LIGHT DEPENDENT RESISTOR (LDR)

A light dependent resistor  is a light controlled  resistor. As the name suggests, LDR is a type of resistor whose working depends upon the intensity of light falling on it. The resistance of a LDR decreases with increasing incident light intensity and resistance increases with decrease in light intensity. This property of an LDR is known as Photo conductivity. LDR can be USED in light   circuits and dark sensors and some light based switching devices.

                                    

SYMBOL OF LDR

LDR is made of a high resistance semiconductor material normally CdS. In the dark, a LDR can have a resistance as high as a few mega ohms , while in the light, a LDR can have a resistance as low as a few hundred ohms. LDR shown in figure have a resistance of 10K in darkness and 300 OHMS in maximum  light intensity. The resistance of LDR can be checked by using multimeter and change in  light intensity causes instantaneous change in reading of resistance.If incident light on the LDR exceeds a certain Energy , photons are absorbed by the semiconductor  resulting in electrons to jump from valence band to conduction band . The increase in conduction causes corresponding decrease in resistance. The resistance range and sensitivity of the LDR Depends upon the various physical and constructional features .LDR can also be used to sense the colour.

LDR IN VOLTAGE DIVIDER CONFIGURATION

we place 10K resistor along with LDR. as we know that the resistance of LDR changes with light intensity

let resistance of LDR at any time= Rth

then the voltage across LDR at any time in the above circuit is given by

                                Vth = { ( Rth ) / ( Rth + 10k )} X 12

12 v battery is applied so thats why 12 is written above.

IN THE ABOVE FIG the intensity of light falling on the LDR is low so resistance is high, therefore voltage across LDR IS HIGH, Output voltage is 11.9V

The light intensity is further increased and correspondingly the resistance decreases and the voltage across LDR decreases as given by OHMS law V=IR  that is smaller the resistance, lesser the voltage

in this case light is further increased and as a result voltage reduces

APPLICATIONS

LDR can be used in various application. you can do a lot of stuff with LDR specially making light sensors and dark sensors. you can make automatic lightening system and much more.

Stay connected, in my next page i will make some circuits based on LDR