设计一个交通信号灯控制电路。要求：

1、主干道和支干道交替放行，主干道每次放行30秒，支干道每次放行20秒。

2、每次绿灯变红灯时，黄灯先亮5秒钟，此时原红灯不变。

3、用十进制数字（递增计数）显示放行和等待时间。-The design of a traffic signal control circuits. Requirements: 1, main roads and branch roads alternately release, the main road each release of 30 seconds, the branch roads each release of 20 seconds. 2, each time the green light change when the red light, yellow light on for 5 seconds, then the original red light unchanged. Three decimal digits (counting) shows the release and waiting time.