lattice and are unable to move.
When light falls on the semi conductive
material it absorbs the light photons and the
energy is transferred to the electrons, which
allow them to break free from the crystal
lattice and conduct electricity and lower the
resistance of the LDR.
A light dependent resistor works on the principle of photo
conductivity. Photo conductivity is an optical phenomenon in
which the materials conductivity is increased when light is
absorbed by the material.
When light falls i.e. when the photons fall on the device, the
electrons in the valence band of the semiconductor material
are excited to the conduction band.
These photons in the incident light should have energy
greater than the band gap of the semiconductor material to
make the electrons jump from the valence band to the
Hence when light having enough energy strikes on the
device, more and more electrons are excited to the conduction
band which results in large number of charge carriers.
The result of this process is more and more current starts
flowing throgh the device when the circuit is closed and hence
it is said that the resistance of the device has been decreased.
This is the most common working principle of LDR.
TYPES OF LIGHT DEPENDENT RESISTORS
Light dependent resistors are classified based on
the materials used.
Intrinsic Photo Resistors
These resistors are pure semiconductor devices
like silicon or germanium. When the light falls on
the LDR, then the electrons get excited from the
valence band to the conduction band and number
of charge carriers increases.
Extrinsic Photo Resistors
These devices are doped with impurities and these
impurities creates a new energy bands above the
valence band. These bands are filled with
electrons. Hence this decrease the band gap and
small amount of energy.
Light dependant resistors have many uses, many of the
uses have to do with objects that have to work in certain
levels of light. Some of the uses of the LDR are in
photographic light meters,
streetlights and various alarms,
light burglar alarms,
re alarms and smoke alarms.
Camera Exposure Control
Auto Slide Focus - dual cell
Photocopy Machines - density of toner
Colorimetric Test Equipment
Electronic Scales - dual cell
Automatic Gain Control – modulated light source
Automated Rear View Mirror
Automatic Headlight Dimmer
Night Light Control
Oil Burner Flame Out
Street Light Control
Absence / Presence (beam breaker)
oLDR CONTROLLED TRANSISTOR CIRCUIT
THE CIRCUIT SHOWN ABOVE SHOWS A SIMPLE WAY OF CONSTRUCTING
A CIRCUIT THAT TURNS ON WHEN IT GOES DARK.
IN THIS CIRCUIT THE LDR AND THE OTHER RESISTOR FORM A SIMPLE
'POTENTIAL DIVIDER' CIRCUIT, WHERE THE CENTRE POINT OF THE
POTENTIAL DIVIDER IS FED TO THE BASE OF THE NPN TRANSISTOR.
WHEN THE LIGHT LEVEL DECREASES, THE RESISTANCE OF THE LDR
AS THIS RESISTANCE INCREASES IN RELATION TO THE OTHER
RESISTOR, WHICH HAS A FIXED RESISTANCE, IT CAUSES THE VOLTAGE
DROPPED ACROSS THE LDR TO ALSO INCREASE. WHEN THIS VOLTAGE
IS LARGE ENOUGH (0.7V FOR A TYPICAL NPN TRANSISTOR), IT WILL
CAUSE THE TRANSISTOR TO TURN ON.
THE VALUE OF THE FIXED RESISTOR WILL DEPEND ON THE LDR USED,
THE TRANSISTOR USED AND THE SUPPLY VOLTAGE.