The above simple circuit design can be utilized as an accurate stopwatch to count up to 100 seconds with a determination of 0.01 second or up to 1000 seconds with a determination of 0.1 second. This stop-watch might be utilized for sports, games and other comparable different exercises.
A 1MHz crystal produce 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. Because of the usage of crystal, the final frequency will be very accurate.
The output of IC4 (7490) is counted and displayed using IC5 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 utilized to select appropriate input frequency and corresponding decimal point position to display up to either 99.99 seconds or 999.9 seconds maximum count. The switch S1 and S2 is push button type.
For proper operation of this stopwatch circuit, the first is by do “reset” by press the switch S3 and then operate the switch S2, according to the resolution/range you choosed (0.1 sec. or 0.01 sec.)/(100 seconds or 1000 seconds). Then to start the time counting, just press switch S1. To stop counting, press switch S1 again. The counting will be stopped and the 7 segment LED display will show the correct time elapsed since the start of counting (the first time switch S1 pressed).