Photoresistor – Symbol, Working, Types & Applications

Photoresistor – A Detailed Guide While walking through the streets in the evening, have you ever noticed how the street lights turn on automatically as it starts getting darker?  This automatic switching ON of the street lights are due to the presence of a special type of variable resistor on its circuit. The resistance of this variable resistor depends on the amount of light that falls on it. Such a resistor is called the photo-resistor, and in this article we shall discuss about some aspects of the same. So let’s start! What is a Photoresistor? Photoresistor is the combination of words “photon” (meaning light particles) and “resistor”. True to its name, a photo-resistor is a device or we can say a resistor dependent on the light intensity. For this reason, they are also known as light dependent a.k.a. LDRs. So to define a photo-resistor in a single line we can write it as: “Photoresistor is a variable resistor whose resistance varies inversely with the intensity of light” From our basic knowledge about the relationship between resistivity (ability to resist the flow of electrons) and conductivity (ability to allow the flow of electrons), we know that both are polar opposites of each other. Thus when we say that the resistance decreases when intensity of light increases, it simply implies that the conductance increases with increase in intensity of light falling on the photo-resistor or the LDR, owing to a property called photo-conductivity of the material. Hence these Photoresistors are also known as photoconductive cells or just photocell. The idea of Photoresistor developed when photoconductivity in Selenium was discovered by Willoughby Smith in 1873. Many variants of the photoconductive devices were then made. Photoresistor Symbol In order to represent a Photoresistor in a circuit diagram, the symbol chosen was that would indicate it to be a light dependent device along with the fact that it is a resistor. While mostly the symbol used is shown in figure 2a (two arrows pointing to a resistor), some prefer to encase the resistor in a circle like that shown in figure 2b.   Working principle of a Photoresistor In order to understand the working principle of a Photoresistor, let’s brush up a little about the valence electrons and the free electrons. As we know valence electrons are those found in the outermost shell of an atom.  Hence, these are loosely attached to the nucleus of the atom. This means that only some small amount of energy is needed to pull it out from the outer orbit. Free electrons on the other hand are those which are not attached to the nucleus and hence free to move when an external energy like an electric field is applied. Thus when some energy makes the valence electron pull out from the outer orbit, it acts as a free electron; ready to move whenever an electric field is applied.  The light energy is used to make valence electron a free electron. This very basic principle is used in the Photoresistor. The light that falls on a photoconductive material is absorbed by it which in turn makes lots of free electrons from the valence electrons. The figure below shows a pictorial representation of the same:   As the light energy falling on the photoconductive material increases, number of valence electrons that gain energy and leave the bonding with the nucleus increases. This leads to a large number of valence electrons jump to the conduction band, ready to move with an application of any external force like an electric field. Thus, as the light intensity increases, the number of free electrons...
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