Digital stopwatch display circuit

The digital stopwatch circuit is shown in the figure. The 5G 5544 (ICl) in the picture is a quartz clock integrated circuit, used as a second signal generator in the circuit
. 5G5544
outputs pulses with a period of 2s from pins ③ and ⑤ . After passing through the non-logical level conversion circuit
composed of VD3, VD4 and VT1 , the second signal output can be obtained. CD4518 (102) has two identical decimal counters packaged inside, so it can form a two-digit count. If more bits of counting are needed, multi-level cascading can be performed.     CD4511 (IC3, IC4) is a BCD-7-segment decoding/driver integrated circuit. It decodes the BCD code output by the decimal counter into a stroke code and drives the LED digital display to display the counted seconds. As the seconds signal continues to be added, the common cathode LED digital display will continue to display the counting number.     Sl is the clear switch, when Sl is pressed. The Cr terminal of CD4 518 can get a positive pulse to clear the circuit. Because the working voltage of the 5G 5544 integrated circuit is 1. 2 ~ 2V, it needs to be provided with working power after being stepped down by VD1 and VD2.