ELECTRONIC CANDLES

A Mini project report submitted to

Jawaharlal Nehru Technological University, Hyderabad

In partial fulfillment of the requirement for the award of B. Tech degree in

Electronics and Communication Engineering

BY

T.Aravind Kumar : 07871A04B3

P.Laxmi Narasimha Reddy : 07871A0480

Under the Guidance of

Mr. ANVAR BASHA.NB

Lecturer

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

RAMAPPA ENGINEERING COLLEGE

(Affiliated to JNTU, Hyderabad)

Hunter Road, Warangal - 506001

2010-11

ELECTRONIC CANDLES

A Mini project report submitted to

Jawaharlal Nehru Technological University, Hyderabad

In partial fulfillment of the requirement for the award of B. Tech degree in

Electronics and Communication Engineering

BY

T.Aravind Kumar : 07871A04B3

P.Laxmi Narasimha Reddy : 07871A0480

Under the Guidance of

Md. ANVAR BASHA.NB

Lecturer

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

RAMAPPA ENGINEERING COLLEGE

(Affiliated to JNTU, Hyderabad)

Hunter Road, Warangal - 506001

2010-11

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

RAMAPPA ENGINEERING COLLEGE

Hunter road, Warangal.

CERTIFICATE

This is to certify that the project entitled ELECTRONIC CANDLES is submitted

by Mr.T.Aravind Kumar and Mr.P. Laxmi Narasimha Reddy bearing 07871A04B3

and 07871A0480 in partial fulfillment of the requirements for the award of the degree

in Bachelor of Technology in Electronics and Communication Engineering during the

academic year 2010-11.

Mr.Anvar Basha,NB Mr. J. Tarun Kumar Dr. V. Janaki

Lecturer Head E.C.E Principal

Guide

External Examiner

ACKNOWLEDGEMENT

The success accomplished in this project would not have been possible by timely help

and guidance by many people. I wish to express my sincere gratitude to all those who have

helped and guided me for the completion of the project.

It is my great pleasure to thank my guide, Mr.NB.Anvar Basha, Lecturer for his

valuable guidance through provoking discussions, vital suggestions and sharing her valuable

expertise through out the project work.

We express our sincere gratitude to Mr.J.Tarun Kumar, Head, ECE for his

wholehearted support.

We express our sincere gratitude to Dr. V. Janaki, Principal, Ramappa Engineering

College, Hanamkonda for her encouragement and providing facilities to accomplish our project

successfully.

Our special thanks to entire faculty members in the department for their suggestions and

support.

Abstract

This is the circuit that can produce effect of candle light in a normal electric bulb.

A candle light as we know, resembles a random flickering light. So the main idea of this project

is to produce a normal flickering light effect in an electric bulb .To achieve this the circuit

performs in three different parts,in the first part it is comprised of the IC’s and is to generate a

randomly changing train of pulses.In the second part it consists of silicon controlled rectifier,an

electric bulb is connected between SCR and mains live wire and gate triggered circuit

components.It is basically half-wave AC power being supplied to the electric bulb.

The third part is the power supply circuit to generate regulated 5V DC from 230V

AC for random signal generator.It comprises a stepdown transformer,full-wave rectifier,filter

capacitor,followed by a regulator.

I

CONTENTS

ABSTRACT I

LIST OF FIGURES V

LIST OF TABLES V

1. INTRODUCTION

1.1Preamble of the project 1

1.2Introduction to the Project 2

2. CIRCUIT DIAG AND WORKING 4

3. THEORY 8

3.1 Transformer 9

3.1.1 History 9

3.1.2 Discovery 9

3.1.3Basic principles 9

3.1.4 Induction law 10

3.1.5 Ideal power equation 11

3.1.6 Detailed operation 12

3.1.7 Practical considerations 13

3.1.8 Leakage flux 13

3.1.9 Energy losses 14

3.2 RESISTORS 16

3.2.1 Units 16

3.2.2 Theory of operation 17

3.2.2.1 Ohm's law 17

II

3.2.2.2 Power dissipation 17

3.2.3 Measurement 18

3.2.4 Electrical and thermal noise 19

3.2.5 Failure modes 20

3.2.6 Uses 20

3.3 CAPACITORS 21

3.3.1 History 21

3.3.2 Theory of operation 22

3.3.2.1 Energy storage 22

3.3.2.2 Current-voltage relation 23

3.3.3 Non-ideal behaviour 24

3.3.3.1 Breakdown voltage 24

3.3.4 Applications 25

3.3.4.1 Energy storage 25

3.3.4.2 Pulsed power and weapons 25

3.4.3 Power conditioning 25

3.4.3.1 Power factor correction 25

3.4 Diodes 26

3.4.1 1N4148 26

3.4.2Specifications 26

3.4.3 1N400X 26

3.5 Integrated Chips (IC) 34

3.5.1 Specifications 34

3.5.2 Advantages 35

III

3.5.2 Disadvantages 35

3.6 Silicon Controlled Rectifer 42

3.6.1 Construction 42

3.6.2 Modes of Operation 42

3.7 Variable resistor 44

3.7.1 Construction 44

3.7.2 Rheostat 45

3.7.3Potentiometer 46

4.0 Future Scope and References 49

5.0 Bibilography 51

IV

LIST OF FIGURES:

Fig 2.1: Circuit Diagram 4

Fig 2.2: IC 555 Astable Mode 6

Fig 3.1.1: Ideal Transformer 10

Fig 3.1.2:Leakage Flux of Transformer 13

Fig 3.3.1:Capacitors of Different types 21

Fig 3.4.1:General Diodes 26

Fig 3.5.1: IC 555 Internal Block Diagram 28

Fig 3.5.2: IC 555 Monostable Mode 29

Fig 3.5.3: IC 555 Bistable Mode 32

Fig 3.5.4:IC74LS164 Shift Register 38

Fig 3.5.4:IC74LS00 Internal Block Diagram 39

Fig 3.7.1:Standard Variable Resistances 44

LIST OF TABLES

Table3.4.1:IN400X family 27

Table3.5.1:IC 555 Timer Specifications 33

Table3.7.1:Electrical Characteristics of IC74L00 39

Table3.7.2:IC74LS86 Maximum Ratings 40

Table3.7.3:IC74LS86 Recommended Ratings 41

V

CHAPTER –1

INTRODUCTION

Preamble to the Project:

Generally when we see a normal candle light we obseve that the glowing light is not

constant and that the light flickers or changes rapidly .And this light is having less brightness.

Here the main objective or the main aim of the project is to bring that flickering light

of normal candle into the electrical bulb so that the bulb glows at random intervals .Here we are

bringing this effect into the candle by using a step-down transformer, which converts a high

voltage to our required levels and a group of 74xx series IC’s which acts as current limiting

components and and positive voltage regulator and using IC555 for pulse generation .So here we

are producing a normal candle’s flickering light into the electrical bulb.

1

Introduction the project :

This is the circuit that can produce effect of candle light in a normal electric bulb. A

candle light as we know resembles a random flickering light. So the main idea of this project is

to produce a normal flickering light effect in an electric bulb .

To achieve this circuit performs in three different parts, in the first part it is Comprised

of the IC’s and is to generate a randomly changing train of pulses . In the second part It consists

of silicon controlled rectifier; an electric bulb is connected between SCR and mains Live wire

and gate triggered circuit components. It is basically half-wave AC power being Supplied to the

electric bulb.

The third part is the power supply circuit to generate regulated 5V DC from 230V AC

for random signal generator.It comprises a step-down transformer,full-wave rectifier, filters

capacitor, followed by a regulator.

The main scope of the project is that, this circuit that can produce effect of candle light

in a normal electric bulb. A candle light as we know resembles a random flickering light. So

themain idea of this project is to produce a normal flickering light effect in an electric bulb.

2

CHAPTER - 2

CIRCUIT DIAGRAM

3

Circuit Diagram:

Fig 2.1: Electronic Candle Circuit

4

Working:

Here is a simple circuit that can produce the effect of candle light in a normal electric bulb.

A candle light,as we all know,resembles a randomly flickering light.So,the objective of this

project activity is to produce a randomly flickering light effect in an electric bulb.To achieve

this, the entire circuit can be divided into three parts.

The first part comprises IC1 (555),IC2 (74LS164),IC3 (74LS86),IC4 (74LS00) and the

associated components.These generate a randomly changing train of pulses.

The second part of the circuit consists of SCR1 (C106),an electric bulb connected between

anode of SCR1 and mains live wire,and gate trigger circuit components.It is basically half-wave

AC power being supplied to the electric bulb.

The third part is the power supply circuit to generate regulated 5V DC from 230V AC for

random signal generator.It comprises a step-down transformer (X1),full-wave rectifier (diodes

D3 and D4), filter capacitor(C9),followed by a regulator (IC5).The random signal generator of

the circuit is built around an 8-bit serial in/parallel out shift register (IC2).Different outputs of the

shift register IC pass through a set of logic gates (N1 through N5) and final output appearing at

pin 6 of gate N5 is fed back to the inputs of pins 1 and 2 of IC2. The clock signal appears at pin 8

of IC2,which is clocked by an astable multivibrator configured around timer (IC1). The clock

frequency can be set using preset VR1 and VR2.It can be set around 100 Hz to provide better

flickering effect in the bulb.The random signal triggers the gate of SCR1.The electric bulb gets

AC power only for the period for which SCR1 is fired.SCR1 is fired only during the positive half

cycles.Conduction of SCR1 depends upon the gate triggering pin 3 of IC2,which is random.

Thus,we see a flickering effect in the light output.

5

IC 555 in Astable Mode :

   

Fig 2.2: IC 555 in Astable Mode and its output waveform

6

Operation :

This circuit diagram shows how a 555 timer IC is configured to function as an astable

multivibrator.An astable multivibrator is a timing circuit whose 'low' and 'high' states are

both unstable.As such,the output of an astable multivibrator toggles between 'low' and

'high' continuously,in effect generating a train of pulses.This circuit is therefore also

known as a 'pulse generator' circuit.

In this circuit,capacitor C1 charges through R1 and R2,eventually building up enough

voltage to trigger an internal comparator to toggle the output flip-flop.Once toggled,the

flip-flop discharges C1 through R2 into pin 7,which is the discharge pin.When C1's

voltage becomes low enough, another internal comparator is triggered to toggle the output

flip-flop.This once again allows C1 to charge up through R1 and R2 and the cycle starts all

over again.

 C1's charge-up time t1 is given by: t1 = 0.693(R1+R2)C1.C1's discharge time t2 is given

by: t2 = 0.693(R2)C1.Thus,the total period of one cycle is t1+t2 = 0.693 C1(R1+2R2). 

The frequency f of the output wave is the reciprocal of this period,and is therefore given

by: f = 1.44/(C1(R1+2R2)),wherein f is in Hz if R1 and R2 are in megaohms and C1 is in

microfarads.  

  

7

CHAPTER - 3

THEORY AND DESIGN

8

Transformer:

History:

Discovery

The phenomenon of electromagnetic induction was discovered independently by Michael

Faraday and Joseph Henry in 1831.However,Faraday was the first to publish the results of his

experiments and thus receive credit for the discovery.The relationship between electromotive

force (EMF) or "voltage" and magnetic flux was formalized in an equation now referred to as

"Faraday's law of induction":

Where: is the magnitude of the EMF in volts

ΦB is the magnetic flux through the circuit (in webers).

Faraday's experiments included winding a pair of coils around an iron ring,thus creating the first

toroidal closed-core transformer.

Basic principles

The transformer is based on two principles: firstly,that an electric current can produce a magnetic

field (electromagnetism) and secondly that a changing magnetic field within a coil of wire

induces a voltage across the ends of the coil (electromagnetic induction).Changing the current in

the primary coil changes the magnetic flux that is developed.The changing magnetic flux induces

a voltage in the secondary coil.

9

An ideal transformer is show in the following figure

Fig 3.1.1: Ideal Transoformer

An ideal transformer is shown in the adjacent figure.Current passing through the primary coil

creates a magnetic field.The primary and secondary coils are wrapped around a core of very high

magnetic permeability,such as iron,so that most of the magnetic flux passes through both the

primary and secondary coils.

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of

induction,which states that:

where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ

equals the magnetic flux through one turn of the coil.If the turns of the coil are oriented

perpendicular to the magnetic field lines,the flux is the product of the magnetic flux density B

and the area A through which it cuts.The area is constant,being equal to the cross-sectional area

10

of the transformer core,whereas the magnetic field varies with time according to the excitation of

the primary.Since the same magnetic flux passes through both the primary and secondary coils in

an ideal transformer,the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for VS and VP gives the basic equation for stepping

up or stepping down the voltage

Ideal power equation:

The ideal transformer as a circuit element

If the secondary coil is attached to a load that allows current to flow,electrical power is

transmitted from the primary circuit to the secondary circuit.Ideally,the transformer is perfectly

efficient; all the incoming energy is transformed from the primary circuit to the magnetic field

and into the secondary circuit. If this condition is met, the incoming electric power must equal

the outgoing power.

Pincoming = IPVP = Poutgoing = ISVS, giving the ideal transformer equation

11

Transformers normally have high efficiency,so this formula is a reasonable approximation.If the

voltage is increased,then the current is decreased by the same factor.The impedance in one

circuit is transformed by the square of the turns ratio.For example,if an impedance ZS is attached

across the terminals of the secondary coil, it appears to the primary circuit to have an impedance

of .This relationship is reciprocal,so that the impedance ZP of the primary circuit

appears to the secondary to be .

Detailed operation:

The simplified description above neglects several practical factors,in particular the primary

current required to establish a magnetic field in the core,and the contribution to the field due to

current in the secondary circuit.

Models of an ideal transformer typically assume a core of negligible reluctance with two

windings of zero resistance.When a voltage is applied to the primary winding,a small current

flows,driving flux around the magnetic circuit of the core.The current required to create the flux

is termed the magnetizing current;since the ideal core has been assumed to have near-zero

reluctance,the magnetizing current is negligible,although still required to create the magnetic

field.

The changing magnetic field induces an electromotive force (EMF) across each winding.Since

the ideal windings have no impedance,they have no associated voltage drop,and so the voltages

VP and VS measured at the terminals of the transformer,are equal to the corresponding EMFs.The

primary EMF,acting as it does in opposition to the primary voltage,is sometimes termed the

"back EMF".This is due to Lenz's law which states that the induction of EMF would always be

such that it will oppose development of any such change in magnetic field.

12

Practical considerations

Leakage flux

Leakage flux of a transformer

Fig3.1.2:Leakage Flux of a Transofrmer

The ideal transformer model assumes that all flux generated by the primary winding links all the

turns of every winding,including itself.In practice,some flux traverses paths that take it outside

the windings.Such flux is termed leakage flux,and results in leakage inductance in series with the

mutually coupled transformer windings.Leakage results in energy being alternately stored in and

discharged from the magnetic fields with each cycle of the power supply.It is not directly a

power loss,but results in inferior voltage regulation,causing the secondary voltage to fail to be

directly proportional to the primary,particularly under heavy load .Transformers are therefore

normally designed to have very low leakage inductance.

However,in some applications,leakage can be a desirable property,and long magnetic paths,air

gaps,or magnetic bypass shunts may be deliberately introduced to a transformer's design to limit

the short-circuit current it will supply.Leaky transformers may be used to supply loads that

exhibit negative resistance,such as electric arcs,mercury vapor lamps,and neon signs;or for safely

handling loads that become periodically short-circuited such as electric arc welders.

13

Air gaps are also used to keep a transformer from saturating,especially audio-frequency

transformers in circuits that have a direct current flowing through the windings.Leakage

inductance is also helpful when transformers are operated in parallel.It can be shown that if the

"per-unit" inductance of two transformers is the same (a typical value is 5%),they will

automatically split power "correctly".

Energy losses :

An ideal transformer would have no energy losses,and would be100% efficient.In practical

transformers energy is dissipated in the windings ,surrounding structures.Larger transformers are

generally more efficient, and those rated for electricity distribution usually perform better than

98%.

Experimental transformers using superconducting windings achieve efficiencies of

99.85%.While the increase in efficiency is small,when applied heavily loaded transformers the

annual savings in energy losses are significant.

A small transformer,such as a plug-in "wall wart" power adapter commonly used for low-power

consumer electronics devices, may be as low as 20% efficient,with considerable energy loss even

when not supplying any power to the device. Though individual losses may be only a few

watts,it has been estimated that the cumulative loss from such transformers in the United States

alone exceeded 32 billion kilowatt-hours (kWh) in 2002.

The losses vary with load current, and may be expressed as "no-load" or "full-load" loss.Winding

resistance dominates load losses,whereas hysteresis eddy currents losses contribute to over 99%

of the no-load loss.The no-load loss can be significant,meaning that even an idle transformer

constitutes on an electrical supply,which encourages development of low-loss transformers (also

see energy efficient transformer).

Transformer losses are divided into losses in the windings,termed copper loss,and those in the

magnetic circuit,termed iron loss.Losses in the transformer arise from:

14

Winding resistance

Current flowing through the windings causes resistive heating of the conductors. At higher

frequencies,skin effect and proximity effect additional winding resistance and losses.

Hysteresis losses

Each time the magnetic field is reversed,a small amount of energy is lost due to hysteresis within

the core.For a given core material,the proportional to the frequency,and is a function of the peak

flux density to which it is subjected.

Eddy currents

Ferromagnetic materials are also good conductors,and a solid core made from such a material

also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore

circulate within the core in a plane normal to the flux,and are responsible for resistive the core

material. The eddy current loss is a complex function of the square of supply frequency and

inverse square of the material thickness.

Magnetostriction

Magnetic flux in a ferromagnetic material,such as the core,causes it to physically expand and

contract slightly with each cycle of the magnetic an effect known as magnetostriction.This

produces the buzzing sound commonly associated with transformers,and in turn causes losses

frictional heating in susceptible cores.

Mechanical losses

In addition to magnetostriction,the alternating magnetic field causes fluctuating electromagnetic

forces between the primary and secondary windings.These incite vibrations within nearby

metalwork,adding to the buzzing noise,and consuming a small amount of power.

15

Stray losses

Leakage inductance is by itself largely lossless,since energy supplied to its magnetic fields is

returned to the supply with the next half-However,any leakage flux that intercepts nearby

conductive materials such as the transformer's support structure will give rise to eddy be

converted to heat.There are also radiative losses due to the oscillating magnetic field, but these

are usually small.

RESISTORS:

A resistor is a two-terminal electronic component that produces a voltage across its terminals that

is proportional to the electric current passing through it in accordance with Ohm's law:

V = IR

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most

electronic equipment.Practical resistors can be made of various compounds and films,as well as

resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).

The primary characteristics of a resistor are the resistance,the tolerance,maximum working

voltage and the power rating.Other characteristics include temperature coefficient,noise,and

inductance.Less well-known is critical resistance,the value below which power dissipation limits

the maximum permitted current flow,and above which the limit is applied voltage.Critical

resistance is determined by the design,materials and dimensions of the resistor.

Resistors can be integrated into hybrid and printed circuits,as well as integrated circuits.Size, and

position of leads (or terminals) are relevant to equipment designers;resistors must be physically

large enough not to overheat when dissipating their power.

16

Theory of operation:

Ohm's law

The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it

where the constant of proportionality is the resistance (R).

Equivalently,Ohm's law can be stated:

This formulation of Ohm's law states that,when a voltage (V) is maintained across a resistance

(R), a current (I) will flow through the resistance.

This formulation is often used in practice.For example,if V is 12 volts and R is 400 ohms, a

current of 12 / 400 = 0.03 amperes will flow through the resistance R.

Power dissipation

The power dissipated by a resistor (or the equivalent resistance of a resistor network) is

calculated using the following:

All three equations are equivalent. The first is derived from Joule's first law. Ohm’s Law derives

the other two from that.

17

The total amount of heat energy released is the integral of the power over time:

If the average power dissipated is more than the resistor can safely dissipate, the resistor may

depart from its nominal resistance and may become damaged by overheating. Excessive power

dissipation may raise the temperature of the resistor to a point where it burns out, which could

cause a fire in adjacent components and materials. There are flameproof resistors that fail (open

circuit) before they overheat dangerously.

Note that the nominal power rating of a resistor is not the same as the power that it can safely

dissipate in practical use. Air circulation and proximity to a circuit board, ambient temperature,

and other factors can reduce acceptable dissipation significantly. Rated power dissipation may be

given for an ambient temperature of 25 °C in free air. Inside an equipment case at 60 °C, rated

dissipation will be significantly less; if we are dissipating a bit less than the maximum figure

given by the manufacturer we may still be outside the safe operating area, and courting

premature failure.

Measurement

The value of a resistor can be measured with an ohmmeter, which may be one function of a

multimeter. Usually, probes on the ends of test leads connect to the resistor.

Measuring low-value resistors, such as fractional-ohm resistors, with acceptable accuracy

requires four-terminal connections. One pair of terminals applies a known, calibrated current to

the resistor, while the other pair senses the voltage drop across the resistor. Some laboratory test

instruments have spring-loaded pairs of contacts, with neighboring contacts electrically isolated

from each other. Better digital multimeters have four terminals on their panels, generally used

with special test leads. These comprise four wires in all, and have special test clips with jaws

insulated from each other. One jaw provides the measuring current, while the other senses the

voltage drop. The resistance is then calculated using Ohm's Law.

18

Electrical and thermal noise

In precision applications it is often necessary to minimize electronic noise. As dissipative

elements, even ideal resistors will naturally produce a fluctuating "noise" voltage across their

terminals. This Johnson–Nyquist noise is a fundamental noise source which depends only upon

the temperature and resistance of the resistor, and is predicted by the fluctuation–dissipation

theorem. For example, the gain in a simple (non-) inverting amplifier is set using a voltage

divider. Noise considerations dictate that the smallest practical resistance should be used, since

the Johnson–Nyquist noise voltage scales with resistance, and any resistor noise in the voltage

divider will be impressed upon the amplifier's output.In addition, small voltage differentials may

appear on the resistors due to thermoelectric effect if their ends are not kept at the same

temperature.

The voltages appear in the junctions of the resistor leads with the circuit board and with the

resistor body. Common metal film resistors show such an effect at a magnitude of about 20

µV/°C. Some carbon composition resistors can go as high as 400 µV/°C, and specially

constructed resistors can go as low as 0.05 µV/°C. In applications where thermoelectric effects

may become important, care has to be taken (for example) to mount the resistors horizontally to

avoid temperature gradients and to mind the air flow over the board.Practical resistors frequently

exhibit other, "non-fundamental", sources of noise, usually called "excess noise." Excess noise

results in a "Noise Index" for a type of resistor. Excess Noise is due to current flow in the resistor

and is specified as μV/V/decade - μV of noise per volt applied across the resistor per decade of

frequency. The μV/V/decade value is frequently given in dB so that a resistor with a noise index

of 0dB will exhibit 1 μV (rms) of excess noise for each volt across the resistor in each frequency

decade. Excess noise is an example of 1/f noise. Thick-film and carbon composition resistors

generate more noise than other types at low frequencies; wire-wound and thin-film resistors,

though much more expensive, are often utilized for their better noise characteristics. Carbon

composition resistors can exhibit a noise index of 0 dB while bulk metal foil resistors may have a

noise index of -40 dB, usually making the excess noise of metal foil resistors insignificant.

19

Thin film surface mount resistors typically have lower noise and better thermal stability than

thick film surface mount resistors. However, the design engineer must read the data sheets for

the family of devices to weigh the various device tradeoffs.

Failure modes

Like every part, resistors can fail in normal use. Thermal and mechanical stress, humidity, etc.,

can play a part. Carbon composition resistors and metal film resistors typically fail as open

circuits. Carbon-film resistors may decrease or increase in resistance. Carbon film and

composition resistors can open if running close to their maximum dissipation. This is also

possible but less likely with metal film and wirewound resistors. If not enclosed, wirewound

resistors can corrode. The resistance of carbon composition resistors are prone to drift over time

and are easily damaged by excessive heat in soldering (the binder evaporates). Variable resistors

become electrically noisy as they wear.

All resistors can be destroyed, usually by going open-circuit, if subjected to excessive current

due to failure of other components or accident.

Uses of Resistors

Though resistors can cause wastage of electricity, it has a lot of advantages and applications in

our daily life.

Resistance is one of the main ingredient in the working of a light bulb. When electricity

passes through the filament of the bulb, it burns bright as it turns extremely hot due to its

smaller size. Though this mechanism wastes a lot of electricity, we are forced to use it to

obtain light. The light used nowadays are highy efficient than the older incandascent

lamps.

The similar filament working is the main ingredient in the working of some of our usual

household stuffs like electric kettles, electric radiators, electric showers, coffee makers,

toasters, and so on.

20

The application of variable resistance is also helpful to us. Our TV’s, radios, loud

speakers and so on work on this principle.

Capacitor:

A capacitor (formerly known as condenser) is a passive electronic component consisting of a pair

of conductors separated by a dielectric (insulator).When a potential difference (voltage) exists

across the conductors,an electric field is present in the dielectric.This field stores energy and

produces a mechanical force between the conductors.The effect is greatest when there is a

narrow separation between large areas of conductor,hence capacitor conductors are often called

plates.

An ideal capacitor is characterized by a single constant value,capacitance,which is measured in

farads.This is the ratio of the electric charge on each conductor to the potential difference

between them.In practice,the dielectric between the plates passes a small amount of leakage

current.The conductors and leads introduce an equivalent series resistance and the dielectric has

an electric field strength limit resulting in a breakdown voltage.

Capacitors are widely used in electronic circuits to block direct current while allowing

alternating current to pass,to filter out interference,to smooth the output of power supplies,and

for many other purposes.They are used in resonant circuits in radio frequency equipment to

select particular frequencies from a signal with many frequencies.

Fig 3.3.1:Various Capacitors

21

Theory of operation

A capacitor consists of two conductors separated by a non-conductive region.The non-

conductive substance is called the dielectric medium,although this may also mean a vacuum or a

semiconductor depletion region chemically identical to the conductors.A capacitor is assumed to

be self-contained and isolated,with no net electric charge and no influence from an external

electric field.The conductors thus contain equal and opposite charges on their facing surfaces,and

the dielectric contains an electric field.The capacitor is a reasonably general model for electric

fields within electric circuits.

An ideal capacitor is wholly characterized by a constant capacitance C,defined as the ratio of

charge ±Q on each conductor to the voltage V between them:

Sometimes charge buildup affects the mechanics of the capacitor,causing the capacitance to

vary.In this case,capacitance is defined in terms of incremental changes:

In SI units,a capacitance of one farad means that one coulomb of charge on each conductor

causes a voltage of one volt across the device.

Energy storage

Work must be done by an external influence to move charge between the conductors in a

capacitor.When the external influence is removed, the charge separation persists and energy is

stored in the electric field.If charge is later allowed to return to its equilibrium position,the

energy is released.

22

The work done in establishing the electric field,and hence the amount of energy stored,is given

by:

Current-voltage relation

The current i(t) through a component in an electric circuit is defined as the rate of flow of the

charge q(t) that has passed through it.Physical charges cannot pass through the dielectric layer of

a capacitor,but rather build up in equal and opposite quantities on the electrodes:as each electron

accumulates on the negative plate,one leaves the positive plate.Thus the accumulated charge on

the electrodes is equal to the integral of the current,as well as being proportional to the voltage

(as discussed above). As with any antiderivative,a constant of integration is added to represent

the initial voltage v (t0).This is the integral form of the capacitor equation,

Taking the derivative of this,and multiplying by C,yields the derivative form,

.

The dual of the capacitor is the inductor,which stores energy in the magnetic field rather than the

electric field.Its current-voltage relation is obtained by exchanging current and voltage in the

capacitor equations and replacing C with the inductance L.

23

Non-ideal behavior:

Capacitors deviate from the ideal capacitor equation in a number of ways.Some of these,such as

leakage current and parasitic effects are linear,or can be assumed to be linear,and can be dealt

with by adding virtual components to the equivalent circuit of the capacitor.

The usual methods of network analysis can then be applied. In other cases,such as with

breakdown voltage,the effect is non-linear and normal (i.e., linear) network analysis cannot be

used,the effect must be dealt with separately.There is yet another group,which may be linear but

invalidate the assumption in the analysis that capacitance is a constant.Such an example is

temperature dependence.

Breakdown voltage

Above a particular electric field,known as the dielectric strength Eds,the dielectric in a capacitor

becomes conductive.The voltage at which this occurs is called the breakdown voltage of the

device,and is given by the product of the dielectric strength and the separation between the

conductors,

Vbd = Edsd

The maximum energy that can be stored safely in a capacitor is limited by the breakdown

voltage.Due to the scaling of capacitance and breakdown voltage with dielectric thickness,all

capacitors made with a particular dielectric have approximately equal maximum energy

density,to the extent that the dielectric dominates their volume.

For air dielectric capacitors the breakdown field strength is of the order 107 V/m and will be

much less when other materials are used for the dielectric.The absolute breakdown voltage of

most capacitors is nowhere near such a high number because of the very small distance between

the plates.Typical ratings for capacitors used for general electronics applications range from a

few volts to 100V or so.For high voltage applications physically much larger capacitors have to

24

be used.In this field, there are a number of factors that can dramatically reduce the breakdown

voltage below the value to be expected by considering the breakdown field strength of the

dielectric alone.For one thing,the geometry of the capacitor conductive parts (plates and

connecting wires) is important.In particular,sharp edges or points hugely increase the electric

field strength at that point and can lead to a local breakdown.Once this starts to happen,the

breakdown will quickly "track" through the dielectric till it reaches the opposite plate and cause a

short circuit. The usual breakdown route is that the field strength becomes large enough to pull

electrons in the dielectric from their atoms thus causing conduction.Other scenarios are

possible,such as impurities in the dielectric,and,if the dielectric is of a crystalline

nature,imperfections in the crystal structure can result in an avalanche breakdown as seen in

semi-conductor devices.Breakdown voltage is also affected by pressure,humidity and

temperature.

Applications

Capacitors have many uses in electronic and electrical systems.They are so common that it is a

rare electrical product that does not include at least one for some purpose.

Energy storage

A capacitor can store electric energy when disconnected from its charging circuit, so it can be

used like a temporary battery.Capacitors are commonly used in electronic devices to maintain

power supply while batteries are being changed. (This prevents loss of information in volatile

memory.)

Conventional electrostatic capacitors provide less than 360 joules per kilogram of energy

density,while capacitors using developing technologies can provide more than 2.52 kilo joules

per kilogram.

25

Diodes

1N4148:

1N4148 diodes

Fig 3.4.1: General Diode

The 1N4148 is a standard small signal silicon diode used in signal processing.Its name follows

the JEDEC nomenclature.The 1N4148 is generally available in a DO-35 glass package and is

very useful at high frequencies with a reverse recovery time of no more than 4ns.This permits

rectification and detection of radio frequency signals very effectively,as long as their amplitude

is above the forward conduction threshold of silicon (around 0.7V) or the diode is biased.

Specifications:

VRRM = 100V (Maximum Repetitive Reverse Voltage)

IO = 200mA (Average Rectified Forward Current)

IF = 300mA (DC Forward Current)

IFSM = 1.0 A (Pulse Width = 1 sec),4.0 A(Pulse Width = 1 uSec) (Non-

Repetitive Peak Forward Surge Current)

PD = 500 mW (power Dissipation)

TRR < 4ns (reverse recovery time)

1N400X:

1N4001 diode

26

The 1N400X diodes are a popular 1.0 amp general purpose rectifier family, commonly used

in AC adapters for common household appliances. Blocking voltage varies from 50-1000V.

Comes in a axial-lead DO-41 plastic package.

The table below shows the blocking voltages of each of the members of the 1N400X family.

Model

number

DC Blocking

voltage / V

1N4001 50

1N4002 100

1N4003 200

1N4004 400

1N4005 600

1N4006 800

1N4007 1000

Table 3.4.1: IN400X family

27

555 timer IC

NE555 from signetics in dual-in-line package is shown as

Internal block diagram

Fig 3.5.1: General 555 IC Internal Block Diagram

The 555 Timer IC is an integrated circuit (chip) implementing a variety of timer and

multivibrator applications.The IC was designed in 1970 and brought to market in 1971 by

Signetics .The original name was the SE555 (metal can)/NE555 (plastic DIP) and the part was

described as "The IC Time Machine".It has been claimed that the 555 gets its name from the

three 5 kΩ resistors used in typical early implementations,but Hans Camenzind has stated that

the number was arbitrary. The part is still in wide use and haslow price and good stability.

28

Depending on the manufacturer,the standard 555 package includes over 20 transistors, 2 diodes

and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).Variants

available include the 556 (a 14-pin DIP combining two 555s on one chip),and the 558 (a 16-pin

DIP combining four slightly modified 555s with DIS & THR connected internally, and TR

falling edge sensitive instead of level sensitive).

Ultra-low power versions of the 555 are also available, such as the 7555 and TLC555.The 7555

requires slightly different wiring using fewer external components and less power.

The 555 has three operating modes:

Monostable mode:

In this mode,the 555 functions as a "one-shot". Applications include timers,missing

pulse detection,bouncefree switches, touch switches,frequency divider,capacitance

measurement,pulse-width modulation (PWM) etc.

Operation

Fig 3.5.2 :IC555 timer in Monostable mode

29

This circuit diagram shows how a 555 timer IC is configured to function as a basic

monostable multivibrator. A monostable multivibrator is a timing circuit that changes state

once triggered, but returns to its original state after a certain time delay.  It got its name

from the fact that only one of its output states is stable.  It is also known as a 'one-shot'.

 In this circuit,a negative pulse applied at pin 2 triggers an internal flip-flop that turns off

pin 7's discharge transistor,allowing C1 to charge up through R1.At the same time,the flip-

flop brings the output (pin 3) level to 'high'.When capacitor C1 as charged up to about 2/3

Vcc, the flip-flop is triggered once again, this time making the pin 3 output 'low' and

turning on pin 7's direction.

Astable - free running mode:

Operation Of IC 555 in Astable Mode :

The 555 can operate as an oscillator.Uses include LED and lamp flashers,pulse generation,logic

clocks,tone generation,security alarms,pulse position modulation etc.

30

Waveforms of IC 555 in Astable Mode :

   

This circuit diagram shows how a 555 timer IC is configured to function as an astable

multivibrator.An astable multivibrator is a timing circuit whose 'low' and 'high' states are

both unstable.As such,the output of an astable multivibrator toggles between 'low' and

'high' continuously,in effect generating a train of pulses.This circuit is therefore also

known as a 'pulse generator' circuit.

 In this circuit,capacitor C1 charges through R1 and R2,eventually building up enough

voltage to trigger an internal comparator to toggle the output flip-flop.Once toggled,the

flip-flop discharges C1 through R2 into pin 7,which is the discharge pin.When C1's

voltage becomes low enough, another internal comparator is triggered to toggle the output

flip-flop.This once again allows C1 to charge up through R1 and R2 and the cycle starts all

over again.

  C1's charge-up time t1 is given by: t1 = 0.693(R1+R2)C1.C1's discharge time t2 is given

by: t2 = 0.693(R2)C1.Thus,the total period of one cycle is t1+t2 = 0.693 C1(R1+2R2). 

The frequency f of the output wave is the reciprocal of this period,and is therefore given

by: f = 1.44/(C1(R1+2R2)),wherein f is in Hz if R1 and R2 are in megaohms and C1 is in

microfarads.  

  

31

Bistable mode or Schmitt trigger:

Operation:

The most common 555 configurations are that of a Astable and Monostable Multivibrator. This

shows how a 555 may be used as a Bistable Multivibrator. While a 555 has a Set/Reset flip flop

built in (another type of Bistable Multivibrator) this circuit uses the concept of hysteresis to

accomplish the same thing

If you use the red wire shown on the illustration as a toggle the LEDs will flip states, and stay

that way until the circuit is toggled again. It will work over the entire power supply voltage range

of the 555, which is 4.5V to 15V. A CMOS 555 will also work well for this circuit, although it

may have the trouble to drive the LEDs directly.

Schematic

Fig 3.5.3: 555 IC Bistalble mode.

32

THEORY OF OPERATION

Because R3 and R4 creates a voltage that is exactly in the middle of the dead zone of the

Schmitt Trigger hysteresis the output of the 555 is stable.It will hold the last state it was

set in indefinitely while there is power.The capacitor C1 is at the same voltage as the

output of the timer.When the toggle button is pushed the capacitor will put the same

voltage on the input,causing the 555 (an inverter),to flip states.The capacitor will quickly

charge or discharge to the voltage level that the network of R3, R4, R5 now presents.Since

R5 is X10 larger than R3 and R4 this voltage will still be in the dead zone of the Schmitt

Trigger and the output of the 555 is stable in its new state.When the toggle button is

released the capacitor will again charge or discharge to its new voltage.The capacitor is

being used as a memory to compliment the 555.It also makes a fairly convenient debounce

for the button.

This circuit concept will work for all inverting Schmitt Triggers, though R5 may have to be

increased to keep the transitions in the dead zone of the hysteresis.

The 555 can operate as a flip-flop,if the DIS pin is not connected and no capacitor is used.Uses

include bounce free latched switches, etc.

33

Specifications

These specifications apply to the NE555. Other 555 timers can have better specifications

depending on the grade (military, medical, etc).

Supply voltage (VCC)

Supply current (VCC = +5 V)

Supply current (VCC = +15 V )

Output current (maximum)

Power dissipation

Operating temperature

4.5 to 15 V

3 to 6 mA

10 to 15 mA

200 mA

600 mW

0 to 70°C

Table 3.5.1:IC 555 Timer Specifications

33

IC 7805:

The 78xx (also sometimes known as LM78xx) series of devices is a family of self-contained

fixed linear voltage regulator integrated circuits.The78xx family is a very popular choice for

many electronic circuits which require a regulated power supply,due to their ease of use and

relative cheapness.When specifying individual ICs within this family,the xx is replaced with a

two-digit number,which indicates the output voltage the particular device is designed to provide

(for example,the 7805 has a 5volt output,while the 7812 produces 12 volts).The 78xx line are

positive voltage regulators,meaning that they are designed to produce a voltage that is positive

relative to a common ground.There is a related line of 79xx devices which are complementary

negative voltage regulators.78xx and 79xx ICs can be used in combination to provide both

positive and negative supply voltages in the same circuit,if necessary.

78xx ICs have three terminals and are most commonly found in the TO220 form factor,although

smaller surface-mount and larger TO3 packages are also available from some

manufacturers.These devices typically support an input voltage which can be anywhere from a

couple of volts over the intended output voltage,up to a maximum of 35 or 40 volts,and can

typically provide up to around 1or 1.5 amps of current (though smaller or larger packages may

have a lower or higher current rating).

Advantages

The 78xx series has several key advantages over many other voltage regulator circuits which

have resulted in its popularity:

34

78xx series ICs do not require any additional components to provide a constant,regulated

source of power,making them easy to use,as well as economical,and also efficient uses of

circuit board real estate.By contrast, most other voltage regulators require several

additional components to set the output voltage level,or to assist in the regulation

process.Some other designs (such as a switching power supply) can require not only a

large number of components but also substantial engineering expertise to implement

correctly as well.

78xx series ICs have built-in protection against a circuit drawing too much power.They

also have protection against overheating and short-circuits, making them quite robust in

most applications.In some cases,the current-limiting features of the 78xx devices can

provide protection not only for the 78xx itself,but also for other parts of the circuit it is

used in,preventing other components from being damaged as well.

Disadvantages

The 78xx devices have a few drawbacks which can make them unsuitable or less desirable for

some applications:

The input voltage must always be higher than the output voltage by some minimum

amount (typically 2 volts).This can make these devices unsuitable for powering some

devices from certain types of power sources(for example, powering a circuit which

requires 5 volts using 6-volt batteries will not work using a 7805).

As they are based on a linear regulator design,the input current required is always the

same as the output current.As the input voltage must always be higher than the output

27voltage,this means that the total power (voltage multiplied by current) going into the

78xx will be more than the output power provided The extra input power is dissipated as

heat.This means both that for some applications an adequate heatsink must be

provided,and also that a(often substantial)portion of the input power is wasted during the

35

process,rendering them less efficient than some other types of power supplies.When the

input voltage is significantly higher than the regulated output voltage (for example,

powering a 7805 using a 24 volt power source), this inefficiency can be a significant

issue.

Even in larger packages,78xx integrated circuits cannot supply as much power as many

designs which use discrete components,and therefore are generally not appropriate for

applications which require more than a few amps of current.

IC74LS164:

A serial-in/parallel-out shift register is similar to the serial-in/ serial-out shift register in that it

shifts data into internal storage elements and shifts data out at the serial-out, data-out, pin. It is

different in that it makes all the internal stages available as outputs. Therefore, a

serial-in/parallel-out shift register converts data from serial format to parallel format. If four data

bits are shifted in by four clock pulses via a single wire at data-in, below, the data becomes

available simultaneously on the four Outputs QA to QD after the fourth clock pulse.

The practical application of the serial-in/parallel-out shift register is to convert data from serial

format on a single wire to parallel format on multiple wires. Perhaps, we will illuminate four

LEDs (Light Emitting Diodes) with the four outputs (QA QB QC QD ).

36

Fig3.7.1:Serial In/Parallelout Shift Register

The above details of the serial-in/parallel-out shift register are fairly simple.It looks like a serial-

in/ serial-out shift register with taps added to each stage output. Serial data shifts in at SI (Serial

Input).After a number of clocks equal to the number of stages,the first data bit in appears at SO

(QD) in the above figure. In general, there is no SO pin.The last stage (QD above) serves as SO

and is cascaded to the next package if it exists.

If a serial-in/parallel-out shift register is so similar to a serial-in/serial-out shift register,why do

manufacturers bother to offer both types?Why not just offer the serial-in/parallel-out shift

register? They actually only offer the serial-in/parallel-out shift register,as long as it has no more

than 8-bits.Note that serial-in/ serial-out shift registers come in gigger than 8-bit lengths of 18 to

to 64-bits.It is not practical to offer a 64-bit serial-in/parallel-out shift register requiring that

many output pins.See waveforms below for above shift register.

37

The shift register has been cleared prior to any data by CLR',an active low signal,which clears all

type D Flip-Flops within the shift register.Note the serial data 1011 pattern presented at the SI

input.This data is synchronized with the clock CLK.This would be the case if it is being shifted

in from something like another shift register,for example,a parallel-in/serial-out shift register (not

shown here).On the first clock at t1,the data 1 at SI is shifted from D to Q of the first shift

register stage.After t2 this first data bit is at QB. After t3 it is at QC.After t4 it is at QD.Four clock

pulses have shifted the first data bit all the way to the last stage QD.The second data bit a 0 is at

QC after the 4th clock.The third data bit a 1 is at QB.The fourth data bit another 1 is at QA.Thus,

the serial data input pattern 1011 is contained in (QD QC QB QA).It is now available on the four

outputs.

It will available on the four outputs from just after clock t4 to just before t5.This parallel data

must be used or stored between these two times, or it will be lost due to shifting out the Q D stage

on following clocks t5 to t8 as shown above.

38

IC74LS00:

Fig 3.7.2:IC74LS00 Internal Parts

Table3: Electircal Charateristics

39

IC74LS86:

Maximum Ratings

40

Recommended Ratings

AC Electrical Characteristics

Dielectric Characteristics

41

Silicon-controlled rectifier (SCR):

A silicon-controlled rectifier (or semiconductor-controlled rectifier)is a four-layer solid state

device that controls current.

Construction of SCR

An SCR consists of four layers of alternating P and N type semiconductor materials.Silicon is

used as the intrinsic semiconductor,to which the proper dopants are added.The junctions are

either diffused or alloyed.The planar construction is used for low power SCRs (and all the

junctions are diffused).The mesa type construction is used for high power SCRs.In this

case,junction J2 is obtained by the diffusion method and then the outer two layers are alloyed to

it,since the PNPN pellet is required to handle large currents.It is properly braced with tungsten or

molybdenum plates to provide greater mechanical strength.One of these plates is hard soldered to

a copper stud,which is threaded for attachment of heat sink.The doping of PNPN will depend on

the application of SCR

Modes of operation

In the normal "off" state,the device restricts current to the leakage current.When the gate-to-

cathode voltage exceeds a certain threshold,the device turns "on" and conducts current.The

device will remain in the "on" state even after gate current is removed so long as current through

the device remains above the holding current. Once current falls below the holding current for an

appropriate period of time,the device will switch "off".If the gate is pulsed and the current

through the device is below the holding current,the device will remain in the "off " state.

If the applied voltage increases rapidly enough,capacitive coupling may induce enough charge

into the gate to trigger the device into the "on" state;this is referred to as "dv/dt triggering." This

is usually prevented by limiting the rate of voltage rise across the device,perhaps by using a

snubber."dv/dt triggering" may not switch the SCR into full conduction rapidly and the partially-

triggered SCR may dissipate more power than is usual, possibly harming the device.

42

SCRs can also be triggered by increasing the forward voltage beyond their rated breakdown

voltage (also called as break over voltage),but again,this does not rapidly switch the entire device

into conduction and so may be harmful so this mode of operation is also usually

avoided .Also ,the actual breakdown voltage may be substantially higher than the rated

breakdown voltage,so the exact trigger point will vary from device to device.

SCRs are made with voltage ratings of up to 7,500 V,and with current ratings up to 3,000 RMS

amperes per device.Some of the larger ones can take over 50 kA in single-pulse operation.SCRs

are used in power switching,phase control,chopper, battery chargers,and inverter circuits.

Industrially they are applied to produce variable DC voltages for motors (from a few to several

thousand HP) from AC line voltage.They control the bulk of the dimmers used in stage

lighting,and can also be used in some electric vehicles to modulate the working voltage in a

Jacobson circuit.Another common application is phase control circuits used with inductive

loads.SCRs can also be found in welding power supplies where they are used to maintain a

constant output current or voltage.Large silicon-controlled rectifier assemblies with many

individual devices connected in series are used in high-voltage DC converter stations.

Two SCRs in"inverse parallel"are often used in place of a TRIAC for switching inductive loads

on AC circuits.Because each SCR only conducts for half of the power cycle and is reverse-biased

for the other half-cycle,turn-off of the SCRs is assured.By comparison,the TRIAC is capable of

conducting current in both directions and assuring that it switches "off" during the brief zero-

crossing of current can be difficult.

Typical electrostatic discharge (ESD) protection structures in integrated circuits produce a

parasitic SCR.This SCR is undesired;if it is triggered by accident,the IC can go into latch up and

potentially be destroyed.

43

Variable Resistors:

Construction

Variable resistors consist of a resistance track with connections at both ends and a wiper which

moves along the track as you turn the spindle.The track may be made from carbon,cermet

(ceramic and metal mixture) or a coil of wire (for low resistances).The track is usually rotary but

straight track versions,usually called sliders,are also available.

Variable resistors may be used as a rheostat with two connections (the wiper and just one end of

the track) or as a potentiometer with all three connections in use. Miniature versions called

presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues.They are specified by

their maximum resistance, linear or logarithmic track,and their physical size.The standard

spindle diameter is 6mm.Some variable resistors are designed to be mounted directly on the

circuit board, but most are for mounting through a hole drilled in the case containing the circuit

with stranded wire connecting their terminals to the circuit board.

Fig 3.7.3:Standard Variable Resistor

44

Linear (LIN) and Logarithmic (LOG) tracks:

Linear (LIN) track means that the resistance changes at a constant rate as you move the

wiper.This is the standard arrangement and you should assume this type is required if a project

does not specify the type of track.Presets always have linear tracks.

Logarithmic (LOG) track means that the resistance changes slowly at one end of the track and

rapidly at the other end, so halfway along the track is not half the total resistance! This

arrangement is used for volume (loudness) controls because the human ear has a logarithmic

response to loudness so fine control (slow change) is required at low volumes and coarser control

(rapid change) at high volumes

It is important to connect the ends of the track the correct way round, if you find that turning the

spindle increases the volume rapidly followed by little further change you should swap the

connections to the ends of the track.

Rheostat:

This is the simplest way of using a variable resistor.Two terminals are used:one connected to an

end of the track,the other to the moveable wiper.Turning the spindle changes the resistance

between the two terminals from zero up to the maximum resistance.

Rheostats are often used to vary current,for example to control the brightness of a lamp or the

rate at which a capacitor charges.

If the rheostat is mounted on a printed circuit board you may find that all three terminals are

connected! However,one of them will be linked to the wiper terminal. This improves the

mechanical strength of the mounting but it serves no function electrically.

45

Rheostat Symbol

Potentiometer:

A potentiometer is constructed with a resistive element formed into an arc of a circle,and a

sliding contact (wiper) travelling over that arc.The resistive element, with a terminal at one or

both ends,is flat or angled, and is commonly made of graphite,although other materials may be

used. The wiper is connected through another sliding contact to another terminal.On panel

potentiometers,the wiper is usually the center terminal of three.For single-turn potentiometers,

this wiper typically travels just under one revolution around the contact."Multiturn"

potentiometers also exist,where the resistor element may be helical and the wiper may move 10,

20,or more complete revolutions,though multiturn potentimeters are usually constructed of a

conventional resistive element wiped via a worm gear. Besides graphite,materials used to make

the resistive element include resistance wire,carbon particles in plastic,and a ceramic/metal

mixture called cermet.

One form of rotary potentiometer is called a String potentiometer. It is a multi-turn potentiometer

operated by an attached reel of wire turning against a spring. It is used as a position transducer.

In a linear slider potentiometer, a sliding control is provided instead of a dial control. The

resistive element is a rectangular strip, not semi-circular as in a rotary potentiometer. Due to the

large opening slot or the wiper, this type of potentiometer has a greater potential for getting

contaminated.

Potentiometers can be obtained with either linear or logarithmic relations between the slider

position and the resistance (potentiometer laws or "tapers"). A letter code ("A" taper, "B" taper,

etc.) may be used to identify which taper is intended, but the letter code definitions are variable

over time and between manufacturers.

46

Manufacturers of conductive track potentiometers use conductive polymer resistor pastes that

contain hard wearing resins and polymers, solvents, lubricant and carbon – the constituent that

provides the conductive/resistive properties. The tracks are made by screen printing the paste

onto a paper based phenolic substrate and then curing it in an oven. The curing process removes

all solvents and allows the conductive polymer to polymerize and cross link. This produces a

durable track with stable electrical resistance throughout its working life

Variable resistors used as potentiometers have all three terminals connected.This arrangement is

normally used to vary voltage,for example to set the switching point of a circuit with a

sensor,or control the volume (loudness) in an amplifier circuit.If the terminals at the ends of the

track are connected across the power supply then the wiper terminal will provide a voltage

which can be varied from zero to the maximum supply

(Potentiometer Symbol)

Presets:

These are miniature versions of the standard variable resistor. They are designed to be mounted

directly onto the circuit board and adjusted only when the circuit is built. For example to set the

frequency of an alarm tone or the sensitivity of a light-sensitive circuit. A small screwdriver or

similar tool is required to adjust presets.

Presets are much cheaper than standard variable resistors so they are sometimes used in projects

where a standard variable resistor would normally be used.

Multiturn presets are used where very precise adjustments must be made. The screw must be

turned many times (10+) to move the slider from one end of the track to the other, giving very

fine control.

47

CHAPTER - 4

FUTURE SCOPE AND REFERENCE

48

Future Scope :

This circuit that can produce effect of candle light in a normal electric bulb. A candle light as

we know, resembles a random flickering light. So the main idea of this project is to produce a

normal flickering light effect in an electric bulb .

Advantages:

Here by the implementation of the this electrical candle we can have many advantages

1 .Normal Candle lasts for some particular time,we cannot have equal brightness at all the

time.But when coming to electronic candle we can have light upto the time we required and the

brightness cannot be altered for some time duration.

2. As the electronic candle is powerd by A.C power and there is not soot formed on the walls

3.In Future by the integration of solar cells in the circuit,we can use the solar power for

illuminating the electronic candle.

4.By the insetion of power storing components we can charge this candle and can be used

whenever we needed.

49

CHAPTER - 5

BIBLIOGRAPHY

50

BIBLIOGRAPHY

References :

Electronic Devices and Circuits - J.Milliman,C.CHakias –Tata McGraw Hill 2nd edition .

Principles Of Electronic Circuits - S.G.Burns Galgotia Publications, 2nd Edition

Solid State Pulse Circuits – David A.Bell,PHI, 4th edition .

Op-Amps & Linear ICs – Ramakanth A.Gayakwad, PHI

Operational Amplifiers – C.G.Clayton

Linear Integrated Circuits – Dr.D.Roy Chowdury .New Age International,2nd edition

Web Sites :

www.microchips.com

www.howstuffworks.com

www.atmel.com

51