用单片机做的交通灯试验，红绿灯各亮3秒，黄灯亮1秒。

代码说明：

用单片机做的交通灯试验，红绿灯各亮3秒，黄灯亮1秒。-Done with single-chip test of traffic lights, traffic lights, the bright 3 seconds, 1 second yellow light.