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To study the operation of light dependant resistor(LDR).

AIM-  To study the operation of light dependant resistor(LDR).

Apparatus-

  • LED ( Red, Green, Yellow, IR LED)
  • Photo Diode (Rx)
  • Power Supply (10V variable & 10Vor 5V fixed)
  • Digital Multimeter (DMM)
  • Resisters (1kΩ, 47kΩ)
  • LDR

THEORY-                                                                          

Fig.6.1b LDR

Introduction-

Light dependant resistor is device in which resistance depends on intensity of light. Resistance of LDR decreases as light intensity increases. Typical example of LDR is Cadmium Sulfide photo cell. Typical resistance for bright light is 200 Ω and for dark condition it is around 10 MΩ. However, these values depends on size and material used for the construction of LDR. LDR can be used as a light sensor. We can construct automatic street light controller, burglar alarm, automatic brightness controller for television etc. applications using LDR.

 

 An LDR is made of semiconductor material with a high resistance. It has a high resistance because there are very few electrons that are free and able to move - the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance.  

 

Procedure-

  • Connected the circuit.
  • Connect the variable AC supply to the lamp. See as we change AC supply, light intensity changes.
  • Now adjust the lamp in such a way that its maximum light falls on LDR.
  • Vary AC supply which will change the light intensity and record the resistance value in the observation table
  • Draw the AC Voltage versus current flowing through LDR & calculate the dynamic resistance of LDR for each setting of AC voltage.

Circuit Diagram-

 

   

 

                                                FIG.6.2b Circuit dig of  LDR

 

Observation table-

 

Resistance at dark condition: ________________________

Resistance at normal light: ___________________________

 

 

 

Application-

 

Light dependent resistors are a vital component in any electric circuit which is to be turned on and off automatically according to the level of ambient light - for example, solar powered garden lights, and night security lighting.

  • Basic Light Dependent Resistors (LDRs), as shown above, are available from Jaycar and SparkFun for as little as $2.00. More advanced light sensors, like the one above from Modern Devices for US$5.00 and this one from SparkFun for US$6.00 are also available.

Result-

From the graph the relationship between Output voltage &Output current is   observed.

It is -------------------------- (Linear/ Nonlinear).

Precaution-

1.The heated LDR will pose a fire hazard.

2.Do not touch the LDR with your bare hands and avoid direct contact with the skin

Click to Download Doc File

 

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