Traffic signal lights are composed of red, green, and yellow lights. A red light means no traffic, a green light means permission to pass, and a yellow light means a warning. Traffic signal lights are used at road intersections. By issuing instructions to vehicles and pedestrians to move or stop, the people and vehicles arriving at the same time can flow through, and mutual interference can be reduced as much as possible, thereby improving the traffic capacity of the intersection and ensuring the smooth and safe intersection.

Therefore, the research and design of the traffic light control system have very important practical significance. The design of the traffic light control system includes: clarify the controlled object and analyze the control requirements of the controlled object, find out the control law, select the control plan, select the PLC and input/output equipment; assign the input/output address, and design the PLC peripheral hardware circuit; program design; system debugging and operation.

(1) Choice of traffic light control system

The traffic light control system can be completed by a variety of circuits. Therefore, there are many design schemes for the traffic signal control system. Here we provide two schemes for selection:

Option one: Composed of ordinary digital circuit integrated chips

The characteristic of this kind of scheme is: the hardware design idea is simple, but uses many components, the circuit is more complicated, the welding debugging is prone to error, and it is not conducive to intelligent control, and the debugging circuit is complicated.

Option two: Use PLC to realize control.

The features of this solution are as follows: strong reliability, strong control function, convenient programming, easy to use, it can be used in harsh environments, strong anti-interference ability, it has various interfaces, is very convenient to connect with external equipment, and adopts building block structure or module type structure, flexible and convenient expansion, convenient maintenance, can modify the program online at any time, no need to modify the hardware circuit, strong adaptability, flexibility and convenience.

Since traffic signal lights are installed at noisy traffic intersections, the requirements of their work stability, reliability and anti-interference ability are all high. At the same time, considering the development, the control system is required to be flexible and convenient to upgrade. So it's better to choose the second design option.

(2) Overall design of traffic signal light control system

The traffic light control system is based on PLC. Its input is connected to the two control buttons for starting and stopping of the control system. The output terminal is connected to the east-west green light, east-west yellow light, east-west red light, north-south green light, north-south yellow light, and north-south red. The lights are used to indicate the passage of vehicles. It uses eight timers for timing control, and two counters control the number of times the green light flashes. Two timers and two counters form an oscillator circuit to control the green light flashing. In addition, an auxiliary relay is used to control the operation of the system.