Light ferdelity report Li-Fi

1

A

Project Report of

Light Fidelity (Li-Fi)

Submitted

In partial fulfilment

For the award of the Degree of

Bachelor of Technology

In Department of Electronics & Communication Engineering

Submitted To: Submitted By:

Name: Prof. Alok Jha Name of Candidate:

Designation (Dept.): Vimal Kumar (11ECIEC037)

Head of Department Girish Kumar Chandan (11ECIEC017)

Department of Electronics & Communication Engineering

CompuCom Institute of Information Technology & Management

Rajasthan Technical University

May 2015

2

Abstract of Li-Fi Technology:-

Whether you’re using wireless internet in a coffee shop, stealing it from the guy next

door, or competing for bandwidth at a conference, you’ve probably gotten frustrated at the slow

speeds you face when more than one device is tapped into the network. As more and more

people and their many devices access wireless internet, clogged airwaves are going to make it

increasingly difficult to latch onto a reliable signal. But radio waves are just one part of the

spectrum that can carry our data. What if we could use other waves to surf the internet? One

German physicist DR. Harald Haas, has come up with a solution he calls “Data Through

Illumination”—taking the fiber out of fiber optics by sending data through an LED light bulb

that varies in intensity faster than the human eye can follow. It’s the same idea behind infrared

remote controls, but far more powerful. Haas says his invention, which he calls D-Light, can

produce data rates faster than 10 megabits per second, which is speedier than your average

broadband connection. He envisions a future where data for laptops, smartphones, and tablets is

transmitted through the light in a room. And security would be a snap—if you can’t see the light,

you can’t access the data.

Li-Fi is a VLC, visible light communication, technology developed by a team of

scientists including Dr Gordon Povey, Prof. Harald Haas and Dr Mostafa Afgani at the

University of Edinburgh. The term Li-Fi was coined by Prof. Haas when he amazed people by

streaming high-definition video from a standard LED lamp, at TED Global in July 2011. Li-Fi is

now part of the Visible Light Communications (VLC) PAN IEEE 802.15.7 standard. “Li-Fi is

typically implemented using white LED light bulbs. These devices are normally used for

illumination by applying a constant current through the LED. However, by fast and subtle

variations of the current, the optical output can be made to vary at extremely high speeds.

Unseen by the human eye, this variation is used to carry high-speed data,” says Dr Povey,

Product Manager of the University of Edinburgh's Li-Fi Program ‘D-Light Project’.

Introduction of Li-Fi Technology:-

In simple terms, Li-Fi can be thought of as a light-based Wi-Fi. That is, it uses light

instead of radio waves to transmit information. And instead of Wi-Fi modems, Li-Fi would use

transceiver- fitted LED lamps that can light a room as well as transmit and receive information.

Since simple light bulbs are used, there can technically be any number of access points.

This technology uses a part of the electromagnetic spectrum that is still not greatly

utilized- The Visible Spectrum. Light is in fact very much part of our lives for millions and

3

millions of years and does not have any major ill effect. Moreover there is 10,000 times more

space available in this spectrum and just counting on the bulbs in use, it also multiplies to 10,000

times more availability as an infrastructure, globally.

It is possible to encode data in the light by varying the rate at which the LEDs flicker on

and off to give different strings of 1s and 0s. The LED intensity is modulated so rapidly that

human eyes cannot notice, so the output appears constant.

More sophisticated techniques could dramatically increase VLC data rates. Teams at the

University of Oxford and the University of Edinburgh are focusing on parallel data transmission

using arrays of LEDs, where each LED transmits a different data stream. Other groups are using

mixtures of red, green and blue LEDs to alter the light's frequency, with each frequency

encoding a different data channel.

Li-Fi, as it has been dubbed, has already achieved blisteringly high speeds in the lab.

Researchers at the Heinrich Hertz Institute in Berlin, Germany, have reached data rates of over

500 megabytes per second using a standard white-light LED. Haas has set up a spin-off firm to

sell a consumer VLC transmitter that is due for launch next year. It is capable of transmitting

data at 100 MB/s - faster than most UK broadband connections.

Genesis of LI-FI:

Harald Haas, a professor at the University of Edinburgh who began his research in the

field in 2004, gave a debut demonstration of what he called a Li-Fi prototype at the TED Global

conference in Edinburgh on 12th July 2011. He used a table lamp with an LED bulb to transmit a

video of blooming flowers that was then projected onto a screen behind him. During the event he

periodically blocked the light from lamp to prove that the lamp was indeed the source of

incoming data. At TED Global, Haas demonstrated a data rate of transmission of around 10Mbps

-- comparable to a fairly good UK broadband connection. Two months later he achieved

123Mbps.

4

Fig 1.0

Back in 2011 German scientists succeeded in creating an800Mbps (Megabits per second)

capable wireless network by using nothing more than normal red, blue, green and

white LED light bulbs (here), thus the idea has been around for a while and various other global

teams are also exploring the possibilities.

Fig 1.1

How Li-Fi Works?

Li-Fi is typically implemented using white LED light bulbs at the downlink transmitter.

These devices are normally used for illumination only by applying a constant current. However,

by fast and subtle variations of the current, the optical output can be made to vary at extremely

high speeds. This very property of optical current is used in Li-Fi setup. The operational

5

procedure is very simple-, if the LED is on, you transmit a digital 1, if it’s off you transmit a 0.

The LEDs can be switched on and off very quickly, which gives nice opportunities for

transmitting data. Hence all that is required is some LEDs and a controller that code data into

those LEDs. All one has to do is to vary the rate at which the LED’s flicker depending upon the

data we want to encode. Further enhancements can be made in this method, like using an array of

LEDs for parallel data transmission, or using mixtures of red, green and blue LEDs to alter the

light’s frequency with each frequency encoding a different data channel. Such advancements

promise a theoretical speed of 10 Gbps – meaning one can download a full high-definition film

in just 30 seconds.

Fig 1.2

To further get a grasp of Li-Fi consider an IR remote. (Fig 3.3). It sends a single data

stream of bits at the rate of 10,000-20,000 bps. Now replace the IR LED with a Light Box

containing a large LED array. This system, fig 3.4, is capable of sending thousands of such

streams at very fast rate.

6

Fig 1.3

Light is inherently safe and can be used in places where radio frequency communication

is often deemed problematic, such as in aircraft cabins or hospitals. So visible light

communication not only has the potential to solve the problem of lack of spectrum space, but can

also enable novel application. The visible light spectrum is unused, it's not regulated, and can be

used for communication at very high speeds.

7

Technology Brief:-

How LI-FI Light Sources Work:-

Introduction:-

LI-FI is a new class of high intensity light source of solid state design

bringing clean lighting solutions to general and specialty lighting. With energy

efficiency, long useful lifetime, full spectrum and dimming, LI-FI lighting

applications work better compared to conventional approaches. This technology

brief describes the general construction of LI-FI lighting systems and the basic

technology building blocks behind their function.

LI-FI CONSTRUCTION:-

The LIFI™ product consists of 4 primary sub-assemblies:

• Bulb

• RF power amplifier circuit (PA)

• Printed circuit board (PCB)

• Enclosure

The PCB controls the electrical inputs and outputs of the lamp and houses

the microcontroller used to manage different lamp functions.

An RF (radio-frequency) signal is generated by the solid-state PA and is

guided into an electric field about the bulb.

The high concentration of energy in the electric field vaporizes the

contents of the bulb to a plasma state at the bulb’s center; this controlled plasma

generates an intense source of light.

All of these subassemblies are contained in an aluminum enclosure

FUNCTION OF THE BULB:-

At the heart of LIFI™ is the bulb sub-assembly where a sealed bulb is

embedded in a dielectric material. This design is more reliable than conventional

light sources that insert degradable electrodes into the bulb. The dielectric

8

material serves two purposes; first as a waveguide for the RF energy transmitted

by the PA and second as an electric field concentrator that focuses energy in the

bulb. The energy from the electric field rapidly heats the material in the bulb to a

plasma state that emits light of high intensity and full spectrum.

SUMMARY:

-

The design and construction of the LIFI™ light source enable efficiency, long

stable life, and full spectrum intensity that is digitally controlled and easy to use.

9

Fig 2.1

10

Application area of li-fi technology

Airways:-

Fig 2.2

Whenever we travel through airways we face the problem in communication media,

because the whole airways communication are performed on the basis of radio

waves to overcome this drawback on radio ways, li-fi is introduce.

Green information technology:-

Green information technology means that unlike radio waves and other

communication waves effects on the birds, human body’s etc. Li-Fi never gives such

side effects on any living thing.

11

Free From Frequency Bandwidth Problem:-

Li-fi is a communication media in the form of light, so no matter about the

frequency bandwidth problem. It does not require the any bandwidth spectrum i.e.

we don’t need to pay any amount for communication and license.

Increase Communication Safety:-

Due to visual light communication, the node or any terminal attach to our

network is visible to the host of network.

Multi User Communication:-

Li-Fi supports the broadcasting of network, it helps to share multiple thing

at a single instance called broadcasting.

Lightings Points Used as Hotspot:-

Any lightings device is performed as a hotspot it means that the light

device like car lights, ceiling lights, street lamps etc. area able to spread internet

connectivity using visual light communication. Which helps us to low cost

architecture for hotspot.

Hotspot is a limited region in which some amount of device can access the

internet connectivity.

Smarter Power Plants:-

Wi-Fi and many other radiation types are bad for sensitive areas. Like

those surrounding power plants. But power plants need fast, inter-connected data

systems to monitor things like demand, grid integrity and (in nuclear plants) core

temperature. The savings from proper monitoring at a single power plant can add

up to hundreds of thousands of dollars. Li-Fi could offer safe, abundant

connectivity for all areas of these sensitive locations. Not only would this save

money related to currently implemented solutions, but the draw on a power

plant’s own reserves could be lessened if they haven’t yet converted to LED

lighting.

12

Undersea Awesomeness:-

Underwater ROVs, those favourite toys of treasure seekers and James

Cameron, operate from large cables that supply their power and allow them to

receive signals from their pilots above.

ROVs work great, except when the tether isn’t long enough to explore an

area, or when it gets stuck on something. If their wires were cut and replaced with

light say from a submerged, high-powered lamp then they would be much

free to explore. They could also use their headlamps to communicate with each

other, processing data autonomously and referring findings periodically back to the

surface, all the while obtaining their next batch of orders.

13

Components Required:

Component Value Quantity in Pieces

Resistor 10k/15k/82ohm 1p/1p/2p

Capacitor 1000uf/470uF/0.1uF 1p/1p/1p

BC 548 - 1p

LM 741 - 1p

Solar Panel - 1p

Laser Light - 1p

Speaker - 1p

PCB - -

14

Resistor:

Fig 3.1

A resistor is a passive two-terminal electrical component that implements electrical resistance as

a circuit element. Resistors act to reduce current flow, and, at the same time, act to lower voltage

levels within circuits. In electronic circuits resistors are used to limit current flow, to adjust

signal levels, bias active elements, terminate transmission lines among other uses. High-power

resistors that can dissipate many watts of electrical power as heat may be used as part of motor

controls, in power distribution systems, or as test loads for generators. Fixed resistors have

resistances that only change slightly with temperature, time or operating voltage. Variable

resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or

as sensing devices for heat, light, humidity, force, or chemical activity.

Resistors are common elements of electrical networks and electronic circuits and are ubiquitous

in electronic equipment. Practical resistors as discrete components can be composed of various

compounds and forms. Resistors are also implemented within integrated circuits.

Measurement/Value of Resistance in use:

2 pieces of Resistor = 82 ohm

1 pieces of Resistor = 10k ohm

1 pieces of Resistor= 15k ohm

Color Coding In Resistor:

15

Fig 3.2

Capacitor:

Fig 3.3 Fig 3.4

A capacitor (originally known as a condenser) is a passive two-terminal electrical

component used to store energy electrostatically in an electric field. The forms of practical

capacitors vary widely, but all contain at least two electrical conductors (plates) separated by

a dielectric (i.e. insulator). The conductors can be thin films, foils or sintered beads of metal or

conductive electrolyte, etc. The no conducting dielectric acts to increase the capacitor's charge

capacity. A dielectric can be glass, ceramic, plastic film, air, vacuum, paper, mica, oxide layer

etc. Capacitors are widely used as parts of electrical circuits in many common electrical devices.

16

Unlike a resistor, an ideal capacitor does not dissipate energy. Instead, a capacitor

stores energy in the form of an electrostatic field between its plates.

When there is a potential difference across the conductors (e.g., when a capacitor is attached

across a battery), an field develops across the dielectric, causing positive charge +Q to collect on

one plate and negative charge −Q to collect on the other plate. If a battery has been attached to a

capacitor for a sufficient amount of time, no current can flow through the capacitor. However, if

a time-varying voltage is applied across the leads of the capacitor, a displacement current can

flow

Transistor Current Components:-

Fig 4.1

The BC548 is a general purpose NPN bipolar junction transistor found commonly in European

electronic equipment and present-day designs in Australian and British electronics magazines

where a commonly-available low-cost NPN transistor is required. It is a part of a family of NPN

and PNP epitaxial silicon transistors that include higher-quality variants, originating in 1966

when Philips introduced the metal-cased BC108 family of transistors which became the most

used transistors in Australia[1] and taken up by many European manufacturers. The BC548 is the

modern plastic packaged BC108, and can be used in any circuit designed for the BC108 or

BC148, which includes many Muller and Philips published designs.

The BC548 is low cost and is available in most European Union and many other countries. It is

often the first type of bipolar transistor hobbyist’s encounter, and is often featured in designs in

hobby electronics magazines where a general-purpose transistor is required. The part number is

assigned by Pro Electron, which allows many manufacturers to offer electrically and physically

interchangeable parts under one identification. As viewed in the image to the right, and going

from left to right, lead 1 (left in diagram) is the collector, lead 2 is the base, and lead 3 is the

emitter.

17

LM 741 IC (Operational Amplifier):

Fig 5.1

An operational amplifier ("op-amp") is a DC-coupled high-gain electronic

voltage amplifier with a differential input and, usually, a single-ended output. In this

configuration, an op-amp produces an output potential (relative to circuit ground) that is typically

hundreds of thousands of times larger than the potential difference between its input terminals.

Operational amplifiers had their origins in analog computers, where they were used to do

mathematical operations in many linear, non-linear and frequency-dependent circuits. The

popularity of the op-amp as a building block in analog circuits is due to its versatility. Due

to negative feedback, the characteristics of an op-amp circuit, its gain, input and output

impedance, bandwidth etc. are determined by external components and have little dependence on

temperature coefficients or manufacturing variations in the op-amp itself.

Op-amps are among the most widely used electronic devices today, being used in a vast array of

consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few cents in

moderate production volume; however some integrated or hybrid operational amplifiers with

special performance specifications may cost over $100 US in small quantities.[3] Op-amps may

be packaged as components, or used as elements of more complex integrated circuits.

The op-amp is one type of differential amplifier. Other types of differential amplifier include

the fully differential amplifier (similar to the op-amp, but with two outputs), the instrumentation

amplifier (usually built from three op-amps), the isolation amplifier (similar to the

18

instrumentation amplifier, but with tolerance to common-mode voltages that would destroy an

ordinary op-amp), and negative feedback amplifier (usually built from one or more op-amps and

a resistive feedback network).

The amplifier's differential inputs consist of a non-inverting input (+) with voltage V+ and an

inverting input (–) with voltage V−; ideally the op-amp amplifies only the difference in voltage

between the two, which is called the differential input voltage. The output voltage of the op-

amp Vought is given by the equation:

Where AOL is the open loop gain of the amplifier (the term "open-loop" refers to the absence

of a feedback loop from the output to the input).

Open loop amplifier:

The magnitude of AOL is typically very large—100,000 or more for integrated circuit op-

amps—and therefore even a quite small difference between V+ and V− drives the amplifier

output nearly to the supply voltage. Situations in which the output voltage is equal to or

greater than the supply voltage are referred to as saturation of the amplifier. The magnitude

of AOL is not well controlled by the manufacturing process, and so it is impractical to use an

operational amplifier as a stand-alone differential amplifier.

Without negative feedback, and perhaps with positive feedback for regeneration, an op-amp

acts as a comparator. If the inverting input is held at ground (0 V) directly or by a resistor

Rag, and the input voltage VIN applied to the non-inverting input is positive, the output will

be maximum positive; if VIN is negative, the output will be maximum negative. Since there is

no feedback from the output to either input, this is an open loop circuit acting as

a comparator.

Closed loop:

Fig 5.1

19

If predictable operation is desired, negative feedback is used, by applying a portion of the

output voltage to the inverting input. The closed loop feedback greatly reduces the gain of

the circuit. When negative feedback is used, the circuit's overall gain and response becomes

determined mostly by the feedback network, rather than by the op-amp characteristics. If the

feedback network is made of components with values small relative to the op amp's input

impedance, the value of the op-amp's open loop response AOL does not seriously affect the

circuit's performance. The response of the op-amp circuit with its input, output, and feedback

circuits to an input is characterized mathematically by a transfer function; designing an op-

amp circuit to have a desired transfer function is in the realm of electrical engineering. The

transfer functions are important in most applications of op-amps, such as in analog

computers. High input impedance at the input terminals and low output impedance at the

output terminal(s) are particularly useful features of an op-amp.

In the non-inverting amplifier on the right, the presence of negative feedback via the voltage

divider Rf, Rg determines the closed-loop gain ACL = Vout / Vin. Equilibrium will be

established when Vout is just sufficient to "reach around and pull" the inverting input to the

same voltage as Vin. The voltage gain of the entire circuit is thus 1 + Rf/Rg. As a simple

example, if Vin = 1 V and Rf = Rg, Vout will be 2 V, exactly the amount required to keep V− at

1 V. Because of the feedback provided by the Rf, Rg network, this is a closed loop circuit.

Another way to analyze this circuit proceeds by making the following (usually valid)

assumptions:[4]

When an op-amp operates in linear (i.e., not saturated) mode, the difference in voltage

between the non-inverting (+) pin and the inverting (−) pin is negligibly small.

The input impedance between (+) and (−) pins is much larger than other resistances in the

circuit.

The input signal Vin appears at both (+) and (−) pins, resulting in a current I through Rg equal

to Vin/Rg.

Since Kirchhoff's current law states that the same current must leave a node as enter it,

and since the impedance into the (−) pin is near infinity, we can assume practically all of

the same current I flows through Rf, creating an output voltage

By combining terms, we determine the closed-loop gain ACL:

20

Pin Description of LM 741:

Fig 6.1

21

Solar Panel:

Fig 7.1

A solar cell is an electronic device that produces electricity when light falls on it. The light is

absorbed and the cell produces dc voltage and current. The device has a positive and a negative

contact between which the voltage is generated and through which the current can flow. You

connect these contacts to whatever it is you want to power. Solar cells have no moving parts.

Effectively they take light energy and convert it into electrical energy in an electrical circuit,

exploiting a physical process known as the photovoltaic effect.

The discovery of the photovoltaic effect is credited to the French physicist, Edmond Becquerel,

in 1839. He found that by concentrating the sun's light on one side of a battery the output current

of the battery could be increased. This revolutionary discovery triggered the idea that one could

produce energy from light by an artificial process. In 1883 an American inventor produced a

solar cell from a material called selenium, but it was very inefficient. Selenium became used in

light-exposure meters for cameras, but not for power production.

It was not until the 1950s that practical solar cells were developed. In 1948 the transistor was

invented, at Bell Laboratories in the United States, and it was found that the same high quality

silicon wafers used for making transistors could be used to make solar cells. This work was

published in 1954. From 1958 onwards the cells were employed in the space race. Solar cells

are still the only sensible source of electrical power for space satellites, because they are in effect

batteries that never run out.

Initially solar cells were too expensive to be used in non-space (i.e. terrestrial) applications,

though Bell Telephone did demonstrate them for rural telephone systems. They are a good idea

for country areas that have no electricity supply network, of which there are many in the

Developing World, and for maritime applications (e.g. to power flashing lights on buoys). If

cells can be made cheap enough (and great efforts are being made to achieve this) they could

even replace our normal methods of making electricity, which are either polluting and/or non-

renewable (burning fossil fuels) or waste poses a long term environmental hazard (radioactive

22

waste from nuclear power plants). Solar cells produce no emissions and do not contribute to the

greenhouse effect, and the amount of energy available from the world's sunlight is far more than

we should ever need.

Individual solar cells are small and therefore not very powerful (though they can run calculators

and watches). More powerful supplies can be made by connecting many cells together in a solar

module. Modules are connected together to form solar panels, and in turn panels are connected

together to form solar arrays.

Efficiency

The efficiency of a solar cell is a measure of the proportion of the light hitting it that is actually

converted into electricity. If the cell were 100% efficient then it would turn all the incident light

into energy, but sadly this is impossible: the maximum allowed within the laws of physics is

between 30% and 40%. Practical solar cells made from silicon wafers (monocrystalline silicon)

can have an efficiency of 16% or so. Thin-film solar cells (e.g. amorphous silicon solar cells)

have lower efficiencies than this, at least for commercial cells, but are much cheaper to produce.

Around mid-day on a clear summer's day the sunlight falling on the earth has a power density of

about 1 kW (1000 watts) for every square meter of surface; (this is typically the power given off

by a one-bar electric fire). A solar module measuring 0.30 m × 0.45 m has an area of 0.135 m²,

and therefore when you point it at the sun the light falling on it has a power of 0.135 × 1000

watts = 135 watts. If the module is 10% efficient, the power available from it is 10% of this, i.e.

13.5 watts. The module is stated to have an output of 13.5 watts peak, i.e. at the peak sunlight of

1000 watts per square meter. The output will be less at other times of the day, in cloudy

conditions, or if the module is in the shade or not pointing directly at the sun.

In space the output is higher because the solar radiation there is stronger, not being affected by

the earth’s atmosphere. It has a power density of 1365 watts per square meter.

How does the light intensity effect the solar cell?

As the intensity of light falls, because of clouds or time of day, solar cell output also falls. The

cell's current is more sensitive to the light intensity than the voltage is. Roughly speaking if you

halve the light intensity you halve the current; but the voltage falls only slightly.

The light intensity can also be reduced just by twisting the cell. The output of a solar cell is at its

maximum when it is perpendicular to the incident light beam, i.e. when it is pointed at the sun.

If you now change the angle, the cell intercepts less of the light beam; however, this smaller

amount of light is still spread out over the same area of cell, so the light intensity on the cell is

reduced.

23

INSIDE A SOLAR PANEL - HOW DOES IT WORK?

Photons

Photons are what make up the light we see. Light is an electromagnetic wave that is transmitted

in tiny pulses of energy. These tiny pulses of energy are referred to as photons.

Semiconductors

All substances can be arranged in order of their ability to conduct electrical charges. Those at the

top of the list are called conductors, and those at the bottom are called insulators. Whether a

substance is classified as a conductor or an insulator depends on its interatomic bonding and on

how tightly the atoms of the substance hold their electrons. The interatomic bonding in some

materials, such as silicon, is intermediate between that of a good conductor and that of a good

insulator.

Fig 7.2

Silicon and germanium belong to group of materials called semiconductors. They are good

insulators in their pure crystalline form at very low temperature. Conductivity increases with

temperature or when they are exposed to light Conductivity can be increased tremendously when

even one atom in ten million is replaced with an impurity that adds or removes an electron from

the crystal structure. The chips used in electronics are made of semiconductor materials, and so

are photovoltaic cells. The most common semiconductor is silicon. Semiconductor materials

will also interact with light (see Figure 1). A photon hitting a silicon atom can give an electron

within the atom enough energy to leave it and move off through the structure. The negatively

charged electron leaves a positively charged hole (a position once occupied by an electron) in its

place; so the photon has created an electron/hole pair. An electron orbiting a surrounding atom

near to a hole can move into the hole leaving a new hole in its place; in this way the positively

charged holes can also move through the structure. In the presence of an electric field the

electrons move in one direction and the holes in the other, because they have opposite electric

charges with holes behaving in nearly all respects as positive particles. In semiconductor

materials, electric current is the flow of oppositely charged electrons and holes.

Rubber, glass, wood

Copper, iron,

aluminium, gold Silicon, germanium

Semi-conductors Poor conductors

Good insulators

Good conductors

Poor insulators

24

The Photovoltaic (PV) Effect

Without an electric field to separate the electrons and holes created by the light they would soon

recombine and there would be no net current. To avoid this a photovoltaic cell (PV cell) is a

wafer or thin film of semiconductor material which is arranged to have an internal electric field,

pointing from the top surface of the wafer or film to the bottom surface (or vice versa). An

electrical contact, usually aluminum, covers the bottom surface. The top surface also has an

electrical contact, but this one is transparent so as to let in the light. When the silicon (or other

semiconductor material) in the PV cell absorbs light, electron/hole pairs are generated. Because

of the internal electric field the electrons move to one contact and holes to the other thus building

up a voltage. The cell acts as a voltage source. If you connect the two contacts with a wire an

electric current will flow in the wire; this is known as the "short-circuit current" of the PV cell;

you can measure it with an ammeter. If you don't connect the contacts the electrons and holes

build up on opposite surfaces of the cell, producing a voltage between the contacts that you can

measure with a voltmeter; this is called the "open-circuit voltage" of the PV cell.

The internal field:

To produce the necessary internal electric field we make use of two types of "doped"

semiconductor material; these are called "n-type" and "p-type" material.

N-type silicon contains a small percentage of phosphorus atoms. These fit quite well into the

structure of the silicon, except that each has one more electron than each silicon atom. These

extra electrons escape from the phosphorus and are free to move round the structure; what they

leave behind are positively charged phosphorus ions, (which are fixed in the structure and can't

move). The phosphorus is called an n-type dopant because of the negative electrons it adds to

the silicon; the resulting material is called n-type silicon because of the electrons it contains

(though you should remember it contains an equal number of positive fixed charges).

P-type silicon contains boron atoms. These fit quite well into the structure of the silicon, except

that each has one fewer electrons than each silicon atom. They therefore grab electrons from the

25

silicon, creating holes that are free to move round the structure; what the holes leave behind are

negatively charged boron ions, (because of the extra electron they've grabbed); the boron ions are

fixed in the structure and can't move. The material is called p-type because of the positive holes

it contains; it also contains an equal number of negative fixed charges. Boron is said to be a p-

type dopant in silicon.

Now consider a wafer of silicon that has excess boron in the top half (p-type silicon) and excess

phosphorus in the bottom half (n-type silicon). In the middle there is what is called a pn-

junction, where the material changes from p-type to n-type. On the n-type side of the junction

there will be electrons and fixed positive charge (phosphorus ions); on the p-type side there will

be holes and fixed negative charge (boron ions). Because there are many electrons in n-type and

very few in p-type material the electrons from the n-side will tend to spread into the p-side,

leaving some net positive charge on the n-side (because of the positive phosphorus ions); this

positive charge will stop the electrons diffusing too far into the p-type material and is further

increased by holes spreading from the p-side, (which also leaves negative charge on the p-side,

because of the negative boron ions). The result is fixed positive electric charges on the n-type

side of the junction and negative fixed charges on the p-type side. This produces an internal

electric field pointing across the junction, which is precisely what is needed for a PV cell.

This accelerates electrons from electron-hole pairs separated by light from the p-type material

into the n-type material where there are many electrons and few holes and so not much chance of

recombining. Similarly the junction accelerates holes from electron-hole pairs in the n-type

material to the p-type material where they are similarly unlikely to recombine.

26

Solar cells

A solar cell is a PV cell designed to convert sunlight to electricity. The simplest cells (Figure 1a)

consist of a circular silicon wafer with a pn-junction sandwiched in the middle, a metallic bottom

contact (e.g. aluminum) and a transparent top contact (either a transparent conducting oxide or a

grid-like metal structure). Solar panels with cells like this have played a vital role in space

technology since the late '50s, powering space satellites. They are expensive to produce because

silicon wafers are expensive to produce (mainly because they are high-purity single crystals) but

their cost was unimportant in the space race.

Fig 7.3

In recent years there has been a continuous search for cheaper forms of PV cell, economical

enough to be used in applications here on earth (terrestrial applications). Attempts have been

made to use cheaper forms of silicon, of lower quality than that used in computer chips, despite

the poorer cell efficiencies that result. One possibility has been to replace the single-crystal

wafer by polycrystalline squares, (consisting of many small grains of crystalline material). A

more radical approach is to use amorphous silicon, having no crystalline structure at all. This

material has the advantage of being much more light-absorbing than crystalline silicon: a thin

film on a suitable substrate only a few microns thick (a thousandth of a millimeter) absorbs most

of the sunlight falling on it; by contrast crystalline cells have to be about 100 microns and in

practice are 0.5mm thick. This means that you need far less amorphous silicon to make the cells,

and they can even be made flexible, whereas crystalline cells are very fragile. The electrons and

holes don't move so easily in amorphous silicon, but this is partly compensated for by the fact

Photon

BACK CONTACT PLATE

FRONT CONTACT GRID

SILICON CONTAINING BORON AS DOPANT

SILICON CONTAINING PHOSPHOROUS AS DOPANT

P-region

N-region

Hole

Electron

Hole

Electron

Photon

Photon

27

that they don't have to move as far (because the cell is so thin). Cell efficiencies are perhaps only

half those in crystalline silicon, but the amorphous cells potentially cost much less than half for

the same surface area, so they seem to be the most economical choice at the moment.

Manufacture of amorphous silicon solar cells

The manufacture of amorphous silicon cells (e.g. by UNI-SOLAR) is very different from that

of crystalline cells. No wafers are involved. Instead the silicon is deposited as a thin film on a

substrate, usually either stainless steel or a glass sheet covered with a layer of tin oxide acting as

a transparent contact.

As shown in Figure 2, the substrate is placed in a steel chamber which is evacuated (i.e. all the

air is pumped out); a small amount of the gas saline (a gaseous compound of silicon and

hydrogen) is then bled in through a valve. Two metal plates within the chamber connect to a

radio-frequency power supply which sets up a purple-colored glow discharge (sometimes called

a plasma) in the saline gas; electrons collide with saline molecules and knock away the hydrogen

atoms, leading to the silicon atoms depositing in a thin amorphous film on the substrate (mixed

with some of the hydrogen atoms, which in fact turn out to be beneficial for the cell). Substrates

used are often 300 mm wide, but in principle they could be larger, limited only by the size of the

deposition chamber.

To make n-type amorphous silicon the same procedure is followed, except that the saline is

mixed with one or two per cent of the gas phosphine, a compound of phosphorus and hydrogen.

To make p-type amorphous silicon the saline is mixed with diorama, a compound of boron and

hydrogen. Either separate chambers or sequential gas streams are used for making each type.

Fig 7.4

Photon

SUPERSTRATE (not necessary if stainless steel back contact used)

N-layer

P-layer

INTRINSIC

LAYER

AMORPHOUS SILICON

AMORPHOUS SILICON CONTAINING BORON

AMORPHOUS SILICON CONTAINING PHOSPHOROUS

Transparent front contact (tin oxide or indium-tin oxide)

Aluminium back contact or stainless steel

Electron

Hole

Photon

Photon

Electron

Hole

28

Unfortunately electron-hole recombination of n- or p-type amorphous silicon to light is very

high. To get round this problem the cell is made mostly from undoped amorphous silicon (i.e.

using just saline): the thin film of undoped amorphous silicon is sandwiched between far thinner

layers of n- and p-type amorphous silicon, as shown in Figure 1b. The n- and p-layers serve to

produce the internal field across the undoped layer, but almost all the light is absorbed in the

undoped layer. (The undoped material is referred to as intrinsic, and the cell is said to have a p-

i-n structure, as opposed to the p-n structure of crystalline silicon cells).

The process used for depositing amorphous silicon lends itself well to mass production

techniques. The substrate (with its electrical contact layer if necessary) passes into a chamber

and receives the n-type deposition, then into a chamber receiving the undoped deposition, and

then to chamber receiving the p-type deposition. (This is simpler to automate than cutting and

polishing wafers).

The PV industry benefits from technological developments in other fields. The development of

silicon coated drums for colour photocopiers is now applied to the production of continuous

metal strips covered with amorphous silicon. If the substrate is flexible stainless steel (as with

Plugging into the Sun laminates) that can be wound into a large roll, it is possible to have a

continuous roll-to-roll production process for amorphous silicon solar cells, (Figure 3). The

stainless steel sheet unwinds from the supply roll and passes though cleaning procedures and the

chambers for n-type, intrinsic, and p-type deposition before reaching the take-up roll. The

resulting cells have the additional advantage of being flexible.

Many manufacturers base their cells on glass substrates. Normally tin-oxide-coated glass is used

since the tin oxide serves as a transparent contact. The p-layer is deposited, followed by the i-

layer and then the n-layer. Aluminum is deposited to form the back contact. In this form of

structure the cell is illuminated through the glass and is protected by it (it is therefore known as a

superstrate).

A single silicon solar cell produces an open-circuit voltage of about 0.5volt. There are

amorphous silicon solar modules that are in fact single cells, producing a low voltage and a

correspondingly high current. However, it is far more common for the module to be divided into

individual strip-shaped cells, which are arranged to be connected in series to produce a working

voltage of around 14 volts, suitable for charging 12-volt lead-acid batteries.

29

We have dealt with the main principles of amorphous silicon solar cell production. There is a

mass of subsidiary detail which is too extensive to cover here. Cell efficiencies can be increased

to some extent by including a second p-i-n structure under the first, using an alloy of amorphous

silicon with germanium. This absorbs a longer wavelength part of the solar spectrum. This is

called a tandem cell or multi-junction cell.

Fig 8.1

Spectrum-splitting

cell, constructed of

three separate p-i-n

type, amorphous

semiconductor solar

sub-cells, each with a

different spectral

response

characteristic. In this

way, the cell can

convert the different

visible and near

infrared wavelengths

of sunlight with

optimal efficiency.

30

How a triple (three layer) UNI-SOLAR amorphous silicon PV cell is made.

Fig 8.2

Adding a third p-i-n structure forms a triple-junction cell. (In production terms this is just a

matter of additional chambers and their gas supplies; see Figure 3b). The right sort of

roughening of the cell surface leads to less reflection from the cell surface, and to corresponding

increases in cell efficiency. The front contact needs careful design, and the whole cell must be

suitably encapsulated and protected against the weather. If everything is done right there is no

reason why the cells should not last for thirty years or more.

N-layer chamber

P-layer chamber

Thicker intrinsic layer chamber

In Roll Out Roll

N-layer chamber

P-layer chamber

Thicker intrinsic layer chamber

N-layer chamber P-layer chamber

Thicker intrinsic layer chamber

N-layer chamber

P-layer chamber Thicker intrinsic layer chamber

Thicker intrinsic layer chamber

P-layer

chamber N-layer chamber

In Roll Out Roll

31

Substrate (stainless steel or glass)

passing through

chamber

Electrode 1

Electrode 2

Excited molecules in a gaseous state containing:

Silicon and either boron (p-

layer) or phosphorus (n-layer)

Regular layer of deposited silicon and >1% doping

material

32

LASER Light:

Fig 8.3

A laser is a device that emits light through a process of optical amplification based on

the stimulated emission of electromagnetic radiation. The term "laser" originated as

an acronym for "light amplification by stimulated emission of radiation". The first laser was

built in 1960 by Theodore H. Mailman at Hughes Laboratories, based on theoretical work

by Charles Hard Townes and Arthur Leonard Schawlow. A laser differs from other sources of

light in that it emits light coherently. Spatial coherence allows a laser to be focused to a tight

spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a

laser beam to stay narrow over great distances (collimation), enabling applications such as laser

pointers. Lasers can also have high temporal coherence, which allows them to emit light with a

very narrow spectrum, i.e., they can emit a single color of light. Temporal coherence can be used

to produce pulses of light as short as a femtosecond.

33

Loud Speaker:

Fig 8.4

A loudspeaker (or loud-speaker or speaker) is an electroacoustic transducer; [1] a device which

converts an electrical audio signal into a corresponding sound.[2] The first crude loudspeakers

were invented during the development of telephone systems in the late 1800s, but electronic

amplification by vacuum tube beginning around 1912 made loudspeakers truly practical. By the

1920s they were used in radios, phonographs, public address systems and theatre sound systems

for talking motion pictures.

The most widely-used type of speaker today is the dynamic speaker, invented in 1925 by Edward

W. Kellogg and Chester W. Rice. The dynamic speaker operates on the same basic principle as

a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an

alternating current electrical audio signal input is applied through the voice coil, a coil of wire

suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move

rapidly back and forth due to Faraday's law of induction, which causes a diaphragm (usually

conically shaped) attached to the coil to move back and forth, pushing on the air to create sound

34

waves. Besides this most common method, there are several alternative technologies that can be

used to convert an electrical signal into sound.

PCB (Printed Circuit Board):

Fig 8.3

A printed circuit board (PCB) mechanically supports and electrically

connects electronic components using conductive tracks, pads and other features etched from

copper sheets laminated onto a non-conductive substrate. PCBs can be single sided (one copper

layer), double sided (two copper layers) or multi-layer (outer and inner layers). Multi- layer PCBs

allow for much higher component density. Conductors on different layers are connected with

plated-through holes called vias. Advanced PCBs may contain components - capacitors, resistors

or active devices - embedded in the substrate.

Process to Design PCB:

Front-end tool data preparation

The board designer has prepared his layout on a Computer Aided Design or CAD

system. Each CAD system uses its own internal data format, so the PCB industry has

developed a standard output format to transfer the layout data to the manufacturer.

Preparing the photo tools.

35

We use laser photo plotters in a temperature and humidity-controlled darkroom to make

the films we will use later to image the PCBs. The photo plotter takes the board data and

converts it into a pixel image. A laser writes this onto the film. The exposed film is

automatically developed and unloaded for the operator.

Print inner layers.

To produce the inner layers of our multilayer PCB, we start with a panel of laminate.

Laminate is an epoxy resin and glass-fibre core with copper foil pre-bonded onto each

side.

Etch inner layers.

We remove the unwanted copper using a powerful alkaline solution to dissolve (or etch

away) the exposed copper. The process is carefully controlled to ensure that the finished

conductor widths are exactly as designed. But designers should be aware that thicker

copper foils need wider spaces between the tracks. The operator checks carefully that all

the unwanted copper has been etched away.

Register punch and Automatic Optical Inspection (AOI)

The inner core of our multilayer is now complete. Next we punch the registration holes

we will use to align the inner layers to the outer layers. The operator loads the core into

the optical punch which lines up the registration targets in the copper pattern and

punches the registration holes.

Lay-up and bond

The outer layers of our multilayer consist of sheets of glass cloth pre-impregnated with

uncured epoxy resin (prepare) and a thin copper foil.

Drilling the PCB

Now we drill the holes for leaded components and the via holes that link the copper

layers together. First we use an X-ray drill to locate targets in the copper of the inner

layers. The machine drills registration holes to ensure that we will drill precisely through

the centre of the inner layer pads.

Electrolyses copper deposition

36

The first step in the plating process is the chemical deposition of a very thin layer of

copper on the hole walls.

Image the outer layers.

We image the outer layers in a clean room to make sure that no dust gets onto the panel

surface where it could cause a short or open circuit on the finished PCB.

Plated gold edge connectors.

For edge-connectors we electroplate hard gold. First the operator puts protective tape on

the board above the connectors. Then he mounts the panel on a horizontal electroplating

bath.

Printed Circuit Board Layout:

37

Fig 9.1

Circuit Board:

38

Fig 9.2

CONCLUSION

The possibilities are numerous and can be explored further. If his technology can be put

into practical use, every bulb can be used something like a Wi-Fi hotspot to transmit wireless

data and we will proceed toward the cleaner, greener, safer and brighter future. The concept of

Li-Fi is currently attracting a great deal of interest, not least because it may offer a genuine and

very efficient alternative to radio-based wireless. As a growing number of people and their many

devices access wireless internet, the airwaves are becoming increasingly clogged, making it more

and more difficult to get a reliable, high-speed signal. This may solve issues such as the shortage

of radio-frequency bandwidth and also allow internet where traditional radio based wireless isn’t

allowed such as aircraft or hospitals. One of the shortcomings however is that it only work in

direct line of sight.

39

References:-

Websites:-

1:- http://timesofindia.indiatimes.com/home/science/Now-just- light-a-bulb-to-

switch-on-your-broadband/articleshow/9713554.cms

2:-

http://oledcomm.com/lifi.html

3:-

http://en.wikipedia.org/wiki/Li-Fi

4:-

https://images.google.com/