The traffic volume on urban roads is typically high, which often leads to traffic congestion. To address the issue of severe traffic jams at city intersections, it is essential to upgrade the traffic light control system. This paper presents a design for a traffic light control circuit system that not only manages the normal operation of the signals but also integrates an emergency vehicle priority control mechanism.
The system utilizes MCGS configuration software as the upper-level monitoring interface and Siemens S7-200 PLC as the lower-level controller to manage the traffic signal system. MCGS (Monitor and Control Generated System) is a widely used industrial automation configuration software in China. It runs on the Windows platform and supports data acquisition from field devices, animation display, alarm management, process control, and report generation. Its flexibility makes it suitable for solving various real-world engineering challenges.
**1. Control System Requirements**
The traffic light control system at the intersection operates with a fixed switching interval of "red - yellow - green" lights. It accounts for different traffic patterns throughout the day and allows for the passage of emergency vehicles such as fire trucks, police cars, and ambulances by incorporating intelligent emergency control features.
**1.1 Normal Timing Control**
When the start switch is turned off, all lights are off. Upon turning the switch on, the north-south direction lights up with red for 9 seconds, while the east-west green light is on for 4 seconds. After 4 seconds, the green light flashes for 3 seconds, then turns off for 2 seconds, allowing the east-west and north-south directions to switch. The cycle continues, with the sequence illustrated in Figure 1.
**1.2 Emergency Vehicle Priority Control**
When an emergency vehicle is detected, the emergency switch is activated, overriding the current traffic light state. The green light in the direction of the emergency vehicle remains on until the vehicle passes. Once the vehicle clears the intersection, the emergency switch is deactivated, and the traffic light returns to the green light for 3 seconds before resuming normal timing. If two emergency vehicles are present, the system prioritizes the first one before handling the second.
**2. Hardware Design of the Control System**
The system uses a Siemens S7-200 PLC to control the traffic lights. Red, green, and yellow LEDs simulate the traffic signals, while a seven-segment digital display shows the working time. The CPU226AC/DC model is selected, with 24 input points and 16 output points. The I/O allocation is detailed in Table 1.
Due to the symmetry in the east-west signal timings, the lights are connected in parallel to save PLC output points. Current-limiting resistors of 2kΩ are used to protect the LEDs and the digital display. A SM4205 common cathode seven-segment display is used, and the hardware wiring diagram is shown in Figure 2.
**3. PLC Ladder Diagram Design**
Traditional traffic light programs often involve multiple timers, making them complex. In this design, a single timer is used, and the timing sequence is managed using data comparison commands. The flashing of the green lights is controlled using the special memory SM0.5. The time is decoded to the seven-segment display using the SEG instruction. Interlocks and emergency signals are managed through four-bit memory flags. The ladder logic is shown in Figure 3, offering a clear and efficient solution for traffic light control.
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