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Accurate
Electronic Stop-Watch
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Here
is a simple circuit which can be used as an accurate stop-watch
to count up to 100 seconds with a resolution of 0.01 second
or up to 1000 seconds with a resolution of 0.1 second.
This stop-watch can be used for sports and similar other
activities. A 1MHz crystal generates stable frequency
which is divided by two stages of 74390 ICs (dual decade
counter) and another stage employing 7490 (decade counter)
IC to obtain a final frequency of 100 Hz or 10 Hz. Due
to the use of crystal, the final frequency is very accurate.
The output of IC4 (7490) is counted and displayed using
IC 74C926 (4-digit counter with multiplexed 7-segment
LED driver). Due to multiplexed display the power consumption
is very low. Switch S2 (2-pole, 2-way) is used to select
appropriate input frequency and corresponding decimal
point position to display up to either 99.99 seconds or
999.9 seconds maximum count.
For proper operation, first press switch S3 (reset) and
then operate switch S2, according to the resolution/range
desired (0.1 sec. or 0.01 sec.)/(100 seconds or 1000 seconds).
Now to start counting, press switch S3. To stop counting,
press switch S3 again. The counting will stop and display
will show the correct time elapsed since the start of
counting. |
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