digital stopwatch

1

Atharva Inamdar Candidate no# 0088 Centre no# 52103 Course no.# 2528

ATHARVA INAMDAR

Candidate no# 0088

AYELSBURY GRAMMAR SCHOOL

AYLESBURY

Centre no# 52103

2


Summary:

The aim of this project is to make a Digital stop watch to count in Seconds and

Minutes. This should be done using only non-programmable components. The stopwatch

should also count accurately. There should be a start/stop facility. The stopwatch is

intended to count from 00:00 to 99:99 and then it will reset back to 00:00.

After making the stopwatch and testing it, my project successfully matched my

specifications. The stopwatch counts accurately to the nearest second. The display also

works fine showing all the numbers correctly, this also shows that my counter and

decoder works. Although I had a few minor problems, my Digital Stop watch was a

success.

3


BRIEF:

IN today‟s world time is crucial. Some people like to time how long the chess

game lasted or how long it took them to beat an opponent in a board game such as

Risk. I am going to build a stopwatch accurate to the nearest second. This stopwatch will

also have a use in schools in science labs, where the time for the experiment to finish is

crucial. It will display minutes and seconds. The stopwatch will have an ON/OFF function.

It will have a Start/Stop switch and a Reset switch.

Specifications:

ON/OFF function

Start/Stop functions

Display minutes(max. 60) and seconds on bright 7 segment LEDs

The resolution will be 1 second

Block Diagram:

Specifications: POWER SUPPLY:

I will be using a +5V DC power supply to power all my ICs and LEDs. The +5V

power supply will be used for digital circuit.

INPUTS:

I will be using 1 SPST switches. One SPST switch will be used as a START/STOP switch.

OUTPUTS:

I will be using four 7-segment LEDs. Two 7segment LEDs will display the seconds

and the other two 7-segment LEDs will display minutes.

EQUIPMENT AVAILABLE:

Breadboard

Set of multi coloured wires

Multimeter

Digital PC oscilloscope

Side cutters, Wire strippers

WIRES: Black: Zero Voltage

Red: +6 positive voltage Orange: Outputs from ICs

Brown: Clock output from 40106B to counters

Yellow: Connections from resistor array to 7 segment LEDs

Blue: Inputs to NAND gate IC

Start Function Pulse

Generator Counter Reset Logic

Display

Driver Display

4


Subsystems:

Pulse Generator:

Specifications:

This subsystem is designed to generate a signal with a frequency of 1Hz. This pulse

will be generated by a relaxation oscillator.

Circuit Diagram:

Component Values:

The relaxation oscillator uses a non-polarised capacitor and a feedback resistor.

The Capacitor to be used will be 10 micro Farads while the feedback resistor is 220 Kilo

Ohms.

How it Works:

When the input voltage is at +9V, the current in the feedback resistor charges the

capacitor. Raising the voltage at the input to 2.8V. At this point the output drops to 0V.

The voltage at input starts dropping to 2.2V. When it reaches 2.2V the output switches

back to +9V.

Testing:

This component was tested using a digital PC oscilloscope. The trace is shown in

the Evaluation section at the end of this report.

Evaluation:

To improve the accuracy of the oscillator I will need to use 44k variable resistor

along with 10 F capacitor to achieve an average output frequency of 1Hz.

Problems:

The relaxation oscillator is not very accurate. Using 200k resistor did not achieve

an accurate frequency. I used a Variable resistor(adjusted to 14k ) and an oscilloscope

to accurately achieve a frequency of 1.058Hz.

5


Photograph:

40106B Hex

Schmitt Inverter

Pin 1

Variable

resistor

100 F

Electrolytic

Capacitor

6


Counter:

Specification:

I will be using four binary counters for seconds and minutes. Each counter will

count one digit. Two counters will count from 0 to 9, however the other two counters will

count from 0 to 6.

Circuit Diagram:

Component Values:

The counters I will be using will be two 4518B dual binary counters. The output will

be 4 bit binary output from each counter.

How It Works:

I am using two 4518B counters, which contain 4 counters. On the first IC, I will be

counting the seconds. For the units digit the counter‟s reset is held low while the Enable is

held high. For the tens digit, I have connected the clock output form the 40106B to the

input of the second counter of the same 4518B. The reset is connected to the reset logic

from the first counter.

The same setup was carried out for the second dual counter. Except this time the

clock inputs were connected to the reset logics of each

Testing:

The function of these counters were tested along with the 7 segment LED drivers

(4511B) and the 7 segment LED display. The counting was checked for the correct order

and if the counters were carrying forward the count after 9 or 6.

Evaluation:

This subsystem function as desired and caused no problems at all.

Problems:

No problems were encountered designing, building or testing this subsystem.

Photograph:

Pin 1

Counter 1

Counter 2

7


Display, Decoder and Driver:

Specifications:

This driver (4511B) will be used for driving the 7 segment LEDs from the binary

output of the counters.

Each display will be used to display each digit of seconds and minutes elapsed.

Circuit Diagram:

How it works:

A binary input is converted to logic signals for each segment of the display. The

outputs are labelled as “a, b, c, d, e, f, g” for the 7 segment display. It also provides

higher current than a counter.

An output from a decoder and driver illuminated certain segments to display a

number between 0 and 9 inclusive. The display is grounded, and thus the segments have

a common cathode. This means that the segments will illuminate when provided with a

high signal from the driver (4511B).

Testing:

To test the 4511B IC, I plugged the binary inputs to positive rail as desired and

tested if the correct number was shown on the 7-segment display. Carrying out this test

also tested the 7-segment displays.

Display testing(further):

At first the decimal point segment and the ground pins were connected to 0V.

Then individually the segments were connected to the positive rail. This tested if the

segments were all functioning normally.

Evaluation:

The results of the test showed that the 4511B driver was functioning normally.

However, one 7-segment display failed the test. Its „a‟ segment was very dim and did not

light up sometimes.

Problems:

One 7-segment LED display had to be replaced due to faulty segment. Other

than this there were no other problems.

8


Photograph:

4511B Display

driver and

Decoder

Pin 1

220 8 resistor

Array 7-Segment

LED Display

9


Reset Logic:

Specification:

I will be using a AND gate in order to reset counters and carry counts to other

counters.

Circuit Diagram:

Component values:

I will be using 4581B IC, which is a quad AND gate IC.

How it works:

To carry a count to the next digit in time, I will be connecting the inputs of a AND

gate to the 1 and 8 binary inputs form the first counter (of the first dual counter chip) and

the output to the reset of the next counter. For the tens counter, I will connect the inputs

to the 2 and 4 binary outputs from the counter and the output from the AND gate to the

clock of the next counter and the reset of the same counter.

This ensures that the units digit count from 0 to 9 and the tens digits count from 0

to 5 and reset at 6 when the units count is 0.

Testing:

Testing the function of the AND gate was very simple. I had connected an LED

and a resistor to the output and used the power supply rails as inputs. Changing the input

from positive to zero and vice versa I checked if the function followed the truth table

given below:

A B Q

0 0 0

0 1 0

1 0 0

1 1 1

Evaluation:

This subsystem was functioning as desired.

Problems:

There were no problems using this subsystem

Photograph:

Where A: first input

B: second input

Q: output

4081B

Quad AND

Gate

Pin 1

10


Fu

ll C

irc

uit D

iag

ram

11


PH

OTO

GR

AP

H O

F C

OM

PLE

TED

BR

EA

DB

OA

RD

12


Evaluation:

To check that my digital stopwatch actually performed its

function, I used several pieces of equipment and procedures.

To ensure the success of my project I built and tested each

subsystem at a time. Firstly, to check the frequency of my relaxation

oscillator, I used a PC oscilloscope. As I was using a relaxation

oscillator, the component values needed we approximate, due to

the approximate formula. Instead of using a fixed resistor, I used a

variable resistor to find out the exact resistance. This oscilloscope was

also able to give a numerical readout of the frequency of the clock.

The trace from the PC oscilloscope is shown below:

To check other components such as 7 segment LED displays, I

used a multimeter to check the voltage to see if I could increase its

brightness. However, I found that they were already at max

brightness as the voltage across them was 2.1V, which is just under

the maximum forward voltage (2.5V).

13


Bibliography:

o CMOS Cook Book (2nd Edition) by Don Lancaster – for pin outs

of various ICs.

Acknowledgements:

o Mr. M Harvey

digital stopwatch

Documents