Information about LDR Sensor (Light Dependent Resistor)

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Information about LDR Sensor

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Check my  video. How to make light sensor using LDR sensor :

                                                                               

         


Circuit Diagram of Darknes sensor                                                         

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Defination of LDR Sensor

             A Photoresistor is an active component that decreases resistance with respect to receving luminosity on the components sensitive surface .The resistance of a photoresistor decreases with incresase in incident light intensity; in other words, it exhibits Photoconductivity.


Function of LDR Sensor

              A Light Dependent Resistor (LDR) or a photo resistor is function of the incident electromagnetic radiation. Hence, they are light sensitive device . They are also called as photo conductors, photo conductive cell or simply photocells.

Structure and Working of LDR Sensor


                   The snake like track shown below is the Cadmium Sulphide (CdS) film which also passes through the sides. On the top and bottom are metal films which are connected to the terminal leads. 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. As explained above, 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. In the absence of light it is designed to have a high resistance inthe range of megaohms. As soon as light falls on the sensor, the electrons are liberated and the conductivity of the material increases. When the light intensity exceeds a certain frequency, the photons absorbed by the semiconductor give band electrons the energy required to jump into the conduction band. This causes the free electrons or holes to conduct electricity and thus dropping the resistance dramatically (< 1 Kilohm).

The equation to show the relation between resistance and illumination can be written as

                                                                                           R = A.E^a
                 where 
                           E – Illumination (lux)
                                                                                          R – Resistance (Ohms)
                                                                                          A,a – constants

 The value of ‘a’ depends on the CdS used and on the manufacturing process. Values usually range between 0.7 and 0.9.

Advantages

LDR are cheap and are readily available in many sizes and shapes. Practical LDR are available in a variety of sizes and package styles, the most popular size having a face diameter of roughly 10 mm. They need very small power and voltage for its operation.

Disadvantages

Highly inaccurate with a response time of about tens or hundreds of millisecond.                                          




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