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Urban roads are arranged in a complex pattern, with vehicles moving smoothly and pedestrians walking briskly. But how is this order maintained? It all comes down to the automatic command system of traffic lights. Traffic lights serve as a crucial control mechanism in urban traffic. As an effective tool for managing traffic flow and increasing road capacity, they significantly reduce traffic accidents. This paper presents the design of an **intelligent traffic light system** that uses the **STC89C52 microcontroller** as its core and incorporates **wireless control via a Zigbee module**. Traffic officers can remotely adjust the duration of traffic signals through a handheld device, allowing them to adapt to varying traffic conditions and crowd levels. Additionally, the system features a digital display showing the remaining time of each signal, embodying the principles of the Internet of Things (IoT) and offering considerable practical value and market potential.
### 1. System Design
The paper introduces a wireless control system for traffic lights at intersections, centered around the **STC89C52 microcontroller**. The system allows traffic officers to change the timing of traffic lights using a **Zigbee module**, while two digital tubes are used to display the current status. The system consists of three main components: the **handheld control module**, the **traffic signal control module**, and the **digital display module**. In real-world scenarios, the distance between the intersection center and the traffic lights is typically no more than 300 meters, and the Zigbee module can reliably transmit data over a range of up to 800 meters. By setting up the Zigbee network in a **Mesh topology**, data can be relayed between nodes, further enhancing the system’s reliability. Traffic officers can directly connect to the traffic light module using a handheld device.
*Figure 1: Wireless terminal layout*
### 2. Hardware Design
#### 2.1. Handheld Device Hardware Design
The hardware of the handheld device includes a **microcontroller**, **liquid crystal display (LCD)**, **wireless communication module**, and a **clock control unit**, as shown in Figure 2. When traffic officers input the desired signal and timing using the button interface, the data is processed by the microcontroller and transmitted to the traffic light control module via the Zigbee module. The CN3065 chip is used for system calibration and timekeeping, while the Zigbee module utilizes the **CC2430**.
*Figure 2: Wireless handheld device hardware topology*
#### 2.2. Traffic Light Control Module Hardware Design
The primary function of the traffic light control module is to receive and interpret data packets, then issue control commands to switch the signals. It communicates with the microcontroller via serial communication, allowing field data to be sent to the city monitoring center for remote management during emergencies. At intersections, traffic lights on the same road are usually synchronized. Therefore, the digital display circuit uses **two common-anode seven-segment displays**, which can show numbers from 0 to 99. Each pin of the P1 port of the STC89C52 is connected to a **300-ohm resistor** before being linked to the display.
*Figure 3: Traffic light control module hardware topology*
### 3. Software Design
The software design of the system is divided into two parts: the **software for the handheld device** and the **software for the traffic light control module**. Both were developed using the **IAR Embedded Workbench** platform.
#### 3.1. Handheld Device Software Design
The handheld device allows traffic officers to input the ID and status of the traffic signal they wish to control and send the command wirelessly. The **CC2430** module supports the **Z-STACK** protocol stack, compliant with the Zigbee standard. As a **Zigbee coordinator**, the handheld device initiates the network and manages node updates, ensuring that only active devices can issue commands. To prevent errors due to packet loss, the system uses the **MODBUS protocol** with **CRC-16** for error checking. Once the Zigbee network is established, the latest list of controllable signals is displayed on the LCD. After the officer enters the command, the control information is transmitted.
*Figure 4: Handheld software communication flow chart*
#### 3.2. Traffic Light Control Module Software Design
The traffic light control module is responsible for joining the Zigbee network initiated by the handheld device and receiving control commands. When connecting to the network, it sends its **ID and date** to the handheld device for synchronization. If it is not part of the Zigbee network, it reverts to the original traffic light logic.
*Figure 5: Control module flow chart*
### 4. Conclusion
This paper addresses the real-world needs of traffic officers in controlling traffic lights, integrating IoT concepts to develop a **wireless handheld traffic light control system**. Through the Zigbee network, the system transmits control commands from the officer to the traffic light module, fulfilling the operational requirements of traffic police on duty. The system is designed with **low power consumption and cost**, ensuring stable and reliable performance. Its flexible and robust wireless network makes it well-suited for large-scale deployment and future expansion.