2dix-The Student Choice
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To study& plot the characteristic of a light emitting diode (LED), LDR, photovoltaic cell, opto-coupler, Photo Diode, Photo Transistor.





Aim- To study& plot the characteristic of a light emitting diode (LED).


  • LED (Red, Green, Yellow).
  • Power Supply (10v Variable).
  • Resisters (1kΩ, 47kΩ).




Light Emitting Diode (LED)

The light emitting diode is an opto electronic device .when forward gased, this diode emits light if a particular colour depending upon the band GAP,LED are made of gallium arsenide (Ga AS),gallium arsenide phosphide  (Ga AS P)&gallium phosphide (Ga P).this materialare doped to get N-type & p-type material. When excited the electrons jump to higher energy levels there by giving off energy in the of radiation.

The different material radiate the following colours of light.

  • Ga As- infrared radiation(invisible)
  • Ga As P-red or yellow (depending upon ratio of ingredients)
  • Ga P- red or green light.

The reverse break down voltage of LED is very low & is around 3v for red colour LEDS therefore we will study only the forward characteristics of a red LED.




  • Connect the power supply, voltmeter, current meter with the LED as shown in the circuit diagram.
  • Increase voltage from the power supply from 0V to 20V in step as shown in the observation table
  • Measure voltage across LED and current through the LED. Note down readings in the observation table and also observes light coming from LED.
  • Reverse DC power supply polarity for reverse bias
  • Repeat the above procedure for the different values of supply voltage for reverse bias
  • Draw VI characteristics for forward bias and reverse bias in one graph


  • Photon energy is related to the emitted frequency by the LED.
  • Photon energy hc hf E
  • (because ) wavelength light of Velocity c f
  • Where h = Plank’s constant = 6.625×10-34 J-S
  • c = Velocity of the light = 3×108 m/s



Circuit diagram (forward bias)

Fig6.1a forward bias




Circuit diagram (reverse bias):

FIG.6.2a Reverse Bias







Observation table:(Forward Bias)

Observation table: (Reverse bias)


1. Do not cover the lamp with paper or fabric, or place it close to flammable objects while lit up.

2.The heated bulb will pose a fire hazard.

3.Do not touch the bulb with your bare hands and avoid direct contact with the skin.





From the graph the relation between LED’s voltage & current is observed.

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












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




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




Fig.6.1b LDR





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.  



  • 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: ___________________________






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.


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

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


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







AIM-  To study the operations of photovoltaic cell.



Photovoltaic cell is a light sensitive device. They produce a voltage when illuminated voltage increases as the intensity of light falling on the semiconductor junction of this cell increases. These are made of silicon or selenium

Photovoltaic cells converts light into electric energy. Photovoltaic cell consists cell of a single semiconductor crystal, which has been doped with both N and P type materials. When light falls on the PN junction which is boundary of these depends a voltage appear across the junction about 0.6 v is developed by the  photovoltaic cell is the  bright sunlight. The amount of power the cell can deliver, depends on the extent of its active surface. These may be used directly to

trigger sensitive relay or where required there output may be amplified electronically.




Fig.6.1cPhotovoltaic Cell








  • Assemble the circuit as shown in fig. check for  circuit assembled.


  • Connected input of table lamp with a for variac for varying the voltage of lamp.Place the ETB under a table lamp so that the light falls directly on the furnace of the photovoltaic cell fitted on the panel switch on the table lamp.


  • Now change the intensity of light falling on the pc by changing lamp voltage with help of variac and note down the corresponding value of current.Plot different sets of reading on a graph paper. The graph will be approx. similar to as shown in fig.


  • It will be observed that the output current in the photo voltaic cell increases as intensity of light increases i.e. the voltage generated by pc increases.



  • There should not be any loose connection.
  • Meter readings should not  be  exceeded beyond their readings
  • Switch off  the circuit when not in use.




RESULT-   From the above experiment we have studied the operations and characteristics of    photovoltaic cell.


    Experiment No. 6(d)

AIM: To study the operations of an opto-coupler.


An optically coupled isolator is a photo emissive which is optically coupled to a photo electric device both contain in the same encapsulation the device provide electrical isolation b/w the two circuits and typically provide a band width from DC to about 100 KHz. Earlier type Contained either a near lamp or a tungsten filament lamp together with a photo conductive cell.


                                    Fig.6.1d Circuit dig of Optocoupler



  • Assemble the circuit as shown is fig. check for the circuit assemble, & switch on the supply to the ETB.
  • Set V2 to 10 V and very V. from 0v  & note down the value of current in the both circuits.
  • It will be observed that the output current in the photo transistor vary in the input current to the LED Part.


Observation table-



Voltage(in volts)




















  • There should not be any loose connection.
  • Meter readings should not  be  exceeded beyond their readings
  • Switch off  the circuit when not in use.




RESULT-  we have studied and performed the operations of an opto coupler 





AIM- To study the operation of  Photo Diode.



  • Infrared LED (Tx)
  • Photo Diode(Rx)
  • Power Supply (10V variable & 10Vor 5V fixed)
  • Digital Multimeter (DMM)
  • Resisters (1kΩ, 47kΩ)





A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode. A small amount of current is also produced when no light is present. Photodiodes may contain optical filters, built-in lenses, and may have large or small surface areas.The common, traditional solar cell used to generate electric

solar power is a large area photodiode. Many diodes designed for use specifically as a

photodiode use a PIN junction rather than a p–n junction,

to increase the speed of response. A photodiode is designed to operate in reverse bias.[1]



                                                Fig.6.1 Symbol of Photo diode


When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up.


Experiment Procedure-

  • Connect the power supply, voltmeter, current meter with the photodiode as shown in the figure
  • Apply 10V from the DC power supply
  • Increase AC power given to lamp to increase intensity
  • Measure reverse leakage current (photo current) of photodiode for different light intensity
  • Draw graph of light intensity versus photocurrent.


Circuit Diagram-

                                                Fig.6.2 e Circuit dig. Of Photo diode

Observation Table-




Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark.




From the graph the relation between LED’s voltage & current is observed.

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



Precaution- Although photodiodes in packages or in chip form are reasonably rugged, avoid touching the cell or package window with bare fingers.















AIM-  To study the operation of Photo Transistor.


  • Infrared LED (Tx)
  • Photo Diode (Rx)
  • Transistor (TR 3904)
  • Power Supply (10V variable & 10Vor 5V fixed)
  • Digital MultiMater (DMM)
  • Resisters (1kΩ, 47kΩ, 470kΩ)


A phototransistor is a device that converts light energy into electric energy. Phototransistors are similar to photoresistors but produce both current and voltage, while photoresistors only produce current. This is because a phototransistor is made of a bipolar semiconductor and focuses the energy that is passed through it. Photons (light particles) activate phototransistors and are used in virtually all electronic devices that depend on light in some way.


How a Phototransistor Works

A phototransistor is a bipolar device that is completely made of silicon or another semi-conductive material and is dependent on light energy. Phototransistors are generally encased in an opaque or clear container in order to enhance light as it travels through it and allow the light to reach the phototransistor’s sensitive parts. A phototransistor generally has an exposed base that amplifies the light that it comes in contact with. This causes a relatively high current to pass through the phototransistor. As the current spreads from the base to the emitter, the current is concentrated and converted into voltage


Phototransistor structure-

A photo transistor can be made to respond to light radiations and work as a sensor. A transistor is like a valve that regulates the amount of electric current that passes through two of its three terminals. The third terminal controls just how much current passes through the other two.

Depending on the type of transistor, the current flow can be controlled by voltage, current, or in thecase of the phototransistor, by light.The drawing below shows the schematic and part drawing of the phototransistor in your Robotics Shield Kit. The brightness of the light shining on the phototransistor’s base (B) terminal determines how much current it will allow to pass into its collector (C) terminal, and out through its emitter (E) terminal. Brighter light results in more current; less-bright light results in less current.

Early photo transistors used germanium or silicon throughout the device giving a homo-junction structure. The more modern phototransistors use type III-V materials such as gallium arsenide and the like. Heterostructures that use different materials either side of the p-n junction are also popular because they provide a high conversion efficiency. These are generally fabricated using epitaxial growth of materials that have matching lattice structures. These photo transistors generally use a mesa structure. Sometimes a Schottky (metal semiconductor) junction can be used for the collector within a phototransistor, although this practice is less common these days because other structures offer better levels of performance.







Fig6.1f  Device of Photo Transistor



Circuit Diagram-


Fig6.2f Circuit dig of Photo transistor






Phototransistors are used for a wide variety of applications. In fact, phototransistors can be used in any electronic device that senses light. For example, phototransistors are often used in smoke detectors, infrared receivers, and CD players. Phototransistors can also be used in astronomy, night vision, and laser range-finding.

Phototransistors are often used as discrete devices, but they will be found combined with other components in some applications.




  • There should not be any loose connection.
  • Meter readings should not  be  exceeded beyond their readings
  • Switch off  the circuit when not in use.



From the graph the relation between LED’s voltage & current is observed.

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





AIM-   To study the operation of Solar Cell.


A solar cell is an electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electric power. This process requires firstly, a material in which the absorption of light raises an electron to a higher energy state, and secondly, the movement of this higher energy electron from the solar cell into an external circuit. The electron then dissipates its energy in the external circuit and returns to the solar cell.

Fig.6.1f  Cross section of a solar cell.

The basic steps in the operation of a solar cell are:


  • the generation of light-generated carriers;
  • the collection of the light-generated carries to generate a current;
  • the generation of a large voltage across the solar cell; and
  • the dissipation of power in the load and in parasitic resistances.


When sunlight hits the semiconductor, an electron springs up and is attracted toward the type semiconductor. This causes more negatives in the n-type semiconductors and more positives in the p-type, thus generating a higher flow of electricity. This is the photovoltaic emf.


Fig.6.2f  Solar Cell

A solar cell made from a monocrystalline silicon wafer with its contact grid made from busbars (the larger strips) and fingers (the smaller ones) that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Solar cells are the building blocks of photovoltaic modules, otherwise known as solar panels.

The solar cell works in several steps:


  • Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.
  • Electrons are excited from their current molecular/ atomic orbital. Once excited an electron can either dissipate the energy as heat and return to its orbital or travel through the cell until it reaches an electrode. Current flows through the material to cancel the potential and this electricity is captured.
  • An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.


1. Do not attempt to service any portion of the system.

2.  not step on the panels or allow objects to fall on the panels.

Result- Thus, We have studied about the Solar Cell.

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