Investigatory Project on LDR

Name

:- Himanshu

Class

:-12

Section

:- A

Roll No

:-

th

Aim:- To study the variations, in current flowing in a circuit containing a LDR.

I would like to thank my Physics teacher Mr. Surender Puli for his constant guidance, and motivation. I would also like to thank Mrs. Neeraj Taneja for her valuable advice. I also want to thank the principal, Mr. John David Shikle, and the administrative head, Captain Vijay Tiwari, for providing the necessary materials. I would also like to extend my gratitude towards the lab attendant, my parents and everyone who has helped me in completing the project successfully.

Himanshu Vidyagyan School

CERTIFICATE     

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A photoresistor or light dependent resistor is a component that is sensitive to light. When light falls upon it then the resistance changes. Values of the resistance of the LDR may change over many orders of magnitude the value of the resistance falling as the level of light increases. It is not uncommon for the values of resistance of an LDR or photoresistor to be several megohms in darkness and then to fall to a few hundred ohms in bright light. With such a wide variation in resistance, LDRs are easy to use and there are many LDR circuits available. The sensitivity of light dependent resistors or photoresistors also varies with the wavelength of the incident light. LDRs are made from semiconductor materials to enable them to have their light sensitive properties. Many materials can be used, but one popular material for these photoresistors is cadmium sulphide, CdS.

Types of photoresistor

Light dependent resistors, LDRs or photoresistors fall into one of two types or categories: Intrinsic photoresistors: Intrinsic photoresistors use un-doped semiconductor materials including silicon or germanium. Photons fall on the LDR excite electrons moving them from the valence band to the conduction band. As a result, these electrons are free to conduct electricity. The more light that falls on the device, the more electrons are liberated and the greater the level of conductivity, and this results in a lower level of resistance. Extrinsic photoresistors: Extrinsic photoresistors are manufactured from semiconductor of materials doped with impurities. These impurities or dopants create a new energy band above the existing valence band. As a result, electrons need less energy to transfer to the conduction band because of the smaller energy gap. Regardless of the type of light dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of incident light.

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LDR symbol

The LDR symbol used in circuits is based around the resistor circuit symbol, but shows the light, in the form of arrows shining on it. In this way it follows the same convention used for photodiode and phototransistor circuit symbols where arrows are used to show the light falling on these components.

Circuit symbols used for the light dependent resistor / photoresistor The light dependent resistor / photoresistor circuit symbols are shown for both the newer style resistor symbol, i.e. a rectagular box and the older zig-zag line resistor circuit symbols.

Lead sulfide (PbS) and indium antimonide (InSb) LDRs are used for the mid infrared spectral region. GeCu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.

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*LDR has a disadvantage that when its temperature changes, its resistance changes drastically for a particular light intensity.

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AIM: To study the variations, in current flowing in a circuit containing a LDR, because of a variation:(a) In the power of the incandescent lamp, used to ‘illuminate’ the LDR. (Keeping all the lamps at a fixed distance). (b) In the distance of a incandescent lamp, (of fixed power), used to ‘illuminate’ the LDR.

APPARATUS: Light Dependent Resistor (LDR) Connecting Wires Source of different power rating (bulbs) Bulb Holder Metre scale Multi Meter   Battery       

1.) When light is incident on it, a photon is absorbed and thereby it excites an electron from valence band into conduction band. Due to such new electrons coming up in conduction band area, the electrical resistance of the device decreases. Thus the LDR or photo-conductive transducer has the resistance which is the inverse function of radiation intensity.

λ0 = threshold wavelength, in meters e = charge on one electron, in Coulombs Ew = work function of the metal used, in Ev

Here we must note that any radiation with wavelength greater than the value obtained in above equation CANNOT PRODUCE any change in the resistance of this device. The band b and gap energy energ y of Cadmium Sulphide is 2.42eV and for Cadmium Selenide it is1.74eV. Due to such large energy gaps, both the materials have extremely high resistivity at room temperature such large energy gaps, both the materials have extremely high resistivity at room temperature.

Now when the device is kept in darkness, its resistance is called as dark 13

resistance. This resistance is typically of the order of 10   ohms. When light falls on it, its resistance decreases up to several kilo ohms or even hundreds of ohms, depending on the intensity of light, falling on it. The spectral response characteristics of two commercial cells were compared in our laboratory. And we found that there is almost no response to the radiation of a wavelength which was shorter than 300nm. It was very 

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interesting to note that the Cadmium Sulphide cell has a peak response nearer or within the green color of the spectrum within a range of 520nm. Thus it can be used nearer to the infra-red region up to 750nm. It was found that the maximum response of Cadmium Sulphoselenide is in the yellow-orange range at 615nm and also it can be used in the infra-red region up to about 970nm.

The sensitivity of a photo detector is the relationship between the light falling on the device and the resulting r esulting output signal. In the case of a photocell, one is dealing with the relationship between the incident light and the corresponding resistance of the cell.

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Like the human eye, the relative sensitivity of a photoconductive cell is dependent on the wavelength (color) of the incident light. Each photoconductor material type has its own unique spectral response curve or plot of the relative response of the photocell versus wavelength of light.

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Choose a specific position position for the source and mount mount it using a holder, make sure it is stable.   Select the bulb with the lowest power rating and connect it to the holder as shown in the figure. Connect the LDR, battery(6V) and the multimeter in series. Set the multimeter to ohm section and select suitable range and measure the resistance with a bulb on.   Similarly switch to current section and move to micro ampere in the multimeter. This gives the value of the current. Repeat these steps with different power sources at different distances and note down observations.

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The experiment has been conducted by using various sources with different power ratings. Voltage of the battery = 6 V

1.) 15 watts (yellow) (wavelength = 570nm) Serial No

DISTANCE FROM SOURCE (cm)

RESISTANCE (Kilo ohm)

CURRENT (micro ampere)

1.

50

142.5

40

2.

40

69

80

3.

30

41

150

4.

20

21

300

2.) 15 watts (incandescent) (mean wavelength = 610nm Serial No

DISTANCE FROM SOURCE (cm)

RESISTANCE (Kilo ohm)

CURRENT (micro ampere)

1.

50

51

120

2.

40

35

170

3.

30

22

270

4.

20

11

540

8

3.) 40 watts (incandescent) (mean wavelength = 610nm) Serial No

DISTANCE FROM SOURCE (cm)

RESISTANCE (Kilo ohm)

CURRENT (micro ampere)

1.

50

20

300

2.

40

13

460

3.

30

8.5

700

4.

20

4.5

1330

4.) 20 watts (CFL) (white light) Serial No

DISTANCE FROM SOURCE (cm)

RESISTANCE (Kilo ohm)

CURRENT (micro ampere)

1.

50

15.5

380

2.

40

10

600

3.

30

6

1000

4.

20

3

2000

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The LDR resistance resistance decreases with increase in intensity intensity of light and hence there is an increase in the flow of current. There is an increase in the current as the distance from the source decreases. The intensity decreases decreases as the distance from the the source increases.

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The LDR may not be perpendicular perpendicular to the source.

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Connections may be faulty.

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The experiment experiment should be conducted in a dark room.

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Measurements should be be taken taken accurately. accurately.

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NCERT Physics www.technologystudent.com/elec1/ ldr 1.htm 1.htm

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www.electrical4u.com/light-dependent-resistor- ldr -working-principle-of-working-principle-of- ldr /

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https://www.electronics-notes.com/articles/electronic_components/resistors/lightdependent-resistor-ldr.php

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