First, reset the circuit by pressing S3 so that the display shows "000". Now open switch S2 for the stop watch to start counting the time. If you want to stop the clock, close S2. Rotary switch S1 is used to select the different time periods at the output of the astable multivibrator.
Circuit Diagram of Digital Stop Watch
Skema Rangkaian Stop Watch Digital Tampilan 7 Segmen