DESIGN AND CONSTRUCTION OF A DIGITAL CLOCK

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DESIGN AND CONSTRUCTION



OF



A DIGITAL CLOCK

BY

SAVAGE OLATUNDE AKINLOLU

MATRIC NUMBER 072041033








AUGUST 2009

DESIGN AND CONSTRUCTION

OF A

DIGITAL CLOCK

BY

SAVAGE OLATUNDE AKINLOLU

(MATRIC NO 072041033)

A PROJECT SUBMITTED TO


THE ELECTRICAL ELECTRONICS DEPARTMENT

SCHOOL OF ENGINEERING, LAGOS STATE POLYTECHNIC IKORODU,
LAGOS.

IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF
NATIONAL DIPLOMA IN ELECTRICAL ELECTRONICS ENGINEERING


AUGUST 2009

5, Yetunde Ibiwunmi

Close,



Ahmadiyya,




Ijaye


Ojokoro,


Lagos

S
tate.



August 2009.


The Head of Department

Electrical Electronics Engineering

Lagos State Polytechnic,

Ikorodu Lagos

Dear Sir,

LETTER OF SUBMISSION


I

hereby submit a project on the construction and

design of a digital clock in
accordance with the regulation of the Lagos state polytechnic in partial fulfillment of the
requirement for the award of national diploma in Electrical Electronics engineering.





Yours faithfully


……………………………



Savage O.A


2007/2008 Session
CERTIFICATION


This is to certify that this project was carried out by Mr. SAVAGE Olatunde Akinlolu
Matric no
072041033 of the department of Electrical Electronics Engineering Lagos State
Polytechnic Ikorodu for the award of National Diploma in Electrical Electronics
Engineering.


Mr. Adaramola …
………………………….

(Project Supervisor) sign/date






Engr Ogundare ……………………………..

(Head of Department)

sign/date






Savage Olatunde A. …………………………..

(
Author of Project
)





sign/date
DEDICATION

I dedicate this project to God Almighty who gave me the knowledge and the natural
interest in Electronics. I also dedicate this project to every member of Savage family,
Beckley family, RCCG Rehoboth Sanctuary
and my friends at school.
ACKNOWLEDGEMENT


I want to use this opportunity to thank all those that contributed to the success of my
project.


Firstly, I thank

the God Almighty
who kept me alive till now and gave me
knowledge wisdom and
understanding of this project. Secondly, I thank my parents and
brothers who encouraged me. Thirdly, I render my special thanks to my pastor, Pastor
Mrs. Yemi Oshilowo who built me faith in God. I am also grateful to my supervisor Mr.
Adaramola who gave me

the necessary assistance that I needed to complete my project.


Finally, my thanks to Google.com and all other website consulted for knowledge and
informations.



ABSTRACT


This project is a digital clock that uses logic
circuit and binary operations in
conjunction with LED seven segment display to display time digitally. This digital clock
was designed for 24 hours time format and also digitally display seconds.


The problem that is usually encountered using

the analog display of time is that
there is not much accuracy in the readings and the person using it has to calculate and
interpret the hours and the minutes. The analog clock is displayed using a scale and a
pointer, due to this, there is much parallax
and human error in the readings which makes
it lass accurate.


The solution to the problem outlined above is to design a digital clock which will
eliminate parallax error an
d reduces human error and thereby giving

a high level of
accuracy in t
he readings.


Another interesting thing about this digital clock is that it uses LED (Light
Emitting Diode)
-

type Seven Segment display and therefore can be seen in dark
n
ess
which most of analog clock do not have.


Though the cos
t of producing a digital clock is more that that of an analog type
but the advantage of a digital clock is much more than that of an analog type.


In consideration of the problems and solutions suggested, it is concluded that a
digital
clock is very useful and it will make life easier.

TABLE OF CONTENTS

Cover page

Title

Certification

Dedication

Acknowledgement

Abstract

Table of contents


CHAPTER 1

Introduction

CHAPTER 2
:
LITERATURE REVIEW

2.0
THEORY AND OPERATIONAL PRINCIPLES OF
DEVICES

2.1 Resistors

2.2 Capacitor

2.3 Diodes

2.4 Transistors

2.5 voltage regulator

2.6 connectors

2.7 7 segment display

2.8

1Cs

2.10

Boards

CHAPTER THREE :




CHAPTER 1

INTRODUCTION

Electronics system is a system that deals with the flow of electrons. Resistance, voltage
and current are the basics of electronics. Electronics system refines, extends, supplements
human facilities and gives us the ability to perceive, communicate, calcul
ate, remember,
reason and so on. Generally, it makes life easier. Electronics device in modern days are
now replacing
electro
mechanical device. For example, an automated electronic switch
that is programmed to operate at a specific

time under a specific co
ndition makes a
mechanical (manual) switch useless. Electronics system is

preferable to mechanical
system

because it has a high degree of accuracy and efficiency.


Electronics system is classified into two part
i.e.

Analog

and Digital electronics

sy
stem
. Analog system is

a system

that deals with discrete values of quantity and changes
its output value linearly with the change in input value. Example is a scale and a
pointer

device such as an analog multimeter, a weight scale, analog clocks e.t.c.

A d
igital system
is the on
e that produces only two output levels

based on the range of values of the input
i.e. its output is represented by two discrete levels only which is

1


and

0

. This 1 and 0
values are called Bits taken from the word ‘’Binary Digit’
’. The bits are used to perform
many arithmetic and logical operations. The 1 and 0 values are otherwise known as logic
levels or logic states. The 0 level is called the LOW level while the 1

level is called the
HIGH level. With
this bit
, it is easier for
computer to process data and
gives

out the
required output because it is only dealing with
1 and 0 values.

This makes it possible for
m
e to design a digital
clock that

will display time
in form of figures instead of scale and
pointer as to the case of an a
nalog clock.


This digital clock makes use of a 555 timer IC wired in an astable mode and it is
designed to give output pulse of 1Hz i.e. one
second

for one complete cycle. The pulse
signal otherwise known as clock is fed into

a BCD counter which
gives a synchronous
8421 (4 bit) output
see figure 1

which
1 is the least significant bit LSB. This synchronous
output if fed into a BCD
-
to
-

Seven Segment Decoder/Driver which will drive the
seven
segment display. The BCD
counters are

cascaded
and various

processes are undergone.
The end product is a

24 Hours

digital clock.


CHAPTER 2

LITERATURE REVIEW



THEORY AND OPERATIONAL PRINCIPLES OF DEVICES

2.1

Resistors

Resistor is a device that offers an opposition to the flow of
current. It is used to limit the
current that flows through a device
. Its value is measured in
ohms.
Resistors have a very
wide range of value, about 0.01ohm to 100Megaohm. A resistor varies in value when
subjected to temperature change. Resistor can be co
nnected in either series or in
parallel
to increase of decrease its values.

It can be connected in either way because it is not
polarized.


2.1.1

Parallel connection of a resistor
:

If we want a lower value of resistance
, we can
connect two or more high value of resistors in parallel to each other

as shown in fig 1

to
give us a total
resistance of: 1/R
T
= 1/R
1

+1/
R
2


2.1.2
Series connection of a resistor:

If we want a higher value of resistance, we can
connect two or more low value of resistor in series to each other as shown in fig1 to give
a total resistance
of: R
T

= R
1

+ R
2

Resistor series connection

Resistor parallel connections

Fig 1

Circuit symbol

2.1.
3
Resistor color code:

The values of modern resistors are being identified by series
of colors printed on them. The value of a resistor is known by the arrangement of these
colors.

Four colors are usually printed on them; First two colors represents a valu
e, third
color represent the multiplier and the last represent the tolerance i.e. the degree of
variation from the nominal value. These colors are shown in the table below:

COLO
R

1
ST

COLOR

2
ND

COLO
R

3
RD

COLO
R

4
TH

COLO
R


Function

Tens

Unit

Mltpy X 10
n

Tolerance


BLACK

0

0

0

+/
-

1%


BROWN

1

1

1

+/
-

2%


RED

2

2

2



ORANGE

3

3

3



YELLOW

4

4

4



GREEN

5

5

5



BLUE

6

6

--



VIOLET

7

7

--



GREY

8

8

--



WHITE

9

9

--



GOLD

--

--

--

+/
-

5%


SILVER

--

--

--

+/
-

10%




Fig 2 Resistor color code
.


2.2
Capacitor

Capacitor stores electric charge
. Its value is measured in
Farads (F)
. They are used with
resistors in timing circuits because it takes time for a capacitor to be filled with charge.
They have a wide range of application such as timing, radio

frequency, smoothing,
coupling, blocking e.t.c. Capacitor blocks AC signals when connected in parallel to a
power supply and blocks DC signals in the case of coupling used in audio circuits.

Capacitors can be a polarized and non
polarized form. The polari
zed capacitor is of a
higher value about 1uF to 6800uF. They have both positive and negative leads and cannot
be connected in either way. The second is the non
-
polarized form is of lower value which
has a range of value of about 1pF to 1uF. They can be con
nected in either way.

Capacitors

can be connected in either series or in parallel to increase of decrease its
values.

2.2.1
Parallel/Series connection of a capacitor
:
Capacitors can be connected either in
series or in parallel to reduce or increas
e is capacitance. The total value of capacitance
when connected in series is Ct = (
Ca Cb
)/(Ca + Cb) while the total capacitance when
connected in parallel is Ct = Ca +
Cb.



2.3

Diodes

Diodes allow electricity to flow in only one direction. It has both
Positive junction

and a
Negative junction
. In the negative junction, electrons are the majority while holes are
the
minority carrier. In the posi
tive junction, holes are the majority carrier and electrons are
the minority carrier. There are different types of diode such as Rectifier diode, Zener
diode, Light Emitting Diode e.t.c. Diodes are classified

as silicon typ
e and germanium
type

owing to their
breakdown voltage.

Diodes have

a wide range of application such as
demodulators, power circuits, display circuits, logic
circuits;

e. t. c.

diodes have voltage
Circuit symbols

Polarized Capacitor

Non
-

polarized capacitor

Fig 3

current and power ratings. Two or more diodes canbe connecte
d in in variouc forms; to
form a transistor, rectifier, bridge rectifier, AND gate, and so on.



2.4
Transistors


Fig 5 Symbols of various types of transistor

A transistor is a device capable of amplifying voltage and current. A transistor is made up
of three leads which is named; Emitter, Base and Collector

and it is of two

types NPN
and PNP transistors
.

Transi
stors are of different types and

sizes
, different packages, different voltage current
and power ratings and also of different gains.

Figure 6 is a water analogy of a transistor.

There are three openings
labeled

"B" (B
ase), "C" (Collector) and "E" (Emitter).

We provide a reservoir of water "C" (the "power supply voltage") but it can't move
Circuit symbol

Fig 4

Bridge Rectifier

Transistor

because there's a black plunger in the way which is blocking the outlet to "E". The
reservoir of water is called the "supply voltag
e". If we increase the amount of water
sufficiently, it will burst our transistor just the same as if we increase the voltage to a real
transistor. We don't want to do this, so we keep that "supply voltage" at a safe level. If we
pour water current into re
servoir "B" (the base “voltage pressure”) this current flows
along the "Base" pipe and pushes the black plunger upwards, allowing quite a lot of water
to flow from "C" to "E". Some of the water from "B" also joins it and flows away. If we
pour even more wa
ter into "B", the black plunger moves up further and a great torrent of
water current flows from "C" to "E".

This is exactly how a transistor works.

Fig 6


Facts derived from these analogy are;

1. A tiny amount of current flowing into "B" allows a large amount to flow from "C" to
"E" so we have an "amplification effect".

We can control a BIG flow of current with a SMALL flow of current. If we continually
change the small amount of water flowing
into "B" then we cause corresponding changes
in the LARGE amount of water flowing from "C" to "E".

The device has a "gain" or "amplification" factor. In a real transistor we measure current
in thousandths of an Ampere or "milliamps". For example, 1mA flow
ing into "B" can
allow
100mA to flow from "C" to "E".

2. The amount of current that can flow from "C" to "E" is limited by the "pipe diameter".
So, no matter how much current we push into "B", there will be a point beyond which we
can't get any more curre
nt flow from "C" to "E". The only way to solve this problem is to
use a larger tra
nsistor. A "power transistor".

3. The transistor can be used to switch the current flow on and off. If we put sufficient
current into "B," the transistor will allow the maxi
mum amount of current to flow from
"C" to "E". The tran
sistor is switched fully "on".

If the current into "B" is reduced to the point where it can no longer lift the black plunger,
the transistor will be "off". Only a small "leakage" current from "B" will

be flowing. To
turn it fully off, we must stop all current flowing into "B". Notice that we need a certain
amount of “voltage pressure” in reservoir “B” before the plunger will move at all. This
voltage is approximately 0.6 volts for a silicon transistor.


If B is less than 0.6 volts, no
current can flow at all. But it can’t be more than 0.6 volts because the black plunger
opens and relieves the pressure. In a real transistor, any restriction to the current flow
causes heat to be produced. This happens wit
h air or water in other things: for example,
your bicycle pump becomes hot near the valve when you pump air through it. A transistor
must be kept cool or it will be damaged. It runs coolest when it is fully OFF and fully
ON. When it is fully ON there is ve
ry little restriction so, even though a lot of current is
flowing, only a small amount of heat is produced. When it is fully OFF then NO heat is
produced. If a transistor is half ON then quite a lot of current is flowing through a
restricted gap and heat i
s produced. To help get rid of this heat, the transistor might be
clamped to a metal plate which draws the heat away and radiates it to the air. Such a plate
is called a "heat sink." It often has fins to increase its surface area and thereby improves
the e
fficiency.

The difference between PNP and NPN transistors is that NPN use
electrons as carriers of current and PNP use a lack of electrons (known as "holes").


2.5
V
oltage regulator
:

One of the most commonly used circuits is

the

VOLTAGE
REGULATOR. The si
mplest design uses just a resistor and a Zener

diode. In the
circuit

below there
is
a

resistor (R1) and a Zener diode (ZD1) connected across a power supply.
The resistor is connected to the positive (+ve) of the supply and the Zener diode anode is
connected to the zero volts (ground). At the junction of these two components the voltage
is
clamped by the Zener diode to its specified voltage
-

in this case
, 5.6 volts is the output

of the circuit.

The resistor R1 can be of a lower value and higher wattage if high current
is needed, or better still we can use a
fixed biased
transistor in place
of the resistor.



fig 7

The output voltage will remain at a constant value of 5.0 volts provided the input voltage
from the supply is more than 6 volts (the
Zener

voltage plus a little to compensate for that
"lost" across the resistor).
The Zener voltage

regulator can be connected to series or
circuits to give desired voltage and current. All the components can be integrated and
packaged in a TO
-
3 or other packages to give a single 3
-
pin voltage regulator as for 7809
used in this project.

2.6

C
onnectors
:
Connector otherwise known as jumpers provides an unimpeded flow of
electricity. It is usually used to connect on
e

part of a circuit to the other where the copper
track on a
PCB or a Vero
board cannot get to
.



Fig 7

2.7

Seven segment Display Decoders

A
Decoder

IC,

is a device which converts one digital format into another and the
most commonly used device for doing this is the BCD (Binary Coded Decimal) to
Connectors or Jumper

on a Vero board

7
-
Segment Display Decoder. 7
-
segment LED (Light Emitting Diode) or LCD
(Liquid Crystal) Displays, provide a ve
ry convenient way of displaying information
or digital data in the form of Numbers, Letters or even Alpha
-
numerical
characters and they consist of 7 individual LEDs (the segments), within one
single display package. In order to produce the required numbers

or characters
from 0 to 9 and A to F respectively, on the display the correct combination of LED
segments need to be illuminated and
Display Decoders

do just that. A standard
7
-
segment LED or LCD display generally has 8 input connections, one for each
LED

segment and one that acts as a common

terminal or connection for all the
internal segments. Some single displays have an additional input pin for the
decimal point in their lower right or left hand corner.

There are two important types of 7
-
segment LED di
gital display.



The Common Cathode Display (CCD)

-

In the common cathode display,
all the cathode connections of the LEDs are joined together to logic "0" and
the individual segments are illuminated by application of a "HIGH", logic "1"
signal to the indivi
dual Anode terminals.



The Common Anode Display (CAD)

-

In the common anode display, all
the anode connections of the LEDs are joined together to logic "1" and the
individual segments are illuminated by connecting the individual Cathode
terminals to a "LOW
", logic "0" signal.

7
-
Segment Display Format


Fig 8

Truth Table for a 7
-
segment display

Individual Segments

Display

a

b

c

d

e

f

g

×

×

×

×

×

×



0



×

×









1

×

×



×

×



×

2

×

×

×

×





×

3



×

×





×

×

4

×



×

×



×

×

5

×



×

×

×

×

×

6

×

×

×









7


Individual Segments

Display

a

b

c

d

e

f

g

×

×

×

×

×

×

×

8

×

×

×





×

×

9

×

×

×



×

×

×

A





×

×

×

×

×

b

×





×

×

×



C



×

×

×

×



×

d

×





×

×

×

×

E

×







×

×

×

F






7
-
Segment Display Elements for all Numbers.

It can be seen that to display any single digit number from 0 to 9 or letter from A to F, we
would need 7 separate segment connections plus one additional connection for the LED's
"common" connection. Also as the segments are basically a standard light emi
tting diode,
the driving circuit would need to produce up to 20mA of current to illuminate each
individual segment and to display the number 8, all 7 segments would need to be lit
resulting a total current of nearly 140mA, (8 x 20mA). Obviously, the use of

so many
connections and power consumption is impractical for some electronic or microprocessor
based circuits and so in order to reduce the number of signal lines required to drive just
one single display, display decoders such as the BCD to 7
-
Segment Dis
play Decoder and
Driver IC's are used instead.

2.8 LED: LED are

light emitting diodes. They are a special type of diode that will radiate
light energy when connected in the forward bias mode. They consume about 30mA. They
are of different colors and some are bi
-
colors i.e. two colors and tri
-
colors i.e. three
colours.


2.9

1Cs
: IC is the abbreviation of Integrated Circuit. It
consists

of ten to one million
transistor

diodes and resistors.

IC can be of a small scale integration, medium scale
integration, large scale integration, very large scale integration and so on.th
e pin out of an
IC ranges from 3 pins

(very small scale integration) to about 100 pins (very large scale
integration)
. ICs are classified into two major classes: Analog IC and Digital IC. We also
have an Analog to digital converter IC and a Digital to Anal
og converter IC.


The ICs
used in this project are

the 555 Timer oscillator, 7404 Quad 2 input AND
GATE, 4510 BCD Synchronous

Counter, 4511 BCD to 7 Segment display. I shall
explain in details the principle of operation of each and every one of them.

2.9.1
555 TIMER IC

:

Fig 9

Picture of the 555 Timer IC


T
he 555, in fig. 1 and fig. 2 above, come in two packages, either the round metal
-
can
called the 'T' package or the more familiar 8
-
pin DIP 'V' package. About 20
-
years ago the
metal
-
can type was
pretty much the standard (SE/NE types). The 556 timer is a dual 555
version and comes in a 14
-
pin DIP package, the 558 is a quad version with four 555's
also in a 14 pin DIP case.


Block diagram of the 555 Timer


I
nside

the 555 timer, at fig. 3, are the equivalent of over 20 transistors, 15 resistors, and
2 diodes, depending of the manufacturer.

The name 555 was taken out from the three 5K
resistor between the Vcc see fig3, the two comparators and the ground.

The equiva
lent
circuit, in block diagram, providing the functions of control, triggering, level sensing or
comparison, discharge, and power output. Some of the more attractive features of the 555
timer are: Supply voltage between 4.5 and 18 volt, supply current 3 to

6 mA, and a
Rise/Fall time of 100 nSec. It can also

withstand quite a bit of abuse.

T
he Threshold current determine the maximum value of Ra + Rb. For 15 volt operation
the maximum total resistance for
R

(Ra +Rb) is 20 Mega
-
ohm.



Pin 1 (Ground):

The gro
und (or common) pin is the most
-
negative supply potential of
the device, which is normally connected to circuit common (ground) when operated from
positive supply voltages.


Pin 2 (Trigger):

This pin is the input to the lower comparator and is used to set

the
latch, which in turn causes the output to go high. Triggering is accomplished by taking
the pin from above to below a voltage level of 1/3 V+ (or, in general, one
-
half the voltage
appearing at pin 5). The action of the trigger input is level
-
sensitive
, allowing slow rate
-
of
-
change waveforms, as well as pulses, to be used as trigger sources. If this pin is held
low longer, the output will remain high until the trigger input is driven high again. One
precaution that should be observed with the trigger in
put signal is that it must not remain
lower than 1/3 V+ for a period of time
longer

than the timing cycle. If this is allowed to
happen, the timer will re
-
trigger itself upon termination of the first output pulse. Thus,
when the timer is driven in the mono
stable mode with input pulses longer than the
desired output pulse width, the input trigger should effectively be shortened by
differentiation. The minimum
-
allowable pulse width for triggering is somewhat
dependent upon pulse level, but in general if it is

greater than the 1uS (micro
-
Second),
triggering will be reliable. A second precaution with respect to the trigger input concerns
storage time in the lower comparator. This portion of the circuit can exhibit normal turn
-
off delays of several microseconds a
fter triggering; that is, the latch can still have a
trigger input for this period of time
after

the trigger pulse. In practice, this means the
minimum monostable output pulse width should be in the order of 10uS to prevent
possible double triggering due t
o this effect. The voltage range that can safely be applied
to the trigger pin is between V+ and ground. A dc current, termed the
trigger

current,
must also flow from this terminal into the external circuit. This current is typically 500nA
(nano
-
amp) and w
ill define the upper limit of resistance allowable from pin 2 to ground.
For an astable configuration operating at V+ = 5 volts, this resistance is 3 Mega
-
ohm; it
can be greater for higher V+ levels.


Pin 3 (Output):

The output of the 555 comes from a hig
h
-
current totem
-
pole stage made
up of transistors Q20
-

Q24. Transistors Q21 and Q22 provide drive for source
-
type
loads, and their Darlington connection provides a high
-
state output voltage about 1.7
volts less than the V+ supply level used. Transistor Q2
4 provides current
-
sinking
capability for low
-
state loads referred to V+ (such as typical TTL inputs. Transistor Q24
has a low saturation voltage, which allows it to interface directly, with good noise
margin, when driving current
-
sinking logic. Exact outp
ut saturation levels vary markedly
with supply voltage, however, for both high and low states. At a V+ of 5 volts, for
instance, the low state Vce(sat) is typically 0.25 volts at 5 mA. Operating at 15 volts,
however, it can sink 200mA if an output
-
low volt
age level of 2 volts is allowable (power
dissipation should be considered in such a case, of course). High
-
state level is typically
3.3 volts at V+ = 5 volts; 13.3 volts at V+ = 15 volts. Both the rise and fall times of the
output waveform are quite fast,
typical switching times being 100nS. The state of the
output pin will always reflect the inverse of the logic state of the la
tch
.

Since the latch
itself is not directly accessible, this relationship may be best explained in terms of latch
-
input trigger con
ditions. To trigger the output to a high condition, the trigger input is
momentarily taken from a higher to a lower level. [see "Pin 2
-

Trigger"]. This causes the
latch to be set and the output to go high. Actuation of the lower comparator is the only
man
ner in which the output can be placed in the high state. The output can be returned to
a low state by causing the threshold to go from a lower to a higher level [see "Pin 6
-

Threshold"], which resets the latch. The output can also be made to go low by tak
ing the
reset to a low state near ground [see "Pin 4
-

Reset"]. The output voltage available at this
pin is approximately equal to the V
cc applied to pin 8 minus 1.7V.

Pin 4 (Reset):

This pin is also used to reset the latch and return the output to a low
state.
The reset voltage threshold level is 0.7 volt, and a sink current of 0.1mA from this pin is
required to reset the device. These levels are relatively independent of operating V+
level; thus the reset input is TTL compatible for any supply voltage. T
he reset input is an
overriding function; that is, it will force the output to a low state regardless of the state of
either of the other inputs. It may thus be used to terminate an output pulse prematurely, to
gate oscillations from "on" to "off", etc. De
lay time from reset to output is typically on
the order of 0.5 µS, and the minimum reset pulse width is 0.5 µS. Neither of these figures
is guaranteed, however, and
may vary
from one manufacturer to another. In short, the
reset pin is used to reset the fli
p
-
flop that controls the state of output pin 3. The pin is
activated when a voltage level anywhere between 0 and 0.4 volt is applied to the pin. The
reset pin will force the output to go low no matter what state the other inputs to the flip
-
flop are in. Wh
en not used, it is recommended that the reset input be tied to V+ to avoid
any
possibility of false resetting.

Pin 5 (Control Voltage):

This pin allows direct access to the 2/3 V+ voltage
-
divider
point, the reference level for the upper comparator. It als
o allows indirect access to the
lower comparator, as there is a 2:1 divider (R8
-

R9) from this point to the lower
-
comparator reference input, Q13. Use of this terminal is the option of the user, but it does
allow extreme flexibility by permitting modifica
tion of the timing period, resetting of the
comparator, etc. When the 555 timer is used in a voltage
-
controlled mode, its voltage
-
controlled operation ranges from about 1 volt less than V+ down to within 2 volts of
ground (although this is not guaranteed).

Voltages can be safely applied outside these
limits, but they should be confined within the limits of V+ and ground for reliability. By
applying a voltage to this pin, it is possible to vary the timing of the device independently
of the RC network. The co
ntrol voltage may be varied from 45 to 90% of the Vcc in the
monostable mode, making it possible to control the width of the output pulse
independently of RC. When it is used in the astable mode, the control voltage can be
varied from 1.7V to the full Vcc.

Varying the voltage in the astable mode will produce a
frequency modulated (FM) output. In the event the control
-
voltage pin is not used, it is
recommended that it be bypassed, to ground, with a capacitor of about 0.01uF (10nF) for
immunity to noise, sinc
e it is a comparator input. This fact is not obvious in many 555
circuits since I have seen many circuits with 'no
-
pin
-
5' connected to anything, but this is
the proper procedure. The small ceramic cap may eliminate false triggering.


Pin 6 (Threshold):

Pi
n 6 is one input to the upper comparator (the other being pin 5)
and is used to reset the latch, which causes the output to go low. Resetting via this
terminal is accomplished by taking the terminal from below to above a voltage level of
2/3 V+ (the normal

voltage on pin 5). The action of the threshold pin is level sensitive,
allowing slow rate
-
of
-
change waveforms. The voltage range that can safely be applied to
the threshold pin is between V+ and ground. A dc current, termed the
threshold

current,
must als
o flow into this terminal from the external circuit. This current is typically 0.1µA,
and will define the upper limit of total resistance allowable from pin 6 to V+. For either
timing configuration operating at V+ = 5 volts, this resistance is 16 Mega
-
ohm.

For 15
volt operation, the maximum value of resistance is 20 MegaOhms.


Pin 7 (Discharge):

This pin is connected to the open collector of a npn transistor (Q14),
the emitter of which goes to ground, so that when the transistor is turned "on", pin 7 is
ef
fectively shorted to ground. Usually the timing capacitor is connected between pin 7
and ground and is discharged when the transistor turns "on". The conduction state of this
transistor is identical in timing to that of the output stage. It is "on" (low re
sistance to
ground) when the output is low and "off" (high resistance to ground) when the output is
high. In both the monostable and astable time modes, this transistor switch is used to
clamp the appropriate nodes of the timing network to ground. Saturati
on voltage is
typically below 100mV (milli
-
Volt) for currents of 5 mA or less, and off
-
state leakage is
about 20nA (these parameters are not specified by all manufacturers, however).
Maximum collector current is internally limited by design, thereby removi
ng restrictions
on capacitor size due to peak pulse
-
current discharge. In certain applications, this open
collector output can be used as an auxiliary output terminal, with current
-
sinking
capability similar to the output (pin 3).


Pin 8 (V +):

The V+ pin

(also referred to as Vcc) is the positive supply voltage terminal
of the 555 timer IC. Supply
-
voltage operating range for the 555 is +4.5 volts (minimum)
to +16 volts (maximum), and it is specified for operation between +5 volts and +15 volts.
The device
will operate essentially the same over this range of voltages without change in
timing period. Actually, the most significant operational difference is the output drive
capability, which increases for both current and voltage range as the supply voltage is

increased. Sensitivity of time interval to supply voltage change is low, typically 0.1% per
volt. There are special and military devices available that operate at voltages as high as
18 volts.













This is a Test circuit to test the IC weather
it is working properly.

2.9.2

4510 BCD (Binary Coded Decimal) counter


BCD stands for
B
inary
C
oded
D
ecimal. A BCD counter has four outputs usually
labe
l
led

A, B, C, D. By convention A is the least significant bit, or
LSB

and D is the most

significant bit
MSB
. The easiest way to understand what a BCD counter does is to follow
the counting sequence in truth table form:

pulses

output D

output C

output B

output A

0

0

0

0

0

1

0

0

0

1

2

0

0

1

0

3

0

0

1

1

4

0

1

0

0

5

0

1

0

1

6

0

1

1

0

7

0

1

1

1

8

1

0

0

0

9

1

0

0

1

Reset
10

0

0

0

0

11

0

0

0

1

:

:

:

:

:

When pulses are delivered to the CLOCK input (and all the other connections needed for
basic operation are made), the outputs of the 4510 follow a sequence starting from 0

0

0

0
up to 1

0

0

1, the
binary equivalent of the decimal number 9. The next pulse causes the
4510 to RESET and counting starts again from 0

0

0

0.

In other words, the counter outputs follow a binary sequence representing the decimal
numbers 0
-
9.... this is why the 4510 is called
a binary coded decimal counter


pin

name

connections

1

LOAD input

normally held LOW, 0 V

2

output D, Q8

output, bit 3

3

load input D, L8

connect LOW

4

load input A, L1

connect LOW

5

carry in (enable)

normally held LOW, 0 V

6

output A, Q1

output, bit
0

7

carry out

no connection needed

8

0 V

power supply 0 V

9

RESET input

normally held LOW

10

UP/DOWN input

HIGH=UP, LOW=DOWN

11

output B, Q2

output, bit 1

12

load input B, L2

connect HIGH

13

load input C, L4

connect HIGH

14

output C, Q4

output, bit

2

15

CLOCK input

pulses in from astable

16

+V

power supply +9 V (range 5
-
15 V)


2.9.2.1

Shortening the count

What do you do when you want to shorten the count sequence, for example, when you
want to use the 4510 as part of the circuit for an electronic

die? For a normal die you
need just six different output states. This can be achieved by RESETTING the counter
when the count reaches 0

1

1

0, the BCD number which represents 6.

Look back briefly at the truth table for BCD counting. When the count reaches

0

1

1

0,
outputs B and C are HIGH together for the first time. You can use an AND gate to reset
the counter, like this:

pulses

output D

output C

output B

output A

0

0

0

0

0

1

0

0

0

1

2

0

0

1

0

3

0

0

1

1

4

0

1

0

0

5

0

1

0

1

6

0

0

0

0

7

0

0

0

1

:

:

:

:

:

2.9
.3

4511 BCD to 7 segment driver

;

This is an ic that converts binary codes BCD to an equivalent binary codes that will drive
a 7 segment display to display figures. Here are the operations of the code conversion.





Outputs of the 4511
-

7 segment driver


Pulse

BCD

a

b

c

d

e

f

g


0

0000

1

1

1

1

1

1

0


1

0001

0

1

1

0

0

0

0


2

0010

1

1

0

1

1

0

1


3

0011

1

1

1

1

0

0

1


4

0100

0

1

1

0

0

1

1


5

0101

1

0

1

1

0

1

1


6

0110

1

0

1

1

1

1

1


7

0111

1

1

1

0

0

0

0


8

1000

1

1

1

1

1

1

1


9

1001

1

1

1

0

0

1

1



2.9.4
BCD to 7
-
Segment Display Decoders

A binary coded decimal (BCD) to 7
-
segment

display decoder such as the
4511,
have 4 BCD inputs and 7 output lines, one for each LED segment. This allows a
smaller 4
-
bit binary number (half a byte) to be used to display all the denary
numbers from 0 to 9 and by adding two displays together, a full range of numbers
from 00 to 9
9 can be displayed with just a single byte of 8 data bits.

BCD to 7
-
Segment Decoder


The use of
packed

BCD allows two BCD digits to be stored within a single byte
(8
-
bits) of data, allowing a single data byte to hold a BCD number in the range of
00 to 99.

An example of the 4
-
bit BCD input (0100) representing the number 4 is given
below.

Examples



2.9.5

2
INPUTS

AND GATE 7408
.

A gate is a digital electronics device used to
perform logical and arithmetical operation. A gate can have as many input as possible but
only one output. An AND gate is a gate such that when all the input is on the output is on
an
d when at least one of the input is off, the
output is off.



CHAPTER THREE

MATERIALS AND METHODOLOGY


3.1
Brief notes on each block.



Time based oscillator
-

contains a 555 timer ic wired in an astable mode with a
frequency of 1Hz. It provides clock pulse for the seconds display.


BCD counter


Converts each clock pulse
received into a

4 bit

8421 synchronous

logic output.


Display decoder/driver
-

decodes the 4 bit 8421 logic and converts it to a seven
segment display codes. It also provides 30mA of current which is able to drive the
seven segment display so no need of a b
uffer.


Seven segment display
-

this is a LED type of display which provides the read out
of the figures 0 to 9.

Dual 7 segment display

BLOCK DIAGRAM FOR A DIGITAL CLOCK (BY SAVAGE OLATUNDE)

unit

unit

unit

ten
s

ten
s

ten
s

BCD

count
er

BCD

count
er

BCD

count
er

BCD

count
er

BCD

count
er

BCD

count
er

Time
based
oscillato
r

SECOND
S

MINUTE
S

HOURS

Display


drivers

Display


drivers

Display


drivers

Display


drivers

Display


drivers

Display


drivers

Dual 7 segment display

Dual 7 segment display

REGULATE
D

POWER

SUPPLY

BATTERY

BACKUP

SYSTEM


Regulated power supply. It contains a transformer which steps down the voltage
from 220V to 12V. it also contains a rectifier which converts the

AC to DC. It
also contains a voltage regulator which stabilizes the voltage to 9VDC.


Battery backup system. This system allows the digital clock to run when there is
no power supply. When the supply is interrupted, data is lost because it contains
no memo
ry. One needs to reset it when there is power supply. This battery system
keeps on charging
when there is power supply and it can supply power to the
digital clock for about 36 hours when there is no mains power supply


3.2
CIRCUIT OPERATION

The digital cl
ock makes use of a 555 timer oscillator which is wired

in an astable mode to
provide

a 1Hz clock pulse. The clock pulse is fed into each BCD counter of the seconds’
stage. The BCD counter provides a 4 bit 8421 synchronous logic output for every clock
pulse

received. This is shown in the figure below.



8

4

2

1


pulses

output
D

output
C

output
B

output
A

Display
output

0

0

0

0

0

0

1

0

0

0

1

1

2

0

0

1

0

2

3

0

0

1

1

3

4

0

1

0

0

4

5

0

1

0

1

5

6

0

1

1

0

6

7

0

1

1

1

7

8

1

0

0

0

8

9

1

0

0

1

9

Reset 10

0

0

0

0

0

11

0

0

0

1

1


12


0

0

1

0

2


The unit of the seconds is enabled via pin 5 of the 4510 BCD counter and it begins to
count from
0


9 the BCD output is fed into

the decoder and the display which provides
the read
s

out of the figures as shown below.



Outputs of the 4511
-

7 segment driver


Pulse

BCD

a

B

c

d

e

f

g


0

0000

1

1

1

1

1

1

0


1

0001

0

1

1

0

0

0

0


2

0010

1

1

0

1

1

0

1


3

0011

1

1

1

1

0

0

1


4

0100

0

1

1

0

0

1

1


5

0101

1

0

1

1

0

1

1


6

0110

1

0

1

1

1

1

1


7

0111

1

1

1

0

0

0

0


8

1000

1

1

1

1

1

1

1


9

1001

1

1

1

0

0

1

1



As soon as it count 9 it resets itself on the count of 10 and in turn enables the BCD
counter of the Tens stage via pin 7 and pin 5 of the BCD counter of the Unit and Tens
display. This makes the clock to display 00


99. The counter is then shortened by the use
of AND gate which resets the counter at the count of 00


59. When the BCD counter
resets, it gives a clock pulse to the Unit of the minutes display. The minutes cou
nts from
00


59 and gives a clock pulse to the Unit of the hours. The hours count from 00 to 23
and then reset itself on the count of 24. This process is being repeated every day and the
rest is a digital clock.



3.3
PROCEDURE FOR CONSTRUCTION

Materials Used


COMPONENTS

QUANTITY

RESISTOR, 2.2K,1K,6.8K, 68K,

1

DIODE IN4001

2

BRIDGE RECTIFIER

1

TRANSISTOR TIP42C

1

TRANSFORMER 220V


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1

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1

fC†㐵㄰ BC䐠䍏a乔bo

S

fC†㐵ㄱ BC䐠a䕃佄bo ‷
p䕇䵅乔⁄fpiA夠vof噅o

S

㔵㔠5f䵅M fC

1

䑕Ai‷⁓䕇䵅乔⁄䥓fi䅙

3

fC‷㐰㠠⁑ 䅄′ f乐k吠q乄
1

GATE

RED LED

2

VERO BOARD

1

IC SOCKET

14




3.4
These are the steps taken in the construction of the digital clock.

1.

Get the entire components ready identifying the right value of
resistor capacitor
and other components.

2.

First of all construct it on the bread board to see how it works and to see maybe
there will be something to add or remove from the circuit.

3.

Insert the components on the bread board according to the circuit diagram.

4.

If the circuit is working properly, transfer it to the Vero board.

5.

Solder the components on the Vero board. These are the steps taken when
soldering


Connect the soldering Iron into the mains power supply and allow it to
heat to the required temperature.


C
lean both surface to be soldered properly with emery cloth or sand paper.


Place the tip of the hot soldering Iron on the joint and place the solder
opposite to the soldering Iron.


Let the solder melt and run freely on both surface


Allow to cool and solidi
fy.

6. Cut out any excess pin of component appearing on the conducting surface of the
Vero board.


3.5
TOOL AND EQUIPMENT USED

1.

Soldering Iron

2.

Soldering lead

3.

Desoldering pump

4.

Cutter

5.

Pliers

6.

Bread board

7.

Connection wire

8.

Drilling Machine

9.

Digital
multimeter

10.

Logic state Analyzer

11.

Wire stripper

12.

Blade

13.

Magnifying Glass

.
CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 DIAGRAMS

This is the internal layout of component of the digital clock


This is the front panel layout of the digital clock



4.2
CALCULATIONS

An astable circuit produces a 'square wave’; this is a digital waveform with sharp
transitions between low (0V) and high (+Vs). Note that the durations of the low and
high states may be different

but in application of it in this digital clock, the high state
duration and the low state duration are the same
. The circuit is called an astable
because it is not stable in any state: the output is continually changing between 'low'
and 'high'.

To calcul
ate the timing period


T = 0.7 × (R1 + 2R2) × C1

To calculate the frequency

F =

1

1.4
f



(R1 + 2R2) X C1


T = time period in seconds (s)

f = frequency in hertz (Hz)

R1 = resistance in ohms ()

R2 = resistance in ohms ()

C1 = capacitance in farads (F)

In the 555 time based oscillator,

R1=6.8K

R2=68K

C1=10U

F=
. 1.4

.



(6800 + 2(68000))

10 X

0.000001


F=

1Hz



CHAPTER FIVE

RECOMMENDATION AND CONCLUSION

RECOMMENDATION

Embarking on this project will gives the author a wide field of knowledge about
digital circuit and ICs. This project alone has produced the knowledge of resistors,
transistors,

capacitors,

diodes, voltage regu
lators, 555 timer IC 4510 BCD counter,
4511 BCD to 7 segment decoder, 7408 AND gate and so on.

I recommend that this project should be improved upon by the use of microcontrollers
and multiplexers to construct this project because this will require less
IC

and the
complexity will be reduced, but the disadvantage of microcontrollers is that they are
dead without it being programmed; but if the programming is learned and well
understood, it will be very easy.

This project was done on the vero board but if it
can be done on printed circuit board
PCB, it will have a greater compactness and the it will be more portable.


CONCLUSION

A working digital clock was achieved by the use of various
components such as
resistors, capacitors, transistors, diodes, voltage reg
ulators, 555 timer IC 4510 BCD
counter, 4511 BCD to 7 segment decoder, 7 segment display 7408 AND gate and so
on cascaded together.



APPENDIX

ABBREVIATIONS

BCD

MSB

LSB

SSD

IC

BIT

REFERENCES

Principle of electronics by VK METHA

www.talkingelectronics.com

www.uoguelgh.ca

www.aaronscake.com

www.ic
-
on
-
line.cn

www.google.com

www
.yahoo.com