18-01-2013, 02:41 PM
Independent Project: Digital Stopwatch
Independent Project.pdf (Size: 20.94 KB / Downloads: 46)
ABSTRACT
This project was chosen because stopwatches are used nearly everywhere, whether for a
sports event or (on a more accurate scale) for experiments.
A stopwatch is basically a clock that can be started, stopped, and reset. So, one way to do
this is to have a 1 Hz signal and then divide the frequency by ten using a decade counter so that it
counts up to nine, then resets to zero (same as the seconds part of a clock). Then the next counter
(connected to the first) should divide by six, so that the counter counts up to five, then resets to
zero (as the tens-of –seconds part should). The ouput of the first counter has divided the
frequency by ten so that a pulse is sent (and counted) by the second counter every ten seconds.
Since the output of the second counter is input into the third, there’s now an input pulse every
sixty seconds that’s counted. This is, of course, the minutes part, and the third counter is
configured as a divide by ten so that it only counts up to nine before being reset to zero. The last
(fourth) counter gets (and counts) a pulse from the third counter every 600 seconds. This is then
the tens-of-minutes part of the stopwatch, and is configured as a divide-by-six so that the counter
only counts up to five before reseting to zero.
Implementation
So, the Wavetek sends the 1 Hz signal into pin 14 of a SN7490 Decade Counter, that’s
modified so that it’s a divide-by-ten counter (the second “hand”). This is accomplished by
grounding pins 7, 6, 3, and 2 and connecting pin 1 to pin 12. The chip is powered by a +5 V
power supply connected to pin 5, and pin 10 is grounded. For ease of design, the four SN7490s
are cascaded, so that the ouput of one is input into the next. So, pin 11 of the divide-by-ten
counter is connected to pin 14 of the next SN7490 (the divide-by-six counter). Then, the seconds part of the stopwatch is connected to a SN7447 BCD-to-seven-segment-decoder/driver. This
involves connecting the pins as shown in Table 1.