设计一个只有四种信号灯的交通灯控制器：由一条主干道和—条支干道汇合成十字路口，在每个入口处设置红、绿、黄、左拐允许四盏信号灯，红灯亮禁止通行，绿灯亮允许通行，黄灯亮则给行驶中的车辆有时间停在禁行线外，左拐灯亮允许车辆向左拐弯。信号灯变换次序为：主支干道交替允许通行，主干道每次放行40s，亮5s红灯让行驶中的车辆有时间停到禁行线外，左拐放行15s，克5s红灯；支干道放行30s，亮5s黄灯，左拐放行15s，亮5s红灯……。各计时电路为倒计时显示。-Only four traffic lights to design a traffic signal controller: from a main road and- bar branch roads merged into the crossroads, the entrance of each set of red, green, yellow, turn left to allow four lights, red light is closed to traffic, the green light to allow passage, the yellow light is to have time to moving vehicles parked outside the line of the cut line, allowing the vehicle to the left turn left turn lights. The order of signal conversion: the main branch roads alternately allow passage, each trunk release 40s, 5s bright red light to moving vehicles have time to stop the entry lane, the left release of 15s, 5s grams of red light　branch trunk release 30s, 5s bright yellow light, turn left release 15s, 5s red light ... .... The countdown timer circuit.