Traffic lights have become a common and effective way to manage vehicle flow on roads today. However, this technology dates back to the 19th century. In 1858, London, England, installed the world’s first traffic light—mechanical signal lights powered by gas, featuring red and blue wrench-shaped signals to control carriages. These were the earliest forms of traffic control. A few years later, in 1868, British engineer Nathan Smith invented the first gas-powered traffic light at the square in front of the Parliament Building in Westminster. This system used red and green rotating lanterns, with red meaning “stop†and green indicating “caution.†Unfortunately, on January 2, 1869, the gas lamp exploded, injuring a police officer and leading to its immediate removal.
Modern urban traffic control systems are computer-integrated management tools that monitor traffic data, control signals, and guide vehicles. They are a critical part of modern city traffic management. The challenge lies in using current computer and automation technologies to efficiently manage traffic, increase intersection capacity, improve vehicle speeds, and reduce accidents. This paper presents a traffic light control system designed using PLC (Programmable Logic Controller), focusing on the calculation and experimental debugging involved in the design process.
**1. System Control Function**
The system includes red, yellow, and green lights at city intersections, operating in all directions (east-west and north-south). The main task is to control the lights in a specific sequence. When the start switch is activated, the system begins. The north-south direction starts with a red light for 25 seconds, followed by a green light for 20 seconds. After 20 seconds, the green light flashes for 3 seconds before turning off. Then, the east-west yellow light turns on for 2 seconds, after which the east-west red light activates. At the same time, the north-south green light comes on for 30 seconds, then turns off after 3 seconds, followed by the yellow light for 2 seconds. This cycle repeats continuously.
**2. Hardware Wiring and I/O Port Allocation**
The system uses a Siemens S7-200-224 PLC as the controller. The start button SB1 is connected to input relay I0.0. Output relays Q0.0 to Q0.5 control the respective lights: Q0.0 for north-south green, Q0.1 for north-south yellow, Q0.2 for north-south red, Q0.3 for east-west green, Q0.4 for east-west yellow, and Q0.5 for east-west red. The output power supply is connected to the 1L terminal.
**3. System Ladder Design**
To ensure clarity and ease of implementation, a sequential control method was adopted. This approach allows the system to operate in predefined steps based on timing and conditions. Each step represents a stage in the control cycle, and the system transitions between these stages according to set parameters. The design uses a dual-flow method with parallel branches, allowing both east-west and north-south directions to operate simultaneously while maintaining coordination through a unified timer.
When the PLC is turned on, it enters the initial state S0.0 and generates a 1-second flashing circuit. Pressing SB1 initiates the system, activating S0.1 and S0.5. The east-west green light turns on, and a 20-second timer (T37) starts. After 20 seconds, the green light begins to flash for 3 seconds using another timer (T38). Then, the yellow light activates for 2 seconds (T39), followed by the red light. After 30 seconds, the cycle returns to the beginning, repeating continuously.
**4. Conclusion**
There are multiple ways to implement traffic light control systems. Given the high demands for stability and reliability in busy intersections, traditional methods often fall short. PLC-based systems offer significant advantages in industrial applications, providing efficiency, ease of modification, and faster debugging. The results of this project meet the required standards and demonstrate practical value for real-world implementation.
For more advanced designs, you can explore free downloadable resources such as PLC-based traffic light circuits, FPGA-based traffic light course projects, and microcontroller-based traffic light controllers. These materials provide valuable insights into modern traffic control solutions.
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