LDR
(Light dependent resistors) are often used in circuits to detect the presence
or the level of light. They can be described by a variety of names:
1. Photoresistor
2. Photocell
3. Photoconductor
Please
note that they are not to be confused with photodiodes or photo-transistor,
which are P-N Junction based devices; LDR are purely resistive with resistance
falling as the level of light increases. These changes in resistance for a particular light level can
be quite large.
LDRs
or photoresistors are a particularly convenient electronics component to use
such as in photographic light meters or even to control when streetlights turn
on.
So what are they made of? And how do they work?
LDR - It's all about
Electrons trapped in a Crystal Lattice
Most
LDR are made of semiconductor materials that have light sensitive properties.
Many materials can be used, but the most popular material for these
photoresistors is cadmium sulphide, CdS.
The
appearance of a typical Photoresistor is as shown below:
There
are two types of LDR or Photoresistors:
1. Intrinsic
photoresistors - Intrinsic photoresistors use un-doped semiconductor materials
including silicon or germanium. Photons fall on the LDR excite electrons moving
them from the valence band to the conduction band. As a result, these electrons
are free to conduct electricity.
2. Extrinsic
photoresistors - Extrinsic
photoresistors are manufactured from semiconductor of materials doped with
impurities. These impurities or dopants create a new energy band above the
existing valence band. As a result, electrons need less energy to transfer to
the conduction band because of the smaller energy gap.
Regardless
of the type of light dependent resistor or photoresistor, both types exhibit an
increase in conductivity or fall in resistance with increasing levels of
incident light.
To understand how they work, it's first necessary to understand that an electrical current is the movement of electrons within a material based on an applied p.d (potential Difference) or Voltage. This allows us to divide material into three classes:
1. Good
conductors - have a large number of free electrons that can drift in a given
direction under the action of a potential difference.
2. Insulators
- high resistance have very few free electrons, and therefore it is hard to
make the them move.
3. Semiconductors
- These are materials that are in-between the properties of Good conductors and
Insulators. They have charge carriers called holes (positive) and electrons
(negative) that only move when a specific voltage is applied in a certain
direction.
3.
As
light falls on the semiconductor, the light photons are absorbed by the
semiconductor lattice. Some of their energy is transferred to the electrons. This gives some of them sufficient energy to
break free from the crystal lattice causing electrons to flow. This results in
a lowering of the resistance of the LDR resistance.
The
relation between Resistance and light intensity is linear, once you plot the
values logarithmically; as more light shines on the LDR semiconductor, the
resistance falls further. This results in more electrons being released from
the crystal lattice of the semiconductor material making up the LRDR, causing
more electricity to flow.
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