203979153-Investigatory-Project-On-LDR.doc

Acknowledgement I would like to thank my chemistry teacher Mrs. Amudha for her constant guidance , motivation, moral encouragement and sympathetic attitude towards the success o f this project. I also want to thank the principal and the institution for provi ding the necessary materials. I would also like to extend my gratitude towards t he lab attendant, my parents and everyone who has helped me in completing the pr oject successfully. DRUPAD P i

Bonafide certificate This is to certify that DRUPAD P of class XII has successfully completed the inv estigatory project on ªTO STUDY VARIATION OF CURRENT USING A LDR" under the guidan ce of Mrs. Amudha. This project is absolutely genuine and doesn't not involve in  any kind of plagiarism. This is in partial fulfillment of Physics practical exa mination AISSCE 2013. Department of Physics (AMUDHA) ii

INDEX ACKNOWLEDGEMENT BONAFIDE CERTIFICATE i ii INTRODUCTION APPLICATIONS AIM & APPARATUS THEORY PROCEDURE OBSERVATIONS RESULT &  CONCLUSION SOURCES OF ERROR BIBLIOGRAPHY 01 02 03 04 09 10 12 13 14 iii

INTRODUCTION The general purpose photoconductive cell is also known as LDR ± light dependent re sistor. It is a type of semiconductor and its conductivity changes with proporti onal change in the intensity of light. There are two common types of materials u sed to manufacture the photoconductive cells. They are Cadmium Sulphide (CdS) an d Cadmium Selenide (CdSe). Extrinsic devices have impurities added, which have a  ground state energy closer to the conduction band since the electrons don't hav e as far to jump, lower energy photons (i.e. longer wavelengths and lower freque ncies) are sufficient to trigger the device. Two of its earliest applications we re as part of smoke and fire detection systems and camera light meters. The stru cture is covered with glass sheet to protect it from moisture and dust and allow s only light to fall on it. 1

Applications Lead sulfide (PbS) and indium antimonide (InSb) LDRs are used for the mid infrar ed spectral region. GeCu photoconductors are among the best farinfrared detector s available, and are used for infrared astronomy and infrared spectroscopy. Anal og Applications · Camera Exposure Control · Auto Slide Focus - dual cell · Photocopy achines - density of toner · Colorimetric Test Equipment · Densitometer · Electronic cales - dual cell · Automatic Gain Control ± modulated light source · Automated Rear iew Mirror Digital Applications · Automatic Headlight Dimmer · Night Light Control · il Burner Flame Out · Street Light Control · Position Sensor *LDR has a disadvantage that when its temperature changes, its resistance change s drastically for a particular light intensity. 2

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AIM & APPARATUS 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 `illumin ate' 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: Ligh t Dependent Resistor (LDR) Connecting Wires Source of different power rating (bu lbs) Bulb Holder Metre scale Multi Meter Battery 3

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

l0 = thresho d wave ength, in meters e = charge on one e ectron, in Cou ombs Ew = work function of the meta  used, in Ev Here we must note that any radiation with wave ength greater than the va ue obta ined in above equation CANNOT PRODUCE any change in the resistance of this devic e. The band gap energy of Cadmium Su phide is 2.42eV and for Cadmium Se enide it  is1.74eV. Due to such arge energy gaps, both the materia s have extreme y high  resistivity at room temperature. 4

Characteristics of photoconductive ce s Now when the device is kept in darkness , its resistance is ca ed as dark resistance. This resistance is typica y of t he order of 1013 ohms. When ight fa s on it, its resistance decreases up to se vera  ki o ohms or even hundreds of ohms, depending on the intensity of ight, f a ing on it. The spectra  response characteristics of two commercia  ce s were  compared in our aboratory. And we found that there is a most no response to th e radiation of a wave ength which was shorter than 300nm. It was very interestin g to note that the Cadmium Su phide ce  has a peak response nearer or within th e green co or of the spectrum within a range of 520nm. Thus it can be used neare r to the infra-red region up to 750nm. It was found that the maximum response of  Cadmium Su phose enide is in the ye ow-orange range at 615nm and a so it can b e used in the infra-red region up to about 970nm. 5

Sensitivity The sensitivity of a photo detector is the re ationship between the ight fa ing on the device and the resu ting output signa . In the case of a ph otoce , one is dea ing with the re ationship between the incident ight and the  corresponding resistance of the ce . 6

Spectra  Response Like the human eye, the re ative sensitivity of a photoconduct ive ce  is dependent on the wave ength (co or) of the incident ight. Each phot oconductor materia  type has its own unique spectra  response curve or p ot of t he re ative response of the photoce  versus wave ength of ight. 7

2.) uminous f ux variation: Considering the source to be a point radiating in a  directions; consider a ste radian (or even a simp e sphere), take a sma  e ement dA on the steradian at a distance `r' from the source. It comprises a sma  part of the energy radiated (dEr) . Now, go further to a distance `R' (R>r) from the source, consider the same area e ement dA, it comprises a much sma er part of energy radiated (dER). [dEr > dER] . It varies inverse y as the square of the distance. 8

PROCEDURE      

Choose a specific position for the source and mount it using a ho der, make sure  it is stab e. Se ect the bu b with the owest power rating and connect it to th e ho der as shown in the figure. Connect the LDR, battery(6V) and the mu timeter  in series. Set the mu timeter to ohm section and se ect suitab e range and meas ure the resistance with a bu b on. Simi ar y switch to current section and move to micro ampere in the mu timeter. This gives the va ue of the current. Repeat t hese steps with different power sources at different distances and note down obs ervations. 9

OBSERVATIONS The experiment has been conducted by using various sources with different power ratings. Vo tage of the battery = 6 V 1.) 15 watts (ye ow) (wave ength = 570nm) Seria  No 1. 2. 3. 4. DISTANCE FROM SOURCE (cm) 50 40 30 20 RESISTANCE (Ki o ohm ) 142.5 69 41 21 CURRENT (micro ampere) 40 80 150 300 2.) 15 watts (incandescent) (mean wave ength = 610nm Seria  No 1. 2. 3. 4. DISTANCE FROM SOURCE (cm) 50 40 30 20 RESISTANCE (Ki o ohm ) 51 35 22 11 CURRENT (micro ampere) 120 170 270 540 10

3.) 40 watts (incandescent) (mean wave ength = 610nm) Seria  No DISTANCE FROM SOURCE (cm) 50 40 30 20 RESISTANCE (Ki o ohm) CURRENT (m icro ampere) 1. 2. 3. 4. 20 13 8.5 4.5 300 460 700 1330 4.) 20 watts (CFL) (white ight) Seria  No DISTANCE FROM SOURCE (cm) 50 40 30 20 RESISTANCE (Ki o ohm) CURRENT (m icro ampere) 1. 2. 3. 4. 15.5 10 6 3 380 600 1000 2000 11

CONCLUSION & RESULT The LDR resistance decreases with increase in intensity of ight and hence there  is an increase in the f ow of current. There is an increase in the current as t he distance from the source decreases. The intensity decreases as the distance f rom the source increases The error ies within the experimenta imit.    

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SOURCES OF ERROR The LDR may not be perpendicu ar to the source. Connections may be fau ty. The e xperiment shou d be conducted in a dark room. Measurements shou d be taken accur ate y.    

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BIBLIOGRAPHY NCERT physics c ass XII Art of E ectronics by pau  worowitz www.wikipedia.com/ w ww.e ectronics2000.co.uk/ inks/education-hobby/ www.ece ab.com/     

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