Stopwatches find use as time keeping devices in many fields, namely sports. Stopwatches may be analog or digital. Digital stopwatches are much more common the analog version owing to their higher accuracy and ease of use. Here we have tried to realize a digital stopwatch of reasonable accuracy and reliability.
This particular stopwatch can count up to 9 minutes and 59.9 seconds .It is accurate up to one tenth of a second. The circuit is relatively simple and easy to realize .The heart of the circuit is an a stable mv followed by counter and decoder stages.
The circuit us explained extensively in the following pages.The circuit operates on 5-v dc supply. It uses a seven segment LED display of common anode type to show time.
This digital stopwatch can count nine minutes and 59.9 seconds. The circuit consists of a 555 astable multivibrator. It generates 10 pulses in one second. We can enable or disable IC 555 by applying a high or low voltage to pin 4 of 555.
This is done by IC 7476. 7476 is a JK flip-flop. Its J and K input are given high level. Therefore this flip-flop toggles at each trailing edge of the clock pulse. Here clock pulses are given by push-to-on switch S1.
IC 7490 counts the pulses of astable multivibrator. It counts upto 9 (1001). When the first counter reaches nine, second counter starts counting, because it gets a trailing edge when first display goes from 9 to 0. so it reaches one second. The third display begins its counting as the second display changes from 9 to 0. Then the display counts 10 seconds.
When the count of the third display reaches 6, both input terminals of NAND gate A of IC 7400 becomes high. So its output becomes low. This output appears as a trailing edge for the fourth display and it starts counting.
Then output of the NAND gate gets inverted. C now becomes high. This is applied to RESET pin of 7490. Therefore all counters except 4 reset to 0. The display shows 1.000. i.e. 1 minute and 0 seconds. IC 7447 is used to convert BCD to 7-segment form.