ABSTRACT

This Project defines a digital stop watch built around timer IC 555 and 4-digit counter IC

MM74C926 with multiplexed 7-Segment display.MM74C926 consists of a 4-digit counter,

an internal output latch, npn output sourcing drivers for common cathode, 7

segment display and internal multiplexing circuitry with four multiplexing output.

The counter advances on the negative edge of the clock are generated by the timer

555. The circuit works on a 5 volt power supply. I t can be easily assembled on a general-

purpose PCB. Enclose the circuit in the wooden box with provisions for 7 -

s e g m e n t d i sp l ays , t ime va ry ing sw i t ch S1 , s t a r t / s t op sw i t ch S2 , and reset

switch S3.

To reset the circuit press S3 so that the display shows ‘0000.’

To start open switch S2 for the stop watch to start counting the time. If you want to stop

the clock, close S2.

Rotary switch S1 is used to select the different time periods at the output of the astable

multivibrator (IC1).

1

Chapter1

INTRODUCTION

1.1 STOP WATCHES

Every stopwatch is composed of four elements: a power source, a time base, a counter, and an

indicator or display. The design and construction of each component depends upon the type of

stopwatch. Stopwatches can be classified into two categories, Type I and Type II. In general,

stopwatches are classified as:

Type I - Figure 1 is show the digital stop watch .Digital design employing quartz oscillators and

electronic circuitry to measure time intervals.

Type II - Stopwatches have an analog design and use mechanical mechanisms to measure time

intervals. Figure 2 is show the analog stop watch.

The power source of a digital stopwatch is usually a silver cell or alkaline battery, which powers

the oscillator, counting and display circuitry. The counter circuit consists of digital dividers that

count the time base oscillations for the period that is initiated by the start/stop buttons. The

display typically has seven or eight digits.

Figure 1. Type I digital stopwatch. Figure 2. Type II analog stopwatch.

2

1.2 Use of stop watch

A stopwatch is a handheld timer used in sporting events such as track meets, swim meets,

triathlons and other time-lapse games. It is designed to be manually started, for example at the

beginning of an event, then stopped with the press of a button at the exact moment a runner

crosses a finish line, a swimmer reaches the end lap or a skier sails past the final gate. Aside

from officials, coaches use stopwatches for training and practices. A stopwatch can be

mechanical, resembling a pocket watch with an analog face, or digital. Digital stopwatches are

more accurate.

Another function of a stopwatch is a countdown timer. The device can be set for a specific

amount of time, and at the press of a button, the timer starts counting down to zero. Calendar,

pacer and alarm functions are other features that might be included in a stopwatch.

Although stopwatches are most closely associated with sports, they are also used for many other

purposes. Stopwatches will also be found in research laboratories and some teachers use

stopwatches to hone debate skills, time spelling bees or issue tests.

These are some special application of digital stop watch.

1. For Experimental purpose in Educational institute.

2. Industries.

3. Cultural Events like “JUST A MINUTE (JAM)”.

4. Sports

3

Chapter 2

DIFFERENCE BETWEEN ANALOG AND DIGITAL WATCH

Analog and digital are common terms used in many fields, including audio. Since they can also

be difficult terms to understand, a few definitions and examples are helpful.

2.1 What are analog?

The word analog is defined as something that is similar to or analogous to something else. When someone

uses an analogy they are comparing one thing with another, such as “her eyes are as blue as the sky”.

Another example of an analogy or analog is an analog clock with hour and minute hands. The clock uses

the positions of the hands to describe the time – the clock is an analog that describes the time of day.

2.2What are digital?

The word digital is defined as something that uses a digit or number to describe something. A

digital clock uses numbers, not hands, to describe the time. For example, a digital clock uses

digits to describe the time as 10 hours, 43 minutes and 12 seconds. Each digit is a specific

numerical value that describes the time of day.

2.3 Comparing Analog to Digital

The clock example is a convenient way to understand the differences between analog and digital

audio. An analog clock is a continuously flowing representation of the time of day. The hands

move smoothly around the clock providing an analogy of the time. A digital clock uses distinct

or individual digits to describe the time – it is not smooth flowing, but is characterized by

discrete numbers that tell the time.

2.4 Digital Samples

Each digital sample is a snapshot in time that represents the size and shape of the sound of the

violin at that precise moment. A digital signal is a series of multiple snapshots or samples, not a

continues wave. Each sound sample is represented by digits, either zeros or ones. There are so

many samples that the human ear perceives the digital sample of the analog wave as a smooth-

4

flowing continuous sound. For example, on a Compact Disc, there are 44,100 digital samples for

each second of music stored on the disc.

5

Chapter 3

COMPONENTS DESCRIPTION

3.1 555 Timer IC:

Fig 3

PIN DESCRIPTION:

Pin Name Purpose

1 GND Ground, low level (0 V)

2

TRIG

OUT rises, and interval starts,

when this input falls below 1/3

VCC.

3 OUT This output is driven to

approximately 1.7V below +VCC

6

or GND.

4

RESET

A timing interval may be reset by

driving this input to GND, but

the timing does not begin again

until RESET rises above

approximately 0.7 volts.

Overrides TRIG which overrides

THR.

5

CTRL

"Control" access to the internal

voltage divider (by default, 2/3

VCC).

6

THR

The interval ends when the

voltage at THR is greater than at

CTRL.

7

DIS

Open collector output; may

discharge a capacitor between

intervals. In phase with output.

8 V+, vcc Positive supply voltage is usually

between 3 and 15 V.

7

Fig 4

DESCRIPTIONS:-

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and

oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-

flop element. The LM555 is a highly stable controller capable of producing accurate timing

pulses. With monostable operation, the time delay is controlled by one external resistor and one

capacitor. With astable operation, the frequency and duty cycle are accurately controlled with

two external resistors and one capacitor. The circuit may be triggered and reset on falling

waveforms and the output circuit can source or sink upto 200mA or drive TTL circuits. The 555

Timer IC is an integrated circuit implementing a variety of timer and multivibrator applications.

PIN 5 is also called control voltage pin. By applying a voltage to the CONTROL VOLTAGE

input, pin 5, you can alter the timing characteristics of the device. In most applications, the

CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF capacitor between pin 5

and 0 V to prevent interference. The CONTROL VOLTAGE input can be used to build an

astable with a frequency modulated output.

8

FEATURES:-

High Current Drive Capability (200mA)

Adjustable Duty Cycle

Temperature Stability of 0.005%/0C

Turn off Time Less Than 2 microsec

APPLICATIONS:-

Precision Timing

Pulse Generation

Time Delay Generation

Sequential Timing

3.1.1 astable mode:-

Fig 5

Fig shows the 555 Timer IC connected to an astable multivibrator. Initially, when the output is

high capacitor C starts charging toward Vcc through R1 and R2. However as soon as voltage

across the capacitor equals 2/3Vcc comparator triggers the flip flop and the output switches

low . Now capacitor C starts discharging through R2 . When the voltage across C equals 1/3Vcc

second comparator’s output triggers the flip flop and the output goes high. Then the cycle

repeats.

9

In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a

specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and

another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and

threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and

R2, and discharged only through R2, since pin 7 has low impedance to ground during output low

intervals of the cycle, therefore discharging the capacitor. In the astable mode, the frequency of

the pulse stream depends on the values of R1, R2 and C

The high time from each pulse is given by

High= ln (2). (R1 + R2). C

And the low time from each pulse is given by

Low = ln (2). R2. C

Where R1 and R2 are the values of the resistors in ohms and C is the value of capacitor in farads.

To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 towards the

capacitor. This bypasses R2 during the high part of the cycle so that the high interval depends

only on R1 and C1.

10

3.2 ICMM74C926

Fig 6

Description:-

The MM74C926 counters consist of a 4- digit counter, an internal output latch, NPN output

sourcing drivers for a 7- segment display, and an internal multiplexing circuitry with four

multiplexing outputs. The multiplexing circuit has its own free-running oscillator, and requires

no external clock. The counters advance on negative edge of clock. A HIGH signal on the reset

input will reset the counter to zero, and reset the carry- out LOW. A LOW signal on the Latch

Enable input will latch the number in the counters into the internal output latches. A HIGH

signal on Display Select input will select the number in the counter to be displayed; a LOW level

signal on the Display Select will select the number in the output latch to be displayed. The

MM74C926 is a 4- decade counter and has Latch Enable, Clock and Reset inputs.

11

Features of MM74C926:

Wide supply voltage range: 3V to 6V

Guaranteed noise margin: 1V

High noise immunity: 0.45 Vcc

High sourcing current: 40 mA

Internal multiplexing circuitry

Functional Description:

Reset - Asynchronous, active high

Display Select - High, displays output of counter

Low, displays output of latch

Latch Enable - High, flow through condition

Low, latch condition

Clock - Negative edge sensitive

Segment Output- Current sourcing with 40 mA

Digit Output - Current sourcing with 1 mA

12

3.3 RESISTANCES:

Fig 7

Resistors are components that have a predetermined resistance. Resistance determines how much

current will flow through a component. Resistors are used to control voltages and currents. A

very high resistance allows very little current to flow. Air has very high resistance. Current

almost never flows through air. A low resistance allows a large amount of current to flow. Metals

13

have very low resistance. That is why wires are made of metal. They allow current to flow from

one point to another point without any resistance. Wires are usually covered with rubber or

plastic. This keeps the wires from coming in contact with other wires and creating short circuits.

High voltage power lines are covered with thick layers of plastic to make them safe, but they

become very dangerous when the line breaks and the wire is exposed and is no longer separated

from other things by insulation. Resistance is given in units of ohms. Common resistor values are

from 100 ohms to 100,000 ohms. Each resistor is marked with colored stripes to indicate its

resistance. Resistance is measured in ohms; the symbol for ohm is an omega (Ω). 1 Ω is quite

small so Resistor values are often given 1 KΩ = 1000Ω , 1 MΩ = 1000000 Ω.

3.3.1 Symbol for variable resistance:-

Fig 8

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

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

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

but straight track versions, usually called sliders, are also available.Variable resistors may be

used as a rheostat with two connections ( the wiper and just one end of the track) or as a

potentiometer with all three connections in use.Variable resistors are often called potentiometers

in books and catalogues. They are specified by their maximum resistance, linear or logarithmic

track, and their physical size. The standard spindle diameter is 6mm.Some variable resistors are

designed to be mounted directly on the circuit board, but most are for mounting through a hole

14

drilled in the case containing the circuit with stranded wire connecting their terminals to the

circuit board.

3.4 CAPACITOR:

A capacitor is an electrical/electronic device that can store energy in the electric field between a

pair of conductors (called "plates"). The process of storing energy in the capacitor is known as

"charging", and involves electric charges of equal magnitude, but opposite polarity, building up

on each plate. Capacitors are often used in electric and electronic circuits as energy-storage

devices.

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: In SI units, a capacitor has a

capacitance of one farad when one coulomb of charge is stored due to one volt applied potential

difference 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). When there is a difference in electric charge

between the plates, an electric field is created in the region between the plates that is proportional to the

amount of charge that has been moved from one plate to the other. This electric field creates a potential

difference V = E·d between the plates of this simple parallel-plate capacitor. 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.

Fig 9

3.5 Seven Segment

15

Fig 10

Seven segment indicator

Fig 11

There are two types of 7-segment displays:

Common Cathode (CC)

Common Anode (CA)

16

The difference between the two displays is the common cathode has all the cathodes of the 7-

segments connected directly together and the common anode has all the anodes of the 7-

segments connected together.

A Common Cathode segment is different from Common Anode segment in that the cathodes of

all the LEDs are connected together. For the use of this seven segment the Common Cathode

connection must be grounded and power must be applied to appropriate segment in order to

illuminate that segment. A seven segment display is a form of electronic display device for

displaying decimal numerals that is an alternative to the more complex dot- matrix displays.

Seven-segment displays are widely used in digital clocks, electronic meters, and other electronic

devices for displaying numerical information. The seven segments are arranged as a rectangle of

two vertical segments on each side with one horizontal segment on the top, middle, and bottom.

Additionally, the seventh segment bisects the rectangle horizontally. The segments of a 7-

segment display are referred to by the letters A to G as shown to the right, where the DP decimal

point is used for the display of non- integer numbers.

3.6 Switches:

Fig 12

Switches are devices that create a short circuit or an open circuit depending on the position of the

switch. For a switch, ON means short circuit (current flows through the switch, and lights light

up.) When the switch is OFF, that means there is an open circuit (no current flows, lights go out).

17

When the switch is ON it looks and acts like a wire. When the switch is OFF there is no

connection.

3.7 Light Emitting Diode:

Fig 13

LEDs are used as indicator lamps in many devices and are increasingly used for other lighting.

When a light-emitting diode is forward biased (switched on), electrons are able

to recombine with electron holes within the device, releasing energy in the form of photons. This

effect is called electroluminescence and the color of the light (corresponding to the energy of the

photon) is determined by the energy gap of the semiconductor. LEDs present

many advantages over incandescent light sources including lower energy consumption,

longer lifetime, improved robustness, smaller size, faster switching, and greater durability and

reliability.

3.8 Diode:

Fig 14

18

Diodes are components that allow current to flow in only one direction. They have a positive side (leg)

and a negative side. When the voltage on the positive leg is higher than on the negative leg then current

flows through the diode (the resistance is very low as it becomes a case of forward bias voltage drop).

When the voltage is lower on the positive leg than on the negative leg then the current does not flow (the

resistance is very high as it becomes a case of reverse bias voltage drop). The negative leg of a diode is

the one with the line closest to it. It is called the cathode. The positive end is called the anode. Usually

when current is flowing through a diode, the voltage on the positive leg is 0.65 volts higher than on the

negative leg. We can connect the diodes to make bridge rectifier to convert AC supply into DC supply.

3.9 Transformer:

Transformers convert AC electricity from one voltage to another with little loss of power.

Transformers work only with AC and this is one of the reasons why mains electricity is AC.

Step-up transformers increase voltage, step-down transformers reduce voltage. Most power

supplies use a step-down transformer to reduce the dangerously high mains voltage to a safer low

voltage.

The input coil is called the primary and the output coil is called the secondary. There is no

electrical connection between the two coils; instead they are linked by an alternating magnetic

field created in the soft-iron core of the transformer. The two lines in the middle of the circuit

symbol represent the core.

Transformers waste very little power so the power out is (almost) equal to the power in. Voltage

is stepped down current is stepped up.

The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the

voltages. A step-down transformer has a large number of turns on its primary (input) coil which

is connected to the high voltage mains supply, and a small number of turns on its secondary

(output) coil to give a low output voltage.

turns ratio = Vp = Np and power out = power in

19

Vs Ns Vs × Is = Vp × Ip

Transformer circuit

symbol (Fig 15)

3.10 Voltage Regulators:

A voltage regulator is an electrical regulator designed to

automatically maintain a constant voltage level. It may use an electromechanical mechanism, or

passive or active electronic components. Depending on the design, it may be used to regulate one

or more AC or DC voltages.

A 5V voltage regulator (7805) is used to ensure that no more than 5V is delivered. The regulator

functions by using a diode to clamp the output voltage at 5V DC regardless of the input voltage -

20

Vp = primary (input) voltage

Np = number of turns on primary coil

Ip = primary (input) current

Vs = secondary (output) voltage

Ns = number of turns on secondary coil

Is = secondary (output) current

excess voltage is converted to heat and dissipated through the body of the regulator. If a DC

supply of greater than 12V is used, excessive heat will be generated, and the board may be

damaged. If a DC supply of less than 5V is used, insufficient voltage will be present at the

regulators output.

Fig 16

3.11 BJT:

A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies on the

contact of two types of semiconductor for its operation. BJTs can be used as amplifiers, switches,

or in oscillators

3.11.1 NPN:

Fig 17

21

NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped

semiconductor (the "base") between two N-doped layers. A small current entering the base is

amplified to produce a large collector and emitter current. That is, when there is a positive

potential difference measured from the emitter of an NPN transistor to its base (i.e., when the

base is high relative to the emitter) as well as positive potential difference measured from the

base to the collector, the transistor becomes active. In this "on" state, current flows between the

collector and emitter of the transistor. Most of the current is carried by electrons moving from

emitter to collector as minority carriers in the P-type base region. To allow for greater current

and faster operation, most bipolar transistors used today are NPN because electron mobility is

higher than hole mobility.

3.11.2 PNP:

Fig 18

The other type of BJT is the PNP, consisting of a layer of N-doped semiconductor between two

layers of P-doped material. A small current leaving the base is amplified in the collector output.

That is, a PNP transistor is "on" when its base is pulled low relative to the emitter.

The arrows in the NPN and PNP transistor symbols are on the emitter legs and point in the

direction of the conventional current flow when the device is in forward active mode.

22

Chapter 4

CIRCUIT DESCRIPTION

4.1 Power supply circuit:

Types of Power Supply

There are many types of power supply. Most are designed to convert high voltage AC mains

electricity to a suitable low voltage supply for electronic circuits and other devices. A power

supply can by broken down into a series of blocks, each of which performs a particular function.

For example a 5V regulated supply:

Each of the blocks is described in more detail below:

Transformer - steps down high voltage AC mains to low voltage AC.

Rectifier - converts AC to DC, but the DC output is varying.

Smoothing - smoothes the DC from varying greatly to a small ripple.

Regulator - eliminates ripple by setting DC output to a fixed voltage.

23

Power supplies made from these blocks are described below with a circuit diagram and a graph

of their output:

Transformer only

Transformer + Rectifier

Transformer + Rectifier + Smoothing

Transformer + Rectifier + Smoothing + Regulator

Transformer only:

The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable

for electronic circuits unless they include a rectifier and a smoothing capacitor.

Transformer + Rectifier:

24

The varying DC output is suitable for lamps, heaters and standard motors. It is not suitable for

electronic circuits unless they include a smoothing capacitor.

Transformer + Rectifier + Smoothing :

The smooth DC output has a small ripple. It is suitable for most electronic circuits.

Transformer + Rectifier + Smoothing + Regulator:

25

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.

4.1.1 Rectifier:

There are several ways of connecting diodes to make a rectifier to convert AC to DC.

The bridge rectifier is the most important and it produces full-wave varying DC. A full-wave

rectifier can also be made from just two diodes if a centre-tap transformer is used, but this

method is rarely used now that diodes are cheaper .A single diode can be used as a rectifier but it

only uses the positive (+) parts of the AC wave to produce half-wave varying DC.

4.1.2 Bridge rectifier:

A bridge rectifier can be made using four individual diodes, but it is also available in special

packages containing the four diodes required. It is called a full-wave rectifier because it uses all

the AC wave (both positive and negative sections). 1.4V is used up in the bridge rectifier because

each diode uses 0.7V when conducting and there are always two diodes conducting, as shown in

the diagram below. Bridge rectifiers are rated by the maximum current they can pass and the

maximum reverse voltage they can withstand.

26

Bridge rectifier is alternative pairs of diode conduct Output: full-wave varying DC

Changing over the connections so the alternative (using all the AC wave)

Direction of AC is convert to the one direction of DC

Fig 19

4.1.3 Single diode rectifier:

A single diode can be used as a rectifier but this produces half-wave varying DC which has gaps when

the AC is negative. It is hard to smooth this sufficiently well to supply electronic circuits unless they

require a very small current so the smoothing capacitor does not significantly discharge during the gaps.

Single diode rectifier Output: half-wave varying DC

(using only half the AC wave)

27

Fig 20

4.1.4 Smoothing:

Smoothing is performed by a large value electrolytic capacitor connected across the DC supply to act as a

reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The

diagram shows the unsmoothed varying DC (dotted line) and the smoothed DC (solid line). The capacitor

charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output.

Note that smoothing significantly increases the average DC voltage to almost the peak value

(1.4 × RMS value). Smoothing is not perfect due to the capacitor voltage falling a little as it

discharges, giving a small ripple voltage. For many circuits a ripple which is 10% of the supply

voltage is satisfactory and the equation below gives the required value for the smoothing

capacitor. A larger capacitor will give fewer ripples. The capacitor value must be doubled when

smoothing half-wave DC.

4.2 Working of 555 Timers IC:

The main heart of the digital stopwatch is the timer IC 555 used as a pulse generator in astable

mode to produce the pulses of 1 second duration or 1Hz frequency. The pulse duration can be

varied by changing the value of variable resistor R2. The output of pulse generator unit is to

control the operation of the switching unit. The pulse duration depends on R1 and R2 and C

value as per the relation given below.

28

Total time period = 0.69(R1+2R2) c

The 555 is a highly stable device for generating accurate time delays or oscillation. Additional

terminals are provided for triggering or resetting if desired. In the time delay mode of operation,

the time is precisely controlled by one external resistor and capacitor. For astable operation as an

oscillator, the free running frequency and duty cycle are accurately controlled with two external

resistors and one capacitor. The may be triggered and reset on falling waveforms and the output

circuit can source or sink up to 200mA or driver TTL circuit.

4.3 Working Of IC MM74C926

29

Fig 21

This is an optional circuit those who do not want to use LCD display they can use this circuit to

interface with pulse generator. A simple display unit using IC 74C926 which comprises a 4 digit

counter and source driver for 7 segment display. It has internal multiplexer with four multiplexed

outputs. The multiplexing circuit has its own free running oscillator and requires no external

clock pulse. This IC has input protection circuit consisting of a series resistor and a diode

connected to the ground. The IC 74C926 features a wide range of supply from 3V to 6V with

guaranteed noise margin of 1V. Connecting the output of pulse generator (IC555) to the input

terminal (pin no.12) of the display unit (IC74C926). As per the pulse rate, display unit will

display the count on seven segments.

The MM74C925 and MM74C926 counter consist of a 4bit counter, an internal output latch, npn

output sourcing drivers for a seven segment display ,and an internal multiplexed circuitry with

four multiplexing outputs .the multiplexing circuit has it own free running oscillator and require

no external clock. The counter advanced on negative edge of clock. A high signal on the reset

input will restart the counter to zero, and reset the carry-out low. A low signal on the latch enable

30

input will latch the number in the counter into the internal output latch. A high signal on

“DISPLAY SELECT” (pin) input will select the number in the counter to be display; a low level

signal on the display select will select the number in the output latch to be displayed. The

MM74C925 is a 4-decade counter and has latch enable, clock and reset inputs. The MM74C926

is like the MM74C925 except that it has a display select and a carry-out is used for cascading

counters. The carry-out signal goes high at 6000, goes back low at 0000.

4.4 CIRCUIT WORKING:

Here’s a digital stop watch built around timer IC LM555 and 4-digit counter IC with multiplexed 7-

segment output drivers (MM74C926). IC MM74C926 consists of a 4-digit counter, an internal output

latch, npn output sourcing drivers for common cathode, 7-segment display and an internal multiplexing

circuitry with four multiplexing outputs. The multiplexing circuit has its own free running oscillator, and

requires no external clock. The counter advances on negative edge of the clock. The clock is generated by

timer IC LM555 (IC1) and applied to pin 12 of IC2. A high signal on reset pin 13 of IC2 resets the

counter to zero. Reset pin 13 is connected to +5V through reset push-on-switch S3. When S2 is

momentarily pressed, the count value becomes0, transistor T1 conducts and it resets IC1. Counting starts

when S2 is in ‘off’ condition. A low signal on the latch-enable input pin 5 (LE) of IC2 latches the number

in the counter into the internal output latches. When switch S2 is pressed, pin 5 goes low and hence the

count value gets stored in the latch. Display-select pin 6 (DS) decides whether the number on the counter

or the number stored in the latch is to be displayed. If pin 6 is low the number in the output latch is

displayed, and if pin 6 is high the number in the counter is displayed. When switch S2 is pressed,

the base of pnp transistor T2 is connected to ground and it starts conducting. The emitter of T2 is

connected to DS pin of IC2. Thus, when switch S3 is pressed, reset pin 13 of IC2 is connected to

ground via transistor T1 and the oscillator does not generate clock pulses. This is done to achieve

synchronization between IC1 and IC2. First, reset the circuit so that the display shows ‘0000.’

Now open switch S2 for the stop watch to start counting the time. If you want to stop the clock,

close switch S2. Rotary switch S1 is used to select the different time periods at the output of the

astable multivibrator (IC1). The circuit works off a 5V power supply. It can be easily assembled

on a general-purpose PCB. Enclose the circuit in a metal box with provisions for four 7-segment

displays, rotary switch S1, start/stop switch S2 and reset switch S3 in the front panel of the box.

31

Fig 22

Chapter 5

TESTING OF PROJECT

32

First time series Digital Stop Watch will be used to do reset the Stop Watch Digital by pressing

the S3 (Reset switch) so it will display data 0000. Then to start doing the counting is done by

pressing the S2 (button start / stop) and to stop the Stop Watch Digital counting process is done

by pressing the S2 (button start / stop) on the Stop Watch Digital. Then to choose the time of

counting on a series Digital Stop Watch is provided by 2 votes counting time 0.1 seconds and 1

second to choose from S1 in the Stop Watch Digital. First reset the circuit by pressing S3 so that

the display shows ‘0000’. Now open the switch S2 for stop watch to start counting timer. If you

want to stop the stop watch, close switch S2. Time varying switch S1 is used to select the

different time periods at the output of astable multivibrator (555)

Chapter 6

CONCULSION

33

From above discussion we concludes that the main heart of the digital stopwatch is the timer IC

555 used as a pulse generator in astable mode to produce the pulses of 1 second duration or 1Hz

frequency. The pulse duration can be varied by changing the value of variable resistor R2. The

output of pulse generator unit is to control the operation of the switching unit. The pulse duration

depends on R1 , R2 and C value as per the relation given below.

Total time period = 0.69(R1+2R2) c

Stop Watch Digital in this article are manufactured using a source clock of timer IC 555 and to

process the timing performance using the IC 74C926. IC 74C926 is a 4-digit counter with output

latches are installed internally to the output viewer 7 segment and for the output driver NPN

transistor to control the viewer 7 segment common cathode. The beating of the Stop Watch

Digital built using a 555 timer IC has output that can be set to tap the Stop Watch Digital is to

count time 0.1 seconds and 1 second.

BIBLIOGRAPHY

LM 555 TIMER IC DATASHEET.

MM74C926 IC DATASHEET.

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BASIC Electronic Devices and Circuits by Rajeev Tiwari

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