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Working of light dependent resistor

Summary

By: 
Himanshu Choudhary

This circuit based project explains the principle of operation of LDR (light dependence resistor). The circuit has various applications like shadow alarm, automatic night/morning lamp. 

As the name suggests, LDR is a type of resistor whose working depends upon only on the light falling on it. The resistor behaves as per amount of light and its output directly varies with it. In general, LDR resistance is minimum (ideally zero) when it receives maximum amount of light and goes to maximum (ideally infinite) when there is no light falling on it.

A critical factor that decides LDR’s working is the frequency of light which should cross a threshold value so as to make LDR respond. Keep on reading to find out how the circuit is designed and how it is made to work.

Description

 

LDR, an acronym for light dependent resistor is a resistor whose resistance is dependent on light. In this project, resistance of LDR is of the order of Mega Ohms in the absence of light and reduces to a few ohms in presence of light. In this circuit when the light falls on LDR, the resistance of LDR becomes low and the entire voltage drop takes place across the variable resistance VR1 (10K). As a result the base of transistor (T1) gets high input and it gets biased thereby glowing the LED. When no light falls on LDR, the resistance of LDR becomes high so almost entire voltage drop takes place across it and the base of transistor is at low potential. So transistor does not gets biased nor it becomes conducting, hence switching off the LED. The sensitivity of the circuit can be adjusted by varying the preset VR1.

 

Circuit Diagram

Components

Resistor Image
Resistor

Resistor is a passive component used to control current in a circuit. Its resistance is given by the ratio of voltage applied across its terminals to the current passing through it. Thus a particular value of resistor, for fixed voltage, limits the current through it. They are omnipresent in electronic circuits.

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LEDs | Light Emitting Diode
LED
 
Light emitting diodes (LEDs) are semiconductor light sources. The light emitted...
LDR | Light Dependent Resistor
LDR

An LDR (Light dependent resistor), as its name suggests, offers resistance in response to the ambient light. The resistance decreases as the intensity of incident light increases, and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light. It can act as a sensor, since a varying voltage drop can be obtained...

Presets | Variable Resistors
Preset
 
A preset is a three legged electronic component which can be made to offer varying resistance in a circuit. The resistance is varied by adjusting the rotary control over it. The adjustment can be done by using a small screw driver or...
BC547 | Transistor BC547  | PDF Datasheet
Transistor BC547
BC547 is an NPN bi-polar junction transistor. A transistor, stands for transfer of resistance, is commonly used to amplify current. A small current at its base controls a larger current...
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