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  1. To Study Variation of Current Using a LDR
  2. Abstract The general purpose photoconductive cell is also known as LDR – light dependent resistor. It is a type of semiconductor and its conductivity changes with proportional change in the intensity of light. There are two common types of materials used to manufacture the photoconductive cells. They are Cadmium Sulphide (CdS) and Cadmium Selenide (CdSe). Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band - since the electrons don't have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device. Two of its earliest applications were as part of smoke and fire detection systems and camera light meters. The structure is covered with glass sheet to protect it from moisture and dust and allows only light to fall on it.
  3. Objective 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.
  4. Theory LDR and its characteristics 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 gap energy 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.
  5. Materials Required: • Light Dependent Resistor (LDR) • Connecting Wires • Source of different power rating (bulbs) • Bulb Holder • Metre scale • Multi Meter • Battery
  6. Procedure: • Choose a specific position for the source and 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
  7. Observations • The experiment has been conducted by using various sources with different power ratings. Voltage of the battery = 6 V
  8. Application :- • Lead sulfide (PbS) and indiumantimonide (InSb) LDRs are used for the mid infrared spectral region. GeCu photoconductors areamong the best farinfrared detectors available, and are used for infrared astronomy and infrared spectroscopy. Analog Applications · Camera Exposure Control · Auto Slide Focus – dual cell · Photocopy Machines – density of toner · Colorimetric Test Equipment · Densitometer · Electronic Scales – dual cell · Automatic Gain Control – modulated lightsource · Automated Rear ViewMirror Digital Applications Automatic Headlight Dimmer · Night Light Control · Oil Burner Flame Out · Street Light Control · PositionSensor
  9. Conclusion • The LDR resistance decreases with increase in 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 as the distance from the source increases • The error lies within the experimental limit. • References • NCERT physics class XII • Art of Electronics by paul worowitz • www.wikipedia.com/ • www.electronics2000.co.uk/links/education-hobby/ • www.ecelab.com/urce