You may have heard or seen such an object called Stopwatch hour. Stopwatch function to find out how long it takes in an activity. Usually the stopwatch is used in a variety of races: race, swimming, or even racing.
Stop watch is made with a digital timer IC LM555 and IC MM74C926 4 digit counter with multiplexed 7-segment LED. MM74C926 consists of 4 digit counter, an internal output latch, NPN output for controlling the source of common cathode, 7-segment display and an internal multiplexing circuitry with four multiplexing outputs. Counter the negative edge clock face. Clock continually be produced by the timer IC LM555. The circuit works with a 5V power supply. It can be easily assembled on the PCB. Enter the circuit in a metal box with provisions for the 7-segment display, turn the switch S1, start / stop switch and reset switch S2 S3.
Digital Stopwatch reset the circuit by pressing S3 so now the display shows '0000 '. Open switch S2 to stop watch to start counting time. If you want to stop the clock, close S2. Rotary switch S1 is used to select different time periods on the output of the astable multivibrator (IC1).