主次干道相交处的交通灯，实现主干道绿灯亮8s,次干道绿灯亮4s,黄灯闪烁3s的周期指示效果，用数码管显示计数倒计时-Traffic lights at the intersection of primary and secondary roads, the green light to achieve the main road 8s, secondary roads green light 4s, 3s effect flashing yellow light cycle instructio