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COMSATS University Islamabad
Wah Campus
Electrical & Computer Engineering Department
Program: BEE
Semester/Section:
2-B
Subject: DIGITAL LOGIC DESIGN
Instructor: Engr. Adnan Saleem Mughal
Project Title:
DIGITAL STOPWATCH
Submitted by:
Nauman
Toheed Ahmad Khattak
Muhammad Afzal Shah
Project progress Report: Final Report
Date of Submission: 14-June-2019
FA18-BEE-007
FA18-BEE-034
FA18-BEE-073
 Objectives
The main objective of digital stopwatch is to count time in seconds, minutes and hours and it can
also be used for different application like count down timer , alarm clock etc.
 Summary
This circuit is based on the principle of 6 phase counter operation, based on synchronous
cascading. The idea is to display clock pulses count from 0 to 59 seconds , minutes and
hours i.e 23:59:59. This is done by using a 555 Timer IC connected in a stable mode to
produce clock pulses of 1 second interval each. While the first display counts from 0 to 9,
the second counter starts its counting operation every time the count value of the first
counter reaches 9. When the 2nd display reach to 6 the encoder give one one output to the
7411 so it set back to 0 and the minutes display change its state from 0 to 1 an so on and
same is the case with hours display. The counter ICs are connected in cascading format
and each counter output is connected to BCD to 7 segment decoder used to drive the 7
segment displays
 Block Diagram
 Circuit Diagram
 Working
555 Timer
The 555 is basically a monostable Multivibrator. The important characteristics of a
Monostable Multivibrator is as long as the pin 2 receives a positive trigger the output at pin 3
will be of low state. And when negative trigger was connected into the pin 2, the output at the pin
3 will go high for a specific period of time. This time was decided by the Resistor and Capacitor
connected with it.
4026 IC
The 4026 IC is a 16-pin CMOS seven-segment counter from the 4000 series. It counts clock
pulses and returns the output in a form which can be displayed on a seven-segment display. This
avoids using a binary-coded decimal to seven-segment decoder, but it can only be used to display
the (decimal) digits 0-9.
IC7411
The IC 7411 is basically AND Gate having three inputs and one output. This IC contains three
AND Gates
WORKING
The working of the circuit starts with 555 timer when it was connected with mono stable
vibrator. The 555 timer generated a clock pulse after every second and its output is connected
with pin 1 of IC 4026 .IC 4026 is a 7 segment decoder and is used to drive 7 segment display
with input clock pulse Here the clock pulse was obtained from the mono stable multi vibrator and
fed into the pin 1 of first ic 4026. Pin 2 was usually grounded since giving high signal to this pin
will inhibit the input clock signal to pin 1 and pin 3(Enable Clock) is always taken High. The pin
3 of the IC 4026 is connected with pin 3 of 555 timer which is the out pin of 555 timer. Initially
when the circuit is switched ON the 7 segments will indicate "00:00:00" count and as soon as the
negative trigger was given to 555 high pulse will be obtained from pin 3. The high pulse was fed
to first IC and therefore it increments its count with each clock, displaying 1 to 9 in its seven
segment. As soon as 10 counts was incremented by IC a high to low signal was obtained from its
pin 5 which indicates the completion of ten increments.
The pin 5 was connected to the clock pin of the next 4026 IC. Therefore whenever 10 counts was
completed by the 7 segment, the high to low signal at the pin 5 will feed a single clock pulse
input to the second IC and therefore the corresponding 7 segment will be incremented one value.
For a digital clock we must reset second IC when it reach to number 6 because we want seconds
count upto "59" therefore we used IC 7411 (Three input AND Gate) when it reaches to 6 all the
three input becomes 1 and it generates output 1 and the next display works. In the same manner
fourth IC will count from 0 to 6 and then value in the fifth IC will be incremented by one. This is
all about for seconds and minute of clock. Now for hours we must reset fifth and sixth IC when
number reached to “23” so we put one more three input AND gate.
 Simulations (in any software)
Simulation are made on proteous
 Hardware Implementation
we construct digital stopwatch on bread board
o Problems encounterd
While constructing the digital stopwatch on bread board the ic7411 not working
properly it does not provide a proper output to the 3rd ic4026 which cannot
changes its state and the display of second counts from 0 to 99 instead of 0 to
59.
o Trouble shooting
The ic7411 only gets input voltage of 0 to 7v and we were giving voltage of 9v to
7411 so by putting the resistor between input voltage and the vcc pin of 7411 it
work properly it and the minutes display change its state.
 Results and Conclusions
After the completion of our circuit of digital stopwatch it counts seconds, minute, and
hours accurately and we came to know the working of different Ic’s . and the circuit has
been implemented on the bread board.