DESIGN AND CONSTRUCTION OF A

MICROCONTROLLER BASED AUTOMATIC SELECTOR

FOR MULTIPLE AC SOURCES

BY

OTUBELU, N.N 2000/97343

OKEKE, I.O 2000/105106

UMOUMOH, I.U 2000/103555

AMADI, P 2000/97344

OBIJIOFOR, O.C 20000/103446

DEPARTMENT OF ELECTRONIC ENGINEERING

UNIVERSITY OF NIGERIA, NSUKKA

AUGUST 2006

TITLE PAGE

DESIGN AND CONSTRUCTION OF A MICROCONTROLLER

BASED AUTOMATIC SELECTOR FOR MULTIPLE AC

SOURCES

DEDICATION

To God Almighty, and to our parents for their understanding, perseverance and financial

support.

ACKNOWLEDGEMENT

Our sincere gratitude goes to all those who in one way or another contributed to the

success of this work. The limit of space will not permit us to mention all.

Most especially, we wish to thank our project supervisor Miss Ogechi Iloanusi for

her guidance, encouragement and support.

We are also very grateful to the following people; Bro Okey Ndu, Emma Guru and

CCP who motivated and supported us in the course of this work.

For life, health, provision, wisdom, and grace, our heart bursts forth in Praise to the

Almighty God who has done awesome things for us.

ABSTRACT

The incidence of power outages in Nigeria is very frequent. The reasons for this

irregularity of supply are numerous and diverse. This is a primary hindrance to economic

growth. In an attempt to remedy this situation, several independent power providers are

springing up. The result is that in the not too distant future, industries and other places

requiring uninterrupted power supply may have multiple sources of power supply to

increase the reliability of power supply. The authors see an automation of the selection of

the available power sources as an efficient way to manage the situation.

In this work, the authors evolved a design of and constructed a microcontroller-

based system that carries out this selection with little or no human supervision. This thesis

documents the library research, design, construction, testing, and software development

processes carried out by the authors to construct a working prototype of this system.

To enhance readability and understanding, the material adopts a top down approach

where most concepts are first introduced generally and then later specified in greater

detail.

To realize this goal, the paper is divided into five chapters. Chapter one discusses

the objective of the project as well as the necessity of programmed design. Chapter two

was devoted to an in-depth study of the components that constitute this system and their

working principles.

Chapter three discussed the system hardware design procedure and construction of

the different modules and consequently hardware integration and testing. Chapter four is

devoted to the software requirements and development as well as the overall system

testing. Finally, Chapter five assesses the project, discussing problems encountered and

recommendations for subsequent improvement.

Relevant components data are provided in the appendix.

TABLE OF CONTENTS Title Page .…………………………………………………………………….…….……...i

Dedication…………………………………………………………………………………..ii

Acknowledgement………………………………………………………………………....iii

Abstract………………………………………………………………………………..…..iv

Table of Contents…………………………………………………………………..…… ..v

CHAPTER ONE-INTRODUCTION

1.1 Overview of Electric Power Supply in Nigeria ………………………..…………....1

1.2 Objective of the Project ………………………………………………………..…...3

1.3 Emphasis on Programmed Design ……………………………………………….....3

1.4 Microprocessing Systems ……………………………………………………….....4

1.4.1 Microprocessor Specifications …………………………………………………...5

1.4.2 Microcontrollers ………………………………………………………………….7

1.4.3 The 8051\8052 Families of Microcontrollers …………………………………….8

CHAPTER TWO-LITERATURE REVIEW

2.1 Power Supplies …………………………………………………………………...10

2.1.1 Transformers ……………………………………………………………………..11

2.1.2 Voltage Rectification ……………………………………………………….……15

2.1.3 Filters ..…………………………………………………………………………....16

2.1.4 Voltage Regulation………………………………………………………………..17

2.1.5 Voltage Divider ……………………………………………………………….….17

2.2 Multiplexers……………………………. ………………………………….…….18

2.3 The Analog to Digital Converter.…………………………...………………..…..20

2.4 The 89C52 Microcontroller………………………………………………………24

2.4.1 Memory Organization……………………………………………………………25

2.4.2 Addressing Modes ……………………………………………………….…….. 27

2.4.3 89C52 Pin Configuration ………………………………………………………. 30

2.5 Relays …………………………………………………………………………. 32

2.6 Seven Segment Displays ………………………………………………………...34

2.7 Transistors and Capacitors………………………………………………………..35

CHAPTER THREE-HARDWARE DEVELOPMENT

3.1 System Overview (Block Diagram)………………………………………………42

3.2 System Specification and Design…………………………………………………43

3.3 Voltage Rectification and Regulation ……………………………………………44

3.4 The Analog-Digital Switch……………………………………………………….45

3.5 The Analog-Digital Interface……………………………………………………. 46

3.6 Microcontroller Module…………………………………………………………..47

3.7 Display Circuitry ………………………………………………………………....49

3.8 The Relay Circuit ………………………………………………………………...51

3.9 Hardware Integration and Testing …………………………………………….....52

CHAPTER FOUR-SOFTWARE DEVELOPMENT AND SYSTEM TESTIN G

4.1 Software requirement specification………………………………………………..55

4.2 Program design (Flowchart)……………………………………………………….55

4.3 Software Development Tools……………………………………………………...57

4.4 Programming the 89C52…………………………………………………………..58

4.5 System Integration and Testing…………………………………………………….59

CHAPTER FIVE-CONCLUSION

5.1 Summary……………………………………………………………………………60

5.2 Challenges Faced…………………………………………………………………...60

5.3 Cost Analysis……………………………………………………………………….61

5.4 Recommendations For Further Study……………………………………………....61

REFERENCES APPENDIX

CHAPTER ONE

INTRODUCTION

1.1 OVERVIEW OF ELECTRIC POWER SUPPLY IN NIGERIA

The unpredictability of power supply in many African countries is one of the main

hindrances to economic growth. The reasons for this irregularity of supply are numerous

and diverse. Wars across the continent have left generation facilities damaged and

transmission lines cut. Apart from the physical damage caused by sabotage and war, many

governments’ budgets have been stretched to the extent that maintenance of facilities has

become a low priority. In addition, many countries have been left with unreliable, aging

equipment with little means of upgrading. Nigeria is a prime example, operating at

approximately one-third of its installed capacity due to aging facilities.

The demand for power in Nigeria is growing at a rate that is leaving the country

unable to keep up. Rural electrification initiatives, low electrification levels at present and

high population growth rates all contribute to this high growth in demand. To meet

demand, new installations are required on a regular basis. Unfortunately the ailing

economy is posing a challenge to this required solution.

In an attempt to address the issue of power supply reliability, countries have

adopted a number of strategies. Many African governments are starting to view

hydroelectric power with scepticism and are viewing a move away from hydropower

dependency as the best long-term solution to reliability problems. Photovoltaic power

generation is a possible supplement to hydroelectric power. Although its conversion

efficiency is not high relative to hydroelectric power, Current conversion efficiencies have

surpassed 30% in the laboratory, and 15% in large-scale production.

Ingenious private sector driven power providers exist whose goal is to construct

and operate Independent Power Plants (IPP) for the generation, transmission and

distribution of power to industrial clusters, commercial entities and the general public in

Nigeria. The Geometric Power Limited (GPL) is an example of such companies. It is

desirous to complement the Nigerian government’s effort to improve the power sector by

bridging the energy gap in a timely manner, and to alleviate the pains of several

industrialists in the country. Such initiatives will revolutionize the power sector in Nigeria

in the near future.

The GPL’s Aba model plant is strategically located in Aba, the commercial capital

city of Abia State, South Eastern Part of Nigeria because of its large cluster of sustainable

industries that need reliable electric power. The plant is designed to provide a model for

the emergence of high reliability in power supply especially for the industrial sector, and

to drive positive economic growth in Nigeria and trade within Sub-Saharan Africa. This

privately owned power plant in Nigeria would serve the industrial and commercial

customers, as well as residences using a specially installed and designated high quality

private distribution network.

International investors led by International Finance Corporation finance the

project. It is strongly supported by several local and International organizations. These

include amongst others, The World Bank; European Investment Bank; Shell Nigeria Gas

Company; The Federal Ministry of Power and Steel in Nigeria; The Bureau of Public

Enterprises (BPE) Nigeria; Owners of the major Industries in Aba and the Aba

Community in general. This project has become a pace setting IPP whose model is

expected to be replicated in other parts of Nigeria.

1.2 OBJECTIVE OF THE PROJECT

The aim of this project is to design a system that will automatically select an

optimum power source from multiple sources for an application. The basis of this

selection is with reference to an optimum voltage of 220V with a specified tolerance range

of ± 20V.In the event of a power failure, a standby generator is switched on. Power failure

in this context implies absence of power supply from the mains or when the power

supplied is not within the tolerance range.

We will employ programmed design to measure and effectively select one out of

six possible ac voltage sources for an application. Its aim is to improve reliability of power

supply to residences, industries, football pitches, hospital theatres, telephone exchanges

and any other situation requiring uninterrupted power supply.

1.3 THE EMPHASIS ON PROGRAMMED DESIGN

Digital systems design can be carried out in two ways; programmed design and

hard-wired approach. In programmed design, the functions realized reside in the program

while that of the hard-wired approach resides in the interconnections of logic components.

Although programmed designs are slower than the hard-wired because of the fetch, decode

and execute of instruction process, it has the following advantages over the hard-wired

Small Scale Integration/Medium Scale Integration (SSI/MSI) IC approach.

Flexibility

The functions realized in the programmed design system reside in the program. The

system is flexible because the program conditions its behaviour and as a result

modifications are easily actualised at less cost. A hard-wired system will need to be

redesigned and reconstructed.

Reliability

Programmed design systems are more reliable than hard-wired logic. This is as a

result of very few large-scale integrated circuits employed in their implementation. Too

many devices cause faults and make troubleshooting difficult.

Less cost

The products realized from the programmed approach require less number of

components than the hardwired logic as they use very few large-scale integrated circuit

ICs. This is cost effective especially when considering large-scale production.

Less time

Programmed design technique offers minimized project implementation duration

because so many device functions are easily incorporated into the software without loss of

functionality and designer’s time. Design complexity is also reduced.

A microprocessor-based solution is considered whenever an application involves

making calculations, making decisions based on external stimulus, and maintaining

memory of past events.

1.4 MICROPROCESSING SYSTEMS

A system consists of several components interconnected in a specific way to fulfill

a specific function. A microprocessing system is a system in which a microprocessor is a

component providing computational control. It is also called a microcomputer, which is a

complete stand-alone computer capable of functioning without any additional equipment.

A microprocessor is a general-purpose digital device that is driven by software

instructions and communicates with several external support chips to perform the

necessary input/output of a specified task. It reads program instructions from memory and

executes those instructions that drive the three external buses with the proper levels and

timing to make the connected devices to perform specific operations. It is

an electronic circuit that functions as the central processing unit (CPU) of a computer,

providing computational control.

The microprocessor is an ultra-large-scale integrated circuit consisting of several

different sections: The Arithmetic Logic Unit (ALU) performs calculations on numbers

and makes logical decisions; the registers are special memory locations for storing

temporary information; the control unit deciphers programs; buses carry digital

information throughout the chip and computer; and local memory supports on-chip

computation. More complex microprocessors often contain other sections—such as

sections of specialized memory, called cache memory, to speed up access to external data-

storage devices.

1.4.1 MICROPROCESSOR SPECIFICATIONS

For the purpose of comparing efficiencies, judging compatibilities, both in physical

and software, safe and optimal application, and ultimately for the purpose of making a

good choice of microprocessors, like in other electronic devices, certain specifications are

usually made on each microprocessor model. Specifications provide information about the

processor’s components, capabilities and special features. Such specifications include:

1. Data- bus width

2. Address-bus width

3. Internal register size

4. Speed (in MHZ)/CPU clock multiples

5. Memory specifications

Data-bus width

The data-bus is a bi-directional set of lines or connectors which carry common

signals (Binary Coded Signals) and which are connected commonly to various devices

such as ROM, RAM, cache, 10-ports and the CPU of a computer such that any

information represented by binary code form by the ordered presence (or absence) of

“high” (1) or “low” (0) voltage-sent to the bus by the CPU or vice versa is made available

to all other devices which are commonly connected to the bus.

If N is the number of connectors that make up the bus, then this can stand for n-bits

binary number, which can represent 2n binary coded data, the bus is said to have bus width

of n-bits. The greater the value of n (i.e. the data bus width) then the greater the amount of

information/data in bytes that can be conveyed in a cycle.

Some processors have been designed such that the data bus width (external)

corresponds to half the internal register size (Intel 8088), the internal register size (Intel

8088) or twice the internal register size (in Intel Pentiums).

Address- bus width

The address bus is a set of conductors or lines that represents binary numbers

which are codes for memory location addresses. The address bus is used by the

microprocessor to select a particular memory location or IC to be active. It is

unidirectional. An n bit wide address bus is capable of uniquely specifying (2n ) different

addresses. The greater the width of the address bus the greater the size of the addressable

hardware memory.

Internal register size

This has two aspects, the size of the register and the number of registers. The size

of the register determines the amount of data that can be processed at a time. As registers

are closer and faster than main memory, the higher the number of internal registers, the

faster the processor is likely to be. Registers could be general purpose or special purpose.

Clock speed

There are two sides to the speed rating of a microprocessor. The first is the clock

speed, the second is the average instruction execution time.

The clock speed is the frequency of the clock. This frequency is measured in

Megahertz, which means the number of cycles per second. A pulse cycle represents the

smallest unit of time or interval of any operation in a system. Hence, the higher the

frequency, the faster its operation which include instruction execution and data transfer.

On the other hand, average instruction execution time expresses the number of

instruction a given processor can execute in one cycle. It can as well be seen as the

number of cycles on the average that it takes for the processor to execute a single

instruction. Intel 80286 has an average instruction execution time of 4.5 cycles per

instruction, where as 80486 has 2 cycles per instruction.

If two of these models, one from each kind are selected which both have the same

frequency, then because of the different average instruction execution time the 80486 will

perform at a rate double that of the 80286.We can say that if all other variables are equal,

including the type of processor, the number of wait-states added to different types of

memory accesses, the width of the data bus, one can compare two systems by their

respective clock rates.

Memory specification

This is specifically related to microcontrollers. They have ROM and RAM

memory included in the chip. Some of them may be ROM-less, some have masked ROMs

and some have on-chip EPROM (UV erasable). One advantage of the ROM-less version is

reduced cost.

1.4.2 MICROCONTROLLERS

A microcontroller is a single computer chip that executes a user program, normally

for the purpose of controlling some device. Its distinguishing feature is the inclusion on a

single chip, of all the resources which permit the IC to serve as a controller in a system or

an instrument. It is a complete computer on a chip containing all of the elements of the

basic microprocessor along with other specialized functions. Its built in facilities has

obvious advantages when building stand-alone front-ends and controllers.

A microcontroller is made up of CPU, RAM, ROM, I\O ports, timers, and serial

communication ports. It has wide applications in embedded systems that require some

amount of computing power but don’t require as much as that provided by a complex

processor. In an embedded system there is only one application software that is typically

burned into ROM. Microcontroller-based systems are generally smaller, more reliable and

cheaper. They are ideal for applications where cost and unit size are important

considerations. A typical example is the 8051\8052 family, which includes 8031,8051,

8052,8751,8752,8951, 8952, etc. the major producers of microcontrollers are Atmel and

Intel.

Certain factors are to be considered when choosing a microcontroller for an

application. They include:

• Speed, size of ROM|RAM, number of I|O ports, timers, power consumption

and cost per unit.

• Availability of software development tools like assemblers, debuggers,

compilers, simulators and technical support.

• Availability and reliable sources of microcontrollers. The 8051\8052

families are readily available.

1.4.3 THE 8051\8052 FAMILIES OF MICROCONTROLLERS

The 8051 is a standard controller widely employed for many applications. It is

manufactured with three variations, 8031,8051,and 8751.The 8031 is a ROM-less version,

requiring external ROMs for making a complete system. The ‘8051’ has masked program

ROM while the 8751contains on-chip EPROM (UV erasable)

The main hardware features of the 8051 series are the on-chip incorporation of

• Data RAM (128 Bytes)

• Special Function Registers (at least 21)

• 32 bit programmable I\O port

• UART (Universal Asynchronous Receiver Transmitter) for serial data input and

output

• Two programmable timers and counters

• Two external interrupts with priority and masking

• Four banks of eight temporary registers

The 8052 series of microcontrollers is very similar to the 8051.The 8052 is an 8-bit

microcontroller originally developed by Intel in the late 1970s.It included an instruction

set of 255 operation codes (opcodes), 32 input\output lines, three user –controllable timers,

an integrated and automatic serial port, and 256 bytes of on-chip RAM as opposed to the

8051’s 128 bytes of RAM with only two timers.

The 8052 was designed such that control of the microcontroller and all

input\output devices is accomplished via Special Function Registers (SFRs). Each SFR

has an address between 128 and 255.Additional functions can be added to new derivative

microcontrollers by adding additional SFRs while remaining compatible with the original

8052.This allows the developers to use the same software with any derivative

microcontroller chip that is 8052-compatible. A derivative chip will generally be able to

execute a standard 8052 program without modification. A derivative chip must be based

on the 8052 instruction set and support the appropriate SFRs which is at least 26 for an

8052.

CHAPTER TWO

LITERATURE REVIEW

2.1 POWER SUPPLIES

A power supply unit (PSU) is a device or system that supplies electrical or other

types of energy to an output load or group of loads. The term is most commonly applied to

electrical energy supplies. Constraints that commonly affect power supplies are the

amount of power they can supply, how long they can supply it without needing some kind

of refueling or recharging, how stable their output voltage or current is under varying load

conditions, and whether they provide continuous power or pulses.

Forms of Electrical power supplies

Electrical power supplies include the mains power distribution system together with any

other primary or secondary sources of energy such as:

� Batteries

� Chemical fuel cells and other forms of energy storage systems

� Solar power

� Generators or alternators

A simple AC power supply uses a transformer to convert the voltage from the wall

outlet to a lower voltage. A diode circuit then rectifies the AC voltage to pulsating DC. A

capacitor smoothens out most of the pulsating of the rectified waveform to give a DC

voltage with some ripple. Finally depending on the requirements of the load a linear

regulator may be used to reduce the voltage to the desired output voltage and remove the

majority of the remaining ripple. It may also provide other features such as current

limiting. This process is shown in figure 2.1 below:

Figure2.1: Block Diagram Showing Parts Of A Regulated Power Supply

2.1.1 TRANSFORMERS

A transformer is an electrical device that transfers energy from one circuit to

another by magnetic coupling with no moving parts. It comprises of two or more coupled

windings or a single tapped winding and, in most cases, a magnetic core to concentrate

magnetic flux. An alternating current in one winding creates a time-varying magnetic flux

in the core, which induces a voltage in the other windings. Transformers are used to

convert between high and low voltages, to change impedance, and to provide electrical

isolation between circuits. All transformers operate with the same basic principles and

with many similarities in their parts.

Basic Principles of Operation of a transformer

A simple transformer consists of two electrical conductors called the primary

winding and the secondary winding. These two windings can be considered as a pair of

mutually coupled coils. Energy is coupled between the windings by the time-varying

magnetic flux that passes through (links) both primary and secondary windings.

Figure 2.2: A Transformer Showing Magnetising Flux In The Core

If a time-varying voltage is applied to the primary winding of turns,

current will flow in it producing a magnetomotive force (MMF). Just as an electromotive

force (EMF) drives current around an electric circuit, so MMF tries to drive magnetic flux

through a magnetic circuit. The primary MMF produces a varying magnetic flux in

the core, and, with an open circuit secondary winding, induces a back electromotive force

(EMF) in opposition to . In accordance with Faraday's law of induction, the voltage

induced across the primary winding is proportional to the rate of change of flux:

and

where

vP and vS are the voltages across the primary winding and secondary winding,

NP and NS are the numbers of turns in the primary winding and secondary winding,

dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and

secondary windings.

Saying that the primary and secondary windings are perfectly coupled is equivalent

to saying that . Substituting and solving for the voltages shows that:

Hence in an ideal transformer, the ratio of the primary and secondary voltages is

equal to the ratio of the number of turns in their windings, or alternatively, the voltage per

turn is the same for both windings. The ratio of the currents in the primary and secondary

circuits is inversely proportional to the turns ratio. This leads to the most common use of

the transformer: to convert electrical energy at one voltage to energy at a different voltage

by means of windings with different numbers of turns. In a practical transformer, the

higher-voltage winding will have more turns, of smaller conductor cross-section, than the

lower-voltage windings.

An ideal transformer also has no loss (100% efficient) but in practice, energy is

dissipated due both to the resistance of the windings (known as copper loss), and to

magnetic effects primarily attributable to the core (known as iron loss). Transformers are,

in general, highly efficient. Transformer losses arise from Winding resistance, Hysteresis

losses, Magnetostriction, mechanical losses, and Stray losses. Also, not all the magnetic

field produced by the primary is intercepted by the secondary. A portion of the leakage

flux may induce eddy currents within nearby conductive objects, such as the transformer's

support structure, and be converted to heat.

Standard Transformer Circuit symbols

Transformer with two windings and iron core.

Transformer with three windings. The dots show the adjacent ends

of the windings.

Step-down or step-up transformer. The symbol shows which

winding has more turns but does not usually show the exact ratio.

Transformer with electrostatic screen, which prevents capacitive

coupling between the windings.

Table 2.1: Transformer Circuit Synbols

Transformer designs

Transformers can be designed in many ways such as autotransformer, polyphase

transformers, resonant transformers, audio transformers etc. autotransformer design will

be discussed in detail.

Autotransformers

An autotransformer has only a single winding, which is tapped at some point along

the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher

(or lower) voltage is produced across another portion of the same winding. While

theoretically separate parts of the winding can be used for input and output, in practice the

higher voltage will be connected to the ends of the winding, and the lower voltage from

one end to a tap. For example, a transformer with a tap at the center of the winding can be

used with 230 volts across the entire winding, and 115 volts between one end and the tap.

It can be connected to a 230 volt supply to drive 115 volt equipment, or reversed to drive

230 volt equipment from 115 volts. As the same winding is used for input and output, the

flux in the core is partially cancelled, and a smaller core can be used. For voltage ratios not

exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient

than a true (two-winding) transformer of the same rating.

In practice, transformer losses mean that autotransformers are not perfectly

reversible; one designed for stepping down a voltage will deliver slightly less voltage than

required if used to step up. The difference is usually slight enough to allow reversal where

the actual voltage level is not critical. By exposing part of the winding coils and making

the secondary connection through a sliding brush, an autotransformer with a near-

continuously variable turns ratio can be obtained, allowing for very small increments of

voltage.

Uses of transformers

• For supplying power from an alternating current power grid to equipment which

uses a different voltage. It may be followed by rectification if direct rather than

alternating power is needed.

• Adaptation of electrical equipment to supply voltages for which it was not made.

For example, to use U.S. equipment, designed for 115 V AC, in European

countries with 230 V AC. A transformer or autotransformer may be used.

• It is also used inside solid-state equipment that requires low voltages to reduce the

main electricity voltage to the required value.

2.1.2 VOLTAGE RECTIFICATION

Rectification is a process whereby an alternating current is converted into direct

current. A rectifier is used to achieve that. A rectifier is an electrical device, comprising

one or more semiconductive devices (such as diodes arranged for converting alternating

current to direct current. One diode can be used to rectify AC by blocking the negative or

positive portion of the waveform.The difference between the term diode and the term

rectifier is merely one of usage, e.g. the term rectifier describes a diode that is being used

to convert AC to DC. All rectifiers comprise a number of diodes in a specific arrangement

for more efficient conversion from AC to DC than is possible with just a single diode.

Rectification can be achieved using only one diode (half wave rectification) or

more than one diode (full wave rectification).

Half-wave rectification

Half wave rectification is the process of removing one half of the input signal to

establish a dc level. Either the positive or negative half of the AC wave is passed while the

other half is blocked, depending on the polarity of the rectifier. Because only one half of

the input waveform reaches the output, it is very inefficient if used for power transfer.

Figure 2.3: Waveform For Half Wave Rectification

Full-wave rectification

Full-wave rectification converts both polarities of the input waveform to DC, and

is more efficient. It can be realized using two rectifiers or four rectifiers (bridge

rectifier).

Figure2.4: Waveform For Full Wave Rectification Using A Bridge Rectifier

A full wave rectifier converts the whole of the input waveform to one of constant

polarity (positive or negative) at its output by reversing the negative (or positive) portions

of the alternating current waveform. The positive (negative) portions thus combine with

the reversed negative (positive) portions to produce an entirely positive (negative)

voltage/current waveform.

For single phase AC, if the AC is center-tapped, then two diodes back-to-back (i.e.

anodes-to-anode or cathode-to-cathode) form a full wave rectifier.

Figure 2.5: Waveform For Full Wave Rectification Using 2 Diodes

2.1.3 FILTERS

Half- wave and full-wave rectification delivers a form of DC output, neither

produces constant voltage DC. In order to produce steady DC from a rectified AC supply,

a smoothing circuit is required. In its simplest form this can be what is known as a

reservoir capacitor or smoothing capacitor, placed at the DC output of the rectifier. There

will still remain an amount of AC ripple voltage where the voltage is not completely

smoothed.

To further reduce this ripple, a capacitor-input filter can be used. This

complements the reservoir capacitor with a choke and a second filter capacitor, so that a

steadier DC output can be obtained across the terminals of the filter capacitor. The choke

presents a high impedance to the ripple current.

Figure 2.6: A Simple Capacitor Filter

2.1.4 VOLTAGE REGULATION

Another factor of importance in a power supply is the amount by which the dc

output voltage changes over a range of circuit operation. The function of the voltage

regulator is to keep the terminal voltage of the dc supply constant when the input voltage

to the transformer varies and the load varies. The regulators can be selected for operation

with load currents from hundreds of milliamperes to tens of amperes, corresponding to

power ratings from milliwatts to tens of watts.

2.1.5 VOLTAGE DIVIDER

The voltage divider circuit is used to reduce the regulated voltage value from the

power supply unit to different values required by various parts of the electronic system. It

completely eliminates the need of providing separate dc power supplies to different

electronic circuits operating at different dc levels in a given electronic system.

2.2 MULTIPLEXERS

A multiplexer is a device that encodes information from two or more data sources

into a single channel for transmission to another point. It is also known as a data selector.

In electronics, the multiplexer combines several electrical signals into a single signal. It

selects one of the inputs and then passes it on to the output. There are different types of

multiplexers for analog and digital circuits. They are used in situations where the cost of

implementing separate channels for each signal is more expensive than the cost and

inconvenience of providing the multiplexing/demultiplexing functions.

A multiplexer could be a 2-line decoder, a 4-line decoder, an 8-line decoder etc

depending on the number of signals to be mulitplexed. The logic circuitry of a four input

multiplexer is shown in figure 2.7 below.

Figure 2.7: Logic Diagram For A 4 Line Multiplexer

The truth table for the logic diagram can also be realized thus; S0 and S1, which are the

control pins are responsible for determining the data input (D0 – D3) that is selected to

transmitted to the data output line.

S0 S1 Data input selected

0 0 D0

0 1 D1

1 0 D2

1 1 D3

Table 2.2: Data Select For A 4 - Line Multiplexer

Multiplexers can be packaged using Small Scale Integration (SSI) or Medium

Scale Integration (MSI). They can be fabricated using Complementary Metal Oxide

Semiconductor (CMOS), and Transistor – Transistor Logic (TTL).

Types of multiplexers

There are different types of multiplexers for analog and digital circuits.

• Digital multiplexers: In digital signal processing, the multiplexer takes several

separate digital data streams and combines them together into one data stream of a

higher data rate. This allows multiple data streams to be carried from one place to

another over one physical link. It forwards one of the input streams to the output

stream based on the values of one or more "selection inputs" or control inputs. At

the receiving end of the link a complementary demultiplexer or demux is

normally required to break the high data rate stream back down into the original.

• Analog multiplexers: In analogue circuit design, a multiplexer is a special type of

analogue switch that connects one signal selected from several inputs to a single

output. They can input and output levels other than just 1 and 0. The input/output

levels can be any analog voltage between the positive and negative supply level.

An example is the 4051 manufactured by Philips components.

Applications of multiplexers

Multiplexer circuits find varied applications in digital systems of all types. These

applications include data selection, data routing, operation sequencing, parallel to serial

conversion, waveform generation and logic function generation.

2.3 THE ANALOG TO DIGITAL CONVERTER

Physical quantities are analog in nature and must be converted into binary form for

it to be understood by a digital system. An analog-to-digital converter is employed in this

conversion. The digital output consists of a number of bits that represents the value of the

analog input.

Most analog to digital converters utilizes a digital to analog converter (DAC) in

their circuitry. The block diagram is shown in figure 2.8 below:

Figure 2.8: Block Diagram Of An ADC

The start command initiates the operation of the ADC and the control unit

continually modifies the binary number stored in the register at a rate determined by the

clock. The input in the register is converted to an analog voltage by the DAC. The

comparator compares the binary number that was converted to an analog voltage VAX with

the analog input VA. If VAX < VA the comparator output stays HIGH but when VAX =VA,

the comparator outputs a LOW. The control logic activates the end of converion when the

conversion is complete.

All ADCs work by sampling their input at discrete intervals of time. This sampling

is done at a rate known as Nyquist rate. The analog signal is sampled with a frequency

slightly greater than twice the highest frequency of the signal. Sampling at this rate

facilitates the perfect reproduction of the original signal by a DAC. Sampling at a rate less

than the Nyquist’s rate causes a problem called aliasing.

Examples of analog to digital IC include ADC0801, ADC0802, ADC0803,

ADC0804, and ADC0805 etc.

Features of an ADC

• Resolution: The resolution of the converter indicates the number of discrete values

it can produce over the range of voltage values. It is usually expressed in bits. For

example, an ADC that encodes an analog input to one of 256 discrete values (0-

255) has a resolution of eight bits, since

28 = 256.

Resolution can also be defined electrically, and expressed in volts. The voltage

resolution of an ADC is equal to its overall voltage measurement range divided by

the number of discrete values. In practice, the resolution of the converter is limited

by the signal-to-noise ratio of the signal in question. If there is too much noise

present in the analog input, it will be impossible to accurately resolve beyond a

certain number of bits of resolution, the "effective number of bits" (ENOB).

• Accuracy: An ADC has several sources of errors. Quantization error and

(assuming the ADC is intended to be linear) non-linearity is intrinsic to any

analog-to-digital conversion. There is also aperture error which is due to a clock

jitter and reveals when digitizing a signal (not a single value). These errors are

measured in a unit called the LSB, which is an abbreviation for least significant bit.

• Conversion time: It is the time interval between the end of the start pulse and the

activation of the end of conversion output. The value of conversion time depends

on the analog signal. A large value will require more steps before the voltage

exceeds the analog voltage.

ADC structures

These are the most common ways of implementing an electronic ADC:

• Direct conversion ADC or flash ADC: It has a comparator that fires for each

decoded voltage range. The comparator bank feeds a logic circuit that generates a

code for each voltage range. Direct conversion is very fast, but usually has only 8

bits of resolution (256 comparators) or fewer, as it needs a large, expensive circuit.

ADCs of this type have a large die size, a high input capacitance, and are prone to

produce glitches on the output (by outputting an out-of-sequence code). They are

often used for video or other fast signals.

• Successive-approximation ADC: It uses a comparator to reject ranges of voltages,

eventually settling on a final voltage range. It works by constantly comparing the

input voltage to a known reference voltage until the best approximation is

achieved. At each step in this process, a binary value of the approximation is stored

in a successive approximation register (SAR).The SAR uses a reference voltage

(which is predetermined and reflects the conditions for which the ADC is used for)

for comparisons. ADCs of this type have good resolutions and quite wide ranges.

They are more complex than some other designs.

• Delta-encoded ADC: has an up-down counter that feeds a digital to analog

converter (DAC). The input signal and the DAC both go to a comparator. The

comparator controls the counter. The circuit uses negative feedback from the

comparator to adjust the counter until the DAC's output is close enough to the

input signal. The number is read from the counter. Delta converters have very wide

ranges, and high resolution, but the conversion time is dependent on the input

signal level. Delta converters are often very good choices to read real-world

signals. Most signals from physical systems do not change abruptly. Some

converters combine the delta and successive approximation approaches; this works

especially well when high frequencies are known to be small in magnitude.

• Digital-ramp ADC: It is so named because the waveform is a step by step ramp

that is a staircase. When the ramp starts, a timer starts counting. When the ramp

voltage matches the input, a comparator fires, and the timer's value is recorded.

Timed ramp converters require the least number of transistors. The ramp time is

sensitive to temperature because the circuit generating the ramp is often just some

simple oscillator. A special advantage of the digital-ramp system is that comparing

a second signal just requires another comparator, and another register to store the

voltage value.

Applications of analog to digital converters

ADCs are used virtually everywhere where an analog signal has to be processed,

stored, or transported in digital form. They include:

• Converting analog signal before sending it to any microprocessor/ microcontroller

Figure 2.9: An ADC Connected To Any Processor

• Digital Signal Processing: A digital Signal Processor performs repetitve

calculations on a stream of digitized data. It is the ADC that feeds the digitized

data to the digital signal processor.

• Music recording: ADCs are integral to much current music reproduction

technology, since much music production is done on computers; even when analog

recording is used, an ADC is still needed to create the PCM data stream that goes

onto a compact disc.

2.4 THE 89C52 MICROCONTROLLER

The 89C52 is an 8-bit microcontroller originally developed by Intel in the late

1970s. It included an instruction set of 255 operation codes (opcodes), 32 input/output

lines, three user-controllable timers, an integrated and automatic serial port, and 256 bytes

of on-chip RAM. It is an improvement on the 89C51 microprocessor. The 89C52 was

designed such that control of the MCU and all input/output between the MCU and external

devices is accomplished via Special Function Registers (SFRs). Each SFR has an address

between 128 and 255.

The 89C52 can be programmed using assembly language or any high level

language.

2.4.1 Memory Organization of The 89C52

The 89C52 has three general types of memory.

• Code memory

• External memory

• On-chip memory which include: Internal RAM and special function

register (SFR) memory.

To program the 89C52 effectively it is necessary to have a basic understanding of

these memory types. The code memory holds the actual 89C52 program that is to be run.

Code memory which is conventionally limited to a size of 64K comes in various shapes

and sizes and may be found on-chip, either burned into the microcontroller as ROM or on

an external EPROM.

The 89C52 has external RAM that is found off-chip. Since the memory is off-chip

the assembly language instructions to access it are slower and less flexible. For example,

to increment an Internal RAM location by 1 requires only 1 instruction and 1 instruction

cycle. To increment a 1-byte value stored in External RAM requires 4 instructions and 7

instruction cycles. This is because of the type of addressing mode required to access the

external RAM. In this case, external memory is 7 times slower and requires 4 times as

much program memory. While Internal RAM is normally limited to 256 bytes, the 89C52

supports External RAM up to 64 Kilobytes.

The 89C52 has on-chip memory that is further divided into 2:

• Internal RAM: The 89C52 has a bank of 256 bytes of Internal RAM and so it is

the fastest RAM available, and it is also the most flexible in terms of reading,

writing, and modifying its contents. Internal RAM is volatile so when the 89C52 is

reset, this memory is undefined. It is further divided into stack, register banks and

bit memory. The stack is used to store values that the user program manually

pushes onto the stack as well as to store the return addresses for CALLs and

interrupt service routines. The register banks are used to assist in manipulating

values and moving data from one memory location to another. The 256 bytes of

Internal RAM are subdivided as shown in the memory map below:

Figure 2.10: Memory Map Of The 89C52

The first 8 bytes (00H- 07H) are "register bank 0". By manipulating a certain SFR,

a program may choose to use register banks 0, 1, 2, or 3. These alternative register

banks are located in internal RAM in addresses 08H through 1FH. The bit

memory actually resides in internal RAM, from addresses 20H through 2FH. Also

the Internal RAM is byte-wide memory, regardless of whether it is used by register

banks or bit memory.

Users may use the 208 bytes remaining of the Internal RAM, from addresses 30H

through FFH for variables that need to be accessed frequently or at high-speed. This area

is also utilized by the microcontroller as a storage area for the operating stack. This fact

severely limits the 89C52’s stack since, as illustrated in the memory map, the area

reserved for the stack is only 208 bytes--and usually it is less since these 208 bytes have to

be shared between the stack and user variables.

• Special Function Register (SFR) memory: Special Function Registers (SFRs)

are areas of memory that control specific functionality of the 89C52 MCU. For

example, four SFRs permit access to the 89C52’s 32 input/output lines (8 lines per

SFR). Another SFR allows a program to read or write to the 89C52’s serial port.

Other SFRs allow the user to set the serial baud rate, control and access timers, and

configure the 89C52’s interrupt system. When programming, SFRs have the

illusion of being Internal Memory.

2.4.2 ADDRESSING MODES OF THE 89C52

It refers to how a given memory location is addressed. The instruction set provides

several means to address memory locations. There are five basic addressing modes in the

8052 instruction set. Each of these addressing modes provides important flexibility.

Immediate Addressing

The value to be stored in memory immediately follows the operation code in

memory. That is, the instruction itself dictates what value will be stored in memory. For

example, the instruction:

MOV A, #20h

The Accumulator will be loaded with the value that immediately follows; in this

case 20 (hexadecimal). Immediate addressing is very fast since the value to be loaded is

included in the instruction. However, since the value to be loaded is fixed at compile-time

it is not very flexible.

Direct Addressing

The value to be stored in memory is obtained by directly retrieving it from another

memory location. For example:

MOV A, 30h

This instruction will read the data out of Internal RAM address 30 (hexadecimal) and store

it in the Accumulator. Direct addressing is generally fast because the value to be loaded is

not included in the instruction. It is quickly accessible since it is stored in the 89C52s

internal RAM. It is also much more flexible than Immediate Addressing since the value to

be loaded is whatever is found at the given address--which may be variable.

Any instruction that refers to an address between 00h and 7Fh is referring to

Internal Memory and any instruction that refers to an address between 80h and FFh is

refering to the SFR control registers that control the 89C52 microcontroller itself.

Indirect Addressing

Indirect addressing is an addressing mode that provides an exceptional level of

flexibility. Indirect addressing is also the only way to access the extra 128 bytes of Internal

RAM found on an 8052.

Indirect addressing appears as follows:

MOV A, @R0

This instruction causes the 8052 to analyze the value of the R0 register. The 89C52

will then load the accumulator with the value from Internal RAM that is found at the

address indicated by R0.

For example, lets say R0 holds the value 40h and Internal RAM address 40h holds

the value 67h. When the above instruction is executed the 89C52 will check the value of

R0. Since R0 holds 40h the 8051 will get the value out of Internal RAM address 40h

(which holds 67h) and store it in the Accumulator. Thus, the Accumulator ends up holding

67h.

External Direct

External Memory is accessed using a suite of instructions that use "External

Direct" addressing. There are only two commands that use External Direct addressing

mode:

MOVX A, @DPTR

MOVX @DPTR, A

Both commands utilize DPTR. In these instructions, DPTR must first be loaded

with the address of external memory that you wish to read or write. Once DPTR holds the

correct external memory address, the first command will move the contents of that

external memory address into the Accumulator. The second command will do the

opposite: it will allow you to write the value of the Accumulator to the external memory

address pointed to by DPTR.

External Indirect

External memory can also be accessed using External Indirect addressing. This

form of addressing is usually only used in relatively small projects that have a very small

amount of external RAM. An example of this addressing mode is:

MOVX @R0, A

Once again, the value of R0 is first read and the value of the Accumulator is

written to that address in External RAM. Since the value of @R0 can only be 00h through

FFh, the project would effectively be limited to 256 bytes of External RAM..

2.4.2 89C52 PIN CONFIGURATION

Figure 2.11: 89C52 Pin Configuration

The 89C52 has a 40-pin DIP pinout. 32 of them are dedicated to I/O lines that have

a one-to-one relation with the SFRs’ P0, P1, P2, and P3. The developer may raise and

lower these lines by writing 1s or 0s to the corresponding bits in the SFRs. All of the ports

have internal pull-up resistors except for port 0. Port 0 is dual-function in that in some

designs, port 0’s I/O lines are available to the developer to access external devices while in

other designs it is used to access external memory. If the circuit requires external RAM or

ROM, the microcontroller will automatically use port 0 to clock in/out the 8-bit data word

as well as the low 8 bits of the address in response to a MOVX instruction and port 0 I/O

lines may be used for other functions as long as external RAM is not being accessed at the

same time. If the circuit requires external code memory, the microcontroller will

automatically use the port 0 I/O lines to access each instruction that is to be executed. In

this case, port 0 cannot be utilized for other purposes since the state of the I/O lines are

constantly being modified to access external code memory.

Port 1 consists of 8 I/O lines that you may use exclusively to interface to external

parts. Port 1 is commonly used to interface to external hardware such as LCDs, keypads,

and other devices. Like port 0, port 2 is dual-function. In some circuit designs it is

available for accessing devices while in others it is used to address external RAM or

external code memory. If the circuit requires external code memory, the microcontroller

will automatically use the port 2 I/O lines to access each instruction that is to be executed.

Port 3 consists entirely of dual-function I/O lines. While the developer may access all

these lines from their software by reading/writing to the P3 SFR, each pin has a pre-

defined function that the microcontroller handles automatically when configured to do so

and/or when necessary.

A crystal connected to XTAL2 and XTAL1 drives the 89C52. Common crystal

frequencies are 11.0592 MHz as well as 12 MHz. A TTL clock source may also be

attached to XTAL1 and XTAL2 to provide the microcontroller’s clock. Pin 9 is the master

reset line for the microcontroller. When this pin is brought high for two instruction cycles,

the microcontroller is effectively reset. SFRs, including the I/O ports, are restored to their

default conditions and the program counter will be reset to 0000H. The Internal RAM is

not affected by a reset. The reset line is often connected to a reset button/switch that the

user may press to reset the circuit.

The address latch enable (ALE) at pin 30 is an output-only pin that is controlled

entirely by the microcontroller and allows the microcontroller to multiplex the low-byte of

a memory address and the 8-bit data itself on port 0. This is because, while the high-byte

of the memory address is sent on port 2, port 0 is used both to send the low byte of the

memory address and the data itself. This is accomplished by placing the low-byte of the

address on port 0, exerting ALE high to latch the low-byte of the address into a latch IC

(such as the 74HC573), and then placing the 8 data-bits on port 0. In this way the 89C52 is

able to output a 16-bit address and an 8-bit data word with 16 I/O lines instead of 24.

The Program Store Enable (PSEN) line at pin 29 is exerted low automatically by

the microcontroller whenever it accesses external code memory. PSEN will not be exerted

by the microcontroller and will remain in a high state if your program is being executed

from internal code memory. The External Access (EA) line at pin 31 is used to determine

whether the 89C52 will execute your program from external code memory or from internal

code memory. If EA is tied high (connected to +5V) then the microcontroller will execute

the program it finds in internal/on-chip code memory.

2.5 RELAYS

A relay is an electrically operated switch. They are used when we need to use a

small amount of power in the electromagnet coming from a low power electronic circuit to

move an armature that is able to switch a much larger amount of power. In other words,

Relays allow one circuit to switch a second circuit that may be completely separate from

the first. For example a low voltage battery circuit can use a relay to switch a 230V AC

mains circuit. No electrical connection exists inside the relay between the two circuits; the

link is magnetic and mechanical. The current flowing through the coil of a relay creates a

magnetic field which attracts a lever and changes the switch contacts. The coil current can

be on or off. Hence, relays have two switch positions and they are double throw

(changeover) switches.

Figure 2.12: Circuit Symbol For A Relay

The relay's switch connections are usually labeled COM, NC and NO:

COM = Common; it is the moving part of the switch.

NC = Normally Closed, COM is connected to this pin when the relay coil is off.

NO = Normally Open, COM is connected to this pin when the relay coil is on.

If we want the switched circuit to be on when the relay coil is on, the circuit is connected

to COM and NO but if we want it to be on when the relay coil is off, we connect the

circuit to COM and NC.

Several features are considered when choosing a relay they include:

� Physical size and pin arrangement: If we are choosing a relay for an existing

PCB we will need to ensure that its dimensions and pin arrangement are suitable.

This information is usually found in the supplier's catalogue.

� Coil voltage: The relay's coil voltage rating and resistance and that of the circuit

powering the relay coil must match. Many relays have a coil rated for a 12V supply.

Some relays however, operate perfectly well with a supply voltage which is a little

lower than their rated value.

� Coil resistance: The circuit must be able to supply the current required by the

relay coil. Ohm's law can be used to calculate the current thus:

Relay coil current = supply voltage / coil resistance For example: A 12V supply relay with a coil resistance of 400 passes a current of

30mA. This is suitable for a 555 timer IC (maximum output current 200mA), but it is

too much for most ICs and they will require a transistor to amplify the current.

� Switch ratings (voltage and current): The relay's switch contacts must be

suitable for the circuit they are to control. The voltage and current ratings must also

match.

� Switch contact arrangement (SPDT, DPDT etc): Most relays are “single pole

changeover" (SPCO) or "double pole changeover" (DPCO).

Generally, Transistors and ICs (chips) must be protected from the brief high

voltage 'spike' produced when the relay coil is switched off. A signal diode (e.g. 1N4148)

can be connected across the relay coil to provide this protection. The diode is connected

'backwards' so that conduction only occurs when the relay coil is switched off, at this

moment current tries to continue flowing through the coil and it is harmlessly diverted

through the diode. Without the diode no current could flow and the coil would produce a

damaging high voltage 'spike' in its attempt to keep the current flowing

Relays have a number of advantages over other switching devices such as

transistors. For instance, Relays can switch AC and DC as well as high voltages. They are

a better choice for switching large currents (> 5A). They can also be used to switch many

contacts at once.

Its drawbacks include bulkiness especially for switching small currents, slow rate

of switching as well as more power which is required to drive them due to the current

flowing through their coil. Also, most relays require more current than many chips can

provide, so a low power transistor may be needed to switch the current for the relay's coil.

2.6 SEVEN SEGMENT DISPLAYS

Most digital equipment have some means for displaying information in form that can

be understood readily by the user or operator. This information is often numerical data but

can also be alphanumeric (number and letters). One of the accepted methods for

displaying numerical digits is the 7-segment display, which is used to form characters 0-9

and sometimes the hexadecimal characters A-F. The individual segments making up a 7-

segment display are identified by letters. The decimal point (an eight segment), which

is optional, is used for the display of non-integer numbers.

Seven segment displays can be achieved using an arrangement of light- emitting

diodes (LEDs) for each segment. By controlling the current through each LED, some

segments will be lighted and others will be dark so that the desired character pattern will

be generated. Other types of 7 segment displays exist using alternative technologies such

as cold cathode gas discharge, vacuum fluorescent, incandescent filament, liquid crystal

display (LCD).

The pin connection for an LED based 7-segment display is shown below:

Figure 2.13: Seven-Segment Display

Types of 7 segment LED display

• Common cathode display: Here the cathodes of all the LEDs are joined together

and connecting them to a HIGH voltage illuminates the individual segments.

• Common anode display: Here the anodes of all the LEDs are joined together and

connecting them to a LOW voltage illuminates the individual segments.

7 segment displays are designed for applications requiring low power consumption. Drive

currents could be as low as 1 mA per segment. The 7-segment display is found in many

displays such as microwaves or fancy toaster ovens

2.7 TRANSISTORS AND CAPACITORS

Passive components are the minor components used in electronic design. For this

project only capacitors and resistors will be considered

TRANSISTORS

The transistor is a three terminal solid state semiconductor device consisting of

either two n-and one p- type layers of material or two p-and one n- type layers of material.

The former is called an npn transistor while the latter is called a pnp transistor.

Transistors can be used for amplification, switching, voltage stabilization, signal

modulation and many other functions. In analog circuits, transistors are used in

amplification (direct current amplification, audio amplification, radio frequency

amplification), and linear regulated power supplies. Transistors are also used in digital

circuits where they function as electrical switches. Digital circuits include logic gates,

random access memory (RAM), and microprocessors.

Transistors are divided into two main categories: bipolar junction transistors

(BJTs) and field effect transistors (FETs). Transistors have three terminals, an input

terminal, a common terminal, and an output terminal. Application of current for BJT or

voltage for FET between the input terminal and the common terminal increases the

conductivity between the common and output terminals, thereby controlling current flow

between them. The physics of this "transistor action" is quite different for the BJT and

FET.

Bipolar Junction Transistors (BJTs)

The bipolar junction transistor (BJT) was the first type of transistor to be mass-

produced. Bipolar transistors are so named because they conduct by using both majority

and minority carriers. The three terminals are named emitter, base and collector. Two p-n

junctions exist inside an NPN BJT: the base/collector junction and base/emitter junction.

The PNP transistor similarly has two n-p junctions.

Figure 2.14: (a) Schematics of a pnp transistor, (b) Schematics of an npn

transistor

The BJT is commonly described as a current-operated device because the

emitter/collector current is controlled by the current flowing between base and emitter

terminals. The BJT is a low input-impedance device. Bipolar transistors can be made to

conduct with light (photons) as well as current. Devices designed for this purpose are

called phototransistors.

Field Effect Transistors (FETs)

The field-effect transistor (FET), sometimes called a unipolar transistor, uses either

electrons (N-channel FET) or holes (P-channel FET) for conduction. The three main

terminals of the FET are named source, gate and drain.

A voltage applied between the gate and source controls the current flowing

between the source and drain. In FETs the source/ drain current flows through a

conducting channel near the gate. This channel connects the source region to the drain

region. The channel conductivity is varied by the electric field generated by the voltage

applied between the gate/source terminals. In this way, the current flowing between the

source and drain is controlled. Like bipolar transistors, FETs can be made to conduct with

light (photons) as well as voltage. Devices designed for this purpose are called

phototransistors.

Field-effect transistor (FET) provides an excellent voltage gain with the added

feature of high input impedance. They are considered low- power consumption

configurations with good frequency range and minimal side and weight.

In summary, while a BJT device controls a large output (collector) current by

means of a relatively small input (base) current, the FET device controls an output (drain)

current by means of a small input (gate voltage) voltage. In both cases, the output current

is the controlled variable.

PNP

P-channel

NPN

N-channel

BJT JFET

Figure 2.15: Transistor Symbols For BJT And JFET

CAPACITORS

A capacitor is a device that stores energy in the electric field created between a pair

of conductors on which electric charges of equal magnitude, but opposite sign, have been

placed. A capacitor is occasionally referred to using the older term condenser. Early

experiments found that conductor would hold much greater electric charges provided that

they were held in close proximitiy to one another yet kept apart. They also found that the

greater the surface area the conductors then the greater the stored charge.

A capacitor consists of two electrodes, or plates, each of which stores an opposite

charge. These two plates are conductive and are separated by an insulator or dielectric.

The charge is stored at the surface of the plates, at the boundary with the dielectric.

Because each plate stores an equal but opposite charge, the net charge on the capacitor as a

whole is always zero. In the diagram below, the rotated molecules create an opposing

electric field that partially cancels the field created by the plates, a process called dielectric

polarization.

Capacitance in a capacitor

When electric charge accumulates on the plates of a capacitor, an electric field is

created in the region between the plates that is proportional to the amount of accumulated

charge. This electric field creates a potential difference, V = E ∗ d between the plates of

this simple parallel-plate capacitor. The electrons within dielectric molecules are

influenced by the electric field, causing the molecules to rotate slightly from their

equilibrium positions.

The capacitor's capacitance (C) is a measure of the amount of charge (Q) stored on

each plate for a given potential difference or voltage (V) which appears between the

plates: that is

In SI units, a capacitor has a capacitance of one farad when one coulomb of charge

causes a potential difference of one volt across the plates. Since the farad is a very large

unit, values of capacitors are usually expressed in microfarads (µF), nanofarads (nF) or

picofarads (pF).

The capacitance is proportional to the surface area of the conducting plate and

inversely proportional to the distance between the plates. It is also proportional to the

permittivity of the dielectric (that is, non-conducting) substance that separates the plates.

The capacitance of a parallel-plate capacitor is given by:

where ε is the permittivity of the dielectric, A is the area of the plates and d is the spacing

between them.

Applications

Capacitors are applied in most electronic and electrical systems

• 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 to supply energy to electronic devices without the memory being

lost when changing their batteries.

• Smoothening: Capacitors are used in power supplies where they smooth the

output of a full or half wave rectifier. They can also be used in charge pump

circuits as the energy storage element in the generation of higher voltages than the

input voltage. Capacitors are connected in parallel with the power circuits of most

electronic devices and larger systems (such as factories) to shunt away and conceal

current fluctuations from the primary power source to provide a "clean" power

supply for signal or control circuits.

• Filtering: Capacitors pass AC but block DC signals (when charged up to the

applied dc voltage), and are often used to separate the AC and DC components of a

signal. This method is known as AC coupling. Capacitors for this purpose are

designed to fit through a metal panel called feed-through capacitors. Capacitors are

also used to filter noise. It is usually connected with a resistor in parallel or series.

• Signal processing: The energy stored in a capacitor can be used to represent

information, either in binary form, as in computers, or in analogue form, as in

switched-capacitor circuits and bucket-brigade delay lines. Capacitors can be used

in analog circuits as components of integrators or filters that are more complex and

in negative feedback loop stabilization. Signal processing circuits also use

capacitors to integrate a current signal.

CHAPTER THREE

HARDWARE DEVELOPMENT

3.1 SYSTEM OVERVIEW

The system consists of different modules which include voltage rectification and

regulation, analog multiplexing circuit, analog to digital converter (ADC), microcontroller

module, display circuitry, relay circuit. The different modules are interconnected as shown

in figure 3.1 below.

Figure 3.1: Block Diagram Of An Automatic Power Selector For Multiple Ac Sources

The functions of the different blocks will now be treated individually before the

actual design of the circuit to realize these functions.

The power supply unit consists of an autotransformer and three step down

transformers. The function of the autotransformer is to simulate the multiple AC power

sources. It generates different voltage levels that will be used to test the system. The step

down transformers steps down the input voltage depending on their turns ratio for

subsequent rectification.

AC voltage signals from the transformers are fed into the rectifier circuit for

rectification. and smoothening. The result is a dc voltage that is analog in nature. Voltage

regulation is carried out to ensure that a constant supply voltage is available for the

Integrated circuit chips and the relay.

The multiplexer is used to eliminate the need for multiple ADCs for the conversion

of the individual sources that will be fed to the microcontroller.

The signal obtained from rectification is essentially a dc voltage with some ripples;

they are inputs to the analog to digital converter (ADC) that converts the analog signal into

digital form that can be processed by the microcontroller.

The microcontroller scans the input voltages, and measures them against the

optimum value, which we set to be 220V ± 20V. The power source with the least

deviation from the optimum value of input voltage that falls within the range is

automatically selected to power the light bulb (load). The relay system switches in order to

power the load at a given time.

The microcontroller also sends a signal that enables the seven segment displays to

display the particular power source in use and also the status of all the other sources at any

particular time. The microcontroller actualizes these functions through the series of micro

programmed instructions that is embedded into it.

3.2 SYSTEM SPECIFICATION AND DESIGN

We are going to design a system that will automatically carry out selection of a

favourable power source to power a load given multiple power sources. The system is

expected to measure three input voltages, display their values and appropriately select one

for use based on a specified voltage range given as 220V±20 for the purpose of this work.

In the event that all input voltages are not within this range, the system is expected to issue

a control signal to another circuitry that will turn on a stand by generator. The maximum

voltage that the system supports is 300V.

However due to the limitation of resources and time, only a display indicating that

the generator is to be on will be realized.

The blocks were first considered individually and tested to ensure that they

satisfied the functional requirements. When the testing of the individual blocks were

completed on a bread board, all the blocks were connected together as in the block

diagram. At this point, problems rising due to improper interfacing between the blocks

were solved.

3.3 VOLTAGE RECTIFICATION AND REGULATION

The voltage rectification and regulation circuit is as shown in figure 3.2 below.

Figure 3.2: Circuit Diagram For Voltage Rectification And Regulation

The step down transformers receives input voltages from the power supply (auto

transformer) and steps down the respective voltages in proportion to a turns ratio of 20:1.

The voltage rectifier IC carries out full wave rectification to produce a dc output voltage.

The advantage of a full wave rectified output is that it has less ripple than a half wave

rectified output. The smaller the ac variation with respect to the dc level, the better the

filter circuit’s operation.

The rectified signal is then passed through a capacitive filter to reduce the ripple

level and produce a steadier dc voltage. Larger values of capacitance provide less ripple

and higher average voltage thereby providing better filter action. The 1000µf capacitor

performs this function.

A dc output voltage changes over a range of circuit operation. The voltage provided

at the output under no load condition is reduced when the load current is drawn from the

supply. There is need for a constant supply of 5V to the ICs and 12V to power the relay.

To realize this, we used three-terminal voltage regulators (7805 and7812); they are fixed

positive voltage regulators that produce a steady output voltage of +5V and +12V

respectively. The three terminals are the input, common and output. The common is

connected to ground. The already rectified and smoothened signal is fed into the input

terminal of the regulator and a capacitor is connected at the output terminal to filter noise.

The variable resistor functions as a voltage divider to scale down the input voltages

such that it falls within the range of 0-5V, which is the maximum range of inputs for the

ADC.

3.4 THE ANALOG –DIGITAL SWITCH

Several ADCs would have been necessary to realize the analog to digital conversion

of the input dc voltages. This would have led to increased cost and complexity of the

circuit. Multiplexing was used to address this problem as shown in figure 3.3.

Due to the unavailability of analog multiplexers in the market, a quad bilateral

switch (4016) was used to achieve analog multiplexing. Since we are testing only three

input voltages at a time, Pin 10,11 and 12 which correspond to the input, output and

control of the fourth input voltage were grounded. Pin 14 (Vcc) is connected to a constant

supply of 5V and Pin 7(ground pin) to ground.

Figure 3.3: Pin Connections Of The Analog-Digital Switch

The 4016 contains four independent switches SWA, SWB, SWC and SWD. Their

respective control pins(ACON, BCON and CCON and DCON) determine the status of each

switch. When ACON is high, then SWA is closed and the voltage at pin 1 is sent to the

ADC through output pin 2 A0.The same applies to the rest of the switches.

The microcontroller determines which control pin is activated at a particular time

by sending signals at specified intervals to the control pins. Two control pins cannot be

activated at the same time. Thus, time-division multiplexing is successfully achieved.

3.5 THE ANALOG-DIGITAL INTERFACE

The analog to digital converter ADC0804 provides the interface between the

analog and the digital modules of the system. Any of the three inputs selected by the

analog to digital switch forms the input to the ADC through its pin 6 (VIN+). The two

analog inputs VIN+ and VIN- allows differential inputs, that is the actual analog input VIN is

the difference in the voltages applied to these two pins. In our design, VIN- is connected to

ground. Therefore, the VIN = VIN+ - OV. The ADC converts the analog input voltage to an

8-bit digital output.

Figure 3.4: Pin Connections Of The ADC

A low pulse is applied to the Pin 3(WR) to signal the start of a conversion. When

conversion is complete, the ADC sends an active low interrupt request signal to the

microcontroller through Pin 5(INTR) indicating the end of conversion process. The

corresponding digital voltage value is currently at the ADC’s data port (DB7-DB0).

Pins 1, 4,7,8,10 and 19 are grounded as shown in the figure above. Pin 20 is

connected to VCC of 5V.A resistor is connected to CLKR to use the internal clock. CLKIN

is connected to a capacitor because the internal clock of the ADC is used in this design.

3.6 MICROCONTROLLER MODULE

The microcontroller module is as shown in figure 3.5. The microcontroller issues

an active low write control signal to the ADC through its Pin 16(WR). Pin 16 is connected

to the WR pin of the ADC. Pin 7(RD) is connected to the interrupt pin of the ADC; a low

pulse from the ADC indicates the end of conversion process. The microcontroller then

reads the corresponding digital voltage value of the input which is currently at the data

port (DB7-DB0) through its Port 0 (P0.0 – P0.7).

Figure 3.5: Microcontroller Module

Pins 10, 11 and 12 (P3.0 – P3.2) are connected to the control pins of the analog-

digital switch. It sends binary data that selects the respective switches of the analog

multiplexer.

Port 1 (P1.0 – P1.7) and Port 2(P2.0-P2.7) of the microcontroller is used as an

output pin to send the required data that will display the status of each power source. P3.3-

P3.5 are output ports that are connected to the relay circuit.

The RESET, VCC and EA pins are tied to a 5V VCC. EA is tied high because the

microcontroller is not using an external memory. While the GND pin is grounded as

shown in the figure above.

A crystal oscillator of 16MHz frequency connected to XTAL1 and XTAL2 pins of

the microcontroller as shown generates the clock required to sequence all the operations of

the system. This high frequency causes the microprocessor to operate at a very high speed.

3.7 DISPLAY CIRCUITRY

This consists of eight common cathode LED seven segment displays. To illuminate

an LED, its cathode must be connected to a high. The common cathodes of the displays

are connected through a pull up transistor to port 2 (P2.0 – P2.7) of the microcontroller.

The pins representing the alphabets (a-g and dot) of each display are connected to port 1

(P1.0 – P1.7) of the microcontroller.

The binary pattern (data) that is required to display the status of each line as well

as the specified line in use is sent to the display circuitry through port 1 (P1.0 – P1.7) of

the microcontroller as shown in the figure below. Our design employs a scrollable display,

the data gets to each of the displays and a binary pattern is also sent to port 2 (P2.0 – P2.7)

of the microcontroller in order to select which of the displays would have its data

displayed.

(P2.0 – P2.7) is connected to the base of the pull up transistors. A high at the

transistor base enables the collector current flow out through the emitter.

The truth table of the common cathode seven segment display is as shown in

table 3.1 below:

a (Pin 1) b (Pin 10) c (Pin

8)

d (Pin

6) e (Pin 5) f (Pin 2) g (Pin 9)

Dp(

0 1 1 1 1 1 1 0 0

1 0 1 1 0 0 0 0 0

2 1 1 0 1 1 0 1 0

3 1 1 1 1 0 0 1 0

4 0 1 1 0 0 1 1 0

5 1 0 1 1 0 1 1 0

6 1 0 1 1 1 1 1 0

7 1 1 1 0 0 0 0 0

8 1 1 1 1 1 1 1 0

9 1 1 1 0 0 1 1 0

A 1 1 1 0 1 1 1 0

b 0 0 1 1 1 1 1 0

C 1 0 0 1 1 1 0 0

d 0 1 1 1 1 0 1 0

E 1 0 0 1 1 1 1 0

F 1 0 0 0 1 1 1 0

Table 3.1: Truth Table For A Common Cathode 7 Segment Display

Figure 3.6: The Display Circuit

3.7 THE RELAY CIRCUIT

The relay circuit was designed as shown in the figure below. The relay is

connected in the normally open mode such that the relay switches to light the bulb when

the relay is ON, and the COM and the NO pins make contact with each other.

For the relay circuit to be activated, a base current is required at the base of the

PNP transistor for the relay to come ON. When the base current flows into the transistor,

the magnetic core of the relay gets magnetized and the normally open (NO) pin of the

relay which is connected to the load (bulb), makes contact with the common (COM) pin

and thus a complete circuit is formed and the load (bulb) lights.

Figure 3.7: The Relay Circuit

The relay is connected in the normally open mode such that the relay switches to

light the bulb when the relay is ON, and the COM and the NO pins make contact with

each other.

The signal diode is connected across the relay to protect it from spikes when the

relay coil is switched on. The diode is connected backward so that it will normally not

conduct. Conduction only occurs when the relay coil is switched on, at this moment

current tries to continue flowing through the coil and it is harmlessly deviated through the

diode.

3.8 HARDWARE INTEGRATION

The different modules specified in the block diagram was designed and tested to

ensure their functionality. The output voltages of the autotransformer were measured in

order to ascertain the actual values. Also the output voltages of the 7812 regulator was

tested to confirm that it could switch the relay and that of the 7805 to make sure that 5V is

supplied to the IC’s. The microcontroller, analog digital switch, ADC and display circuit

were not be tested at this level because their functionality depends on the software

embedded in the microcontroller. The software at this point was yet to be programmed

into the 89C52.

All the modules were integrated as in figure 3.8.

CHAPTER FOUR

SOFTWARE DEVELOPMENT AND SYSTEM INTEGRATION

We employed a top down methodology in building our software. With this

approach the design is presented in stages of increasing details. It starts with an overview

of the system using the software specification requirement, and then its design details

using a flow chart, this last stage is conventionally specified using pseudocodes.

4.1 SOFTWARE REQUIREMENT SPECIFICATION

The assembly language codes is expected to capture the following functions:

• Specify the optimum voltage of 220V

• Maximum deviation from the optimum voltage is set as 20 volts

• Capture the input voltages (in digital form) and convert and scale to the analog

value.

• Store the analog input voltage values in registers

• Display the different analog values of the inputs

• Compare input voltages with the optimum voltage value and obtain the positive

difference

• Determine the input with the least value of positive difference

• Compare this value with the set value of maximum deviation from optimum

voltage

• If it is less than 20volts, then the input is selected for use and is indicated in the

display panel

• If it is more than 20 volts, a stand by generator is switched on.

4.2 PROGRAM DESIGN (FLOW CHART)

The flow chart of the automatic power selector is shown in the figure below

Figure 4.1: Flowchart For Automatic Power Selector For Multiple AC Sources

4.3 SOFTWARE DEVELOPMENT TOOLS

The 8052 can be programmed with any language as long as a translator is available

to translate the codes to machine language opcodes (operation codes) of the

8052.Assembly language is a low level language that is closest to machine language

because they have one to one relationship with machine language codes. To code in

assembly language it is necessary to understand the architecture of the microprocessor.

Another way of writing a program for a microcomputer is with a high-level

language such as BASIC, Paschal or C. These languages use program statements that are

more English-like than those of assembly language. Each high level language may

represent many machine code instructions. A compiler program is used to translate higher-

level language statements to machine codes which can be loaded into memory and

executed. To code in C for example we need a C compiler for 8052.

Programs can usually be written faster in high-level languages than assembly

language because the high-level language works with bigger building blocks. However,

such programs usually execute more slowly and require more memory than the same

programs written in assembly. Therefore, programs that involve a lot of hardware control

such as robots and factory control systems or programs that must run as quickly as

possible are best written in assembly language.

. The following kits are required to program in assembly language

• PG302 Programmer

• PG302 Programmer Software

• ADT 87 Adapter (for programming 89C51, 89C52 etc)

• ADT 90 Adapter

• DB9 Serial Port Cable

• Turbo Assembler (TASM)

4.4 PROGRAMMING THE 89C52

• Program is written with a text editor

• It is saved as filename.asm

• Click the start button on computer

• Click on run

• Browse to location of TASM

• Click on tasm.exe

• Type in TASM-51 file path\filename.asm filename.hex (this command assembles

the code to produce a hex file)

• The appropriate adapter is connected to the PG302 programmer

• The programmer is connected to the serial port of the computer using the serial

cable

• The microprocessor chip is plugged in and power is supplied to the programmer

• The PG302 is loaded and appropriate settings carried out

• The status of the chip is checked to confirm it is blank. If it is not blank, it must be

erased

• The hex file to be burnt into the chip is located

• Click on “Program device” (the programming takes a few seconds)

• When the process is complete, the chip is unplugged and fixed in the IC socket for

in-circuit testing

The TASM is an assembler that coverts the coded program into its hexadecimal

equivalent by converting the program into hex files, object file and list file. The hex file

shows the hexadecimal equivalent of the coded program. The list file shows the list of all

the errors that occurred while running the program and the line in which they occurred for

purposes of debugging. The “PG302” is the hardware that converts the hex codes into

machine language 1’s and 0’s.

The assembly codes are attached in the appendix

4.5 SYSTEM TESTING

The programmed 89C52 is placed in its socket and in circuit testing was carried out.

It was observed that the design objective was realized with a little drawback. When any 3

input voltages were tested, the voltage closet to the optimum voltage within a tolerance of

±20 was selected to power the load. When none of the input voltages were within the

tolerance range, ‘gen on’ was displayed. However, due to cost limitations we could not

actualize the start up of a generator.

CHAPTER FIVE

CONCLUSION

5.5 SUMMARY

The aim of this project which is to design and construct a microcontroller based

automatic power selector was realized. Programmed designed was employed to reduce

design complexity and achieve cost effectiveness in commercial production.

An analog –digital interface was necessary because supply voltages are analog in

nature but digital processing was employed. Digital processing is chosen over the analog

because of high efficiency, cost effectiveness and high accuracy of digital systems. The

system was designed to operate within a range of 0 – 300V (ac).

The system provides a solution to the problem of managing multiple power sources

that arose out of a need for constant power supply in developing countries like Nigeria. It

can be used in industries, base stations, football pitch, hospital theatres, residential

buildings etc.

5.6 CHALLENGES FACED

The major challenge faced was that of the unavailability of certain components. A

design of this nature required the use of an 89C51 microcontroller which has 128 byte of

RAM. However due to its unavailability in the local market, a trade off was made between

transport cost and the use of 89C52 with 256 byte of RAM which was under utilized. The

ADC0804 was also not available in the east leading to considerable travel expenses.

A design alternative was used to implement multiplexing due to scarcity of analog

multiplexers. This led to an increase in the size of codes required. Also some components

did not produce the nominal values of output specified by the manufacturer.

5.7 COST ANALYSIS

The estimate of the cost of the individual components is shown in table 5.1 below.

From the table, it can be inferred that the total cost for the actualization of this project is

put at about N 20,820 .00 including miscellaneous, transportation and other logistic

expenses.

S/N DESCRIPTION QUANTITY UNIT COST(N ) COST (N)

1 Autotransformer 1 4000 4000

2 Rectifier IC 3 100 300

Hire of PG302 Kit 4500

3 4016 1 200 200

4 ADC(0804) 1 1000 1000

5 89C52 microcontroller 1 2000 2000

6 Dual seven segment displays 4 200 800

7 Relays 3 70 210

8 Resistors 12 10 120

9 Capacitors 10 10 100

10 Crystal 1 50 50

11 Connectors 9yds 20 180

12 Light bulb 1 60 60

13 Dot Matrix Vero board 3 200 600

14 Transistors 12 10 120

15 Diodes 6 5 30

16 IC Sockets 5 50 250

18 Packaging 1300

19 Transport / Miscellaneous 5000

TOTAL 20,820

5.8 RECOMMENDATIONS FOR FURTHER STUDY

This design is only a model that can be modified in order to accommodate some

other specifications that may be required in the future.

The design of a circuit to automatically power on the generator in the event of

“power failure” and the possibility of expanding the number of voltage inputs to the

system are left as recommendation for further study and improvement.

It is also recommended that an uninterrupted power supply unit be provided for the

microcontroller so that in the event of absolute power failure and the unavailability of a

stand by generator, the microcontroller can still indicate this status.

REFERENCES

Altman, L. and Scrupski, S.E(Ed.), Applying Microprocessors, Electronics Magazine

Book Series, McGraw Hill.N.Y.1976.

Boylestad, R.L and Nashelsky, L., Electronic Device and Circuit theory & ed,

Hall of India Private Limited, New Delhi, 2003.

Hayes, T.C. and Horowitz, P.G., The Art Of Electronics, 2nd ed, Cambridge University

Press,Great Britain (1980)

Hordeski, M., Interfacing Microcomputers in Control Systems, Instruments and control

Systems,Vol.52, No.5, pp40-48, May 1979.

Kleitz, W., Digital and Microprocessor Fundamentals, Theory and Applications, 4th ed,

Prentice Hall Inc., Pearson Education, Japan, 2003.

Special Issue on Microprocessors, Proc. IEEE, vol.66. No.2, 1978.

Theraja, A. K and Theraja, B.L., A Textbook of Electrical Technology, S.Chand and

CompanyLimited, New Delhi, 1999.

The Online 8052 Resource, 8052 Tutorial and Reference, www.8052.com

Tocci, R.J. and Laskowski, L.P., Microprocessors and Microcomputers: Hardware and

Software,Prentice Hall Inc., New Jersey, 1979.

Weller, W.J., Assembly Level Programming for Small Computers, D.C Health and Co.,

Lexington, Massachusetts, 1975.

Zaks, R., Microprocessor: From Chips to Systems, SYBEX, Berkley, Carlifornia, 1977.