Four IC 4026 (Decade counter with decoder which converts counter values to 7 segment outputs) was used to drive each of the 7 segment displays. The 1Hz clock from oscillator is fed into CLK pin of U1. With each incoming pulse in to the CLK pin of this IC ,the counter increments by one which in turn gets decoded and displays the value in 7 segment accordingly. When the count reaches the maximum value of 9 a high signal was sent out through the pin 5 CO of U1. This will the clock input to the next IC U2. So thus when U1 segment counts up to 9 CO signal will sent out to U2. Then it starts displaying the value 1 in its segment.
The segment associated with U2 will count up to 9 before returning to 0. But to stay withing the limit of 60 seconds we need to stop U2 before it hits 6 mark. So here comes along another IC 4017 (Johnson counter increments count values from Q0 to Q9 with each incoming input pulse) which is used to reset the IC U2 before it hits the 6 mark in its segment. To do so we have connected the Q6 pin to Reset pin (MR) of U2 and Reset pin of U5 itself. The clock from U1 was used by both U2 and U5 in order to keep the count similar for providing correct reset point. When the count in U2 and U5 reaches from 0 to 6 the Q6 pin in U5 goes high resetting itself and U2. Thus this sets the boundary of 60 seconds for our stopwatch.
The output from Q6 acts as clock source for the IC U3. So when 60 seconds count gets elapsed the minute segment associated with U3 increments to 1. When this U3 counts up to 9 the CO pin goes high which feeds clock to the chip U4. This is similar to the way U1 fed clock to U2. Then U4 starts counting from 1 with every clock input from CO of U3 and can count up to 9. Thus U3 and U4 segment combined can count up to 99. Therefore this DIY stopwatch has a count limitation of 99 minutes after it starts counting again from 0 minutes.
Hope you all will have fun building this stopwatch. Please comment below if you have any comments, suggestions and improvements with this project.