Design of automatic bus stop announcement system based on ZigBee

Aiming at the characteristics of the bus system in small and medium-sized cities and the actual operating environment, a single-chip automatic station announcement system based on ZigBee (Purple Bee Protocol) technology was designed in consideration of cost performance and practicality. The station transmitter uses CC2430 chip to realize the automatic transmission of station address coding information. The peripheral circuit is simple and can be powered by batteries for a long time. The on-board part has the function of real-time switching between manual and automatic station announcement modes. The bus line can be easily changed through the line information stored in the U disk. While the station is automatically announced, the station name, calendar, time, week, temperature, service terms and other information can be scrolled. The system is low-cost, highly scalable, reliable in operation, easy to install and maintain, and suitable for promotion and use.

With the rapid development of my country's economy, people have more opportunities to travel, and the role of public transportation in daily life has become more and more prominent, which has put forward higher requirements for the safe operation of urban public transportation systems. At present, many buses in medium and large cities have realized unmanned ticket sales, and bus drivers have a greater responsibility for driving safety. This urgently needs to install automatic station announcement devices on each bus. In recent years, domestic scholars have gradually carried out research in this area, using single-chip microcomputers, embedded processors, wireless sensor networks and GPS technologies to design various automatic bus station announcement systems, which have played a good role in promoting the development of urban public transportation. However, most bus routes in small and medium-sized cities have not yet fully adopted automatic station announcement systems, and there is an urgent need for automatic station announcement devices that are low-cost, reliable, easy to maintain and easy to promote.

In view of the characteristics of bus systems in small and medium-sized cities, the single-chip microcomputer is combined with ZigBee technology to design a low-cost automatic station announcer based on ZigBee technology. It has the advantages of both manual and automatic station announcers and is suitable for large-scale popularization and application.

1 Overall plan

There are many bus stops on the main streets of the city. One stop may accommodate buses of multiple routes, and the same stop may be named differently on different routes. Therefore, the station address coding information needs to be dynamically set using an 8-bit DIP switch so that each bus can automatically identify its current position on the route. Figure 1 shows a schematic diagram of an automatic bus stop announcement system with n stops.

Figure 1 Automatic bus stop announcement system

Due to the narrow roads in small and medium-sized cities, buses may receive multiple station address coding information at the same time during travel. For example, a bus traveling to the right in Figure 1 will receive the coding information of platform 1 and platform n at the same time. At this time, the coding information of platform 1 can be automatically determined to be valid based on the driving direction. The vehicle's driving direction can be identified by prefabrication or station address increment (decrement). If a sudden change in coding occurs, it means that the vehicle may have used manual station announcement during travel because it could not automatically announce the station. In this way, the system can record the station code for timely maintenance.

The system is divided into two parts: the vehicle-mounted part and the platform part. The vehicle-mounted part uses a control module with the MSP430F149 single-chip microcomputer as the core; the platform part only needs to transmit station address coding information, requiring low power consumption, low cost, high reliability and easy maintenance, so the ZigBee transmission module is composed of the CC2430 chip as the core. Figure 2 shows the design block diagram of the bus automatic station announcement system.

Figure 2 Design block diagram of the automatic bus stop announcement system

2 Hardware Design

2.1 Station address code information transmitter

The hardware design of the station address coding information transmitter is shown in Figure 3, which is mainly composed of ZigBee transmission module, crystal oscillator, impedance matching network, antenna and battery. ZigBee uses Chipcon's CC2430 chip, which continues the architecture of the previous CC2420 chip and integrates the ZigBee RF front end, memory and microcontroller (enhanced 8051 core) on a single chip. It is produced using 0.18 mCMOS process. The current consumption in the transmission mode is less than 25mA, and it has the ultra-short time characteristic of switching from sleep mode to active mode. It is particularly suitable for occasions that require very long battery life. Two No. 5 batteries can be used for up to 2a.

Figure 3 Hardware schematic diagram of the site transmitter

The CC2430 chip is connected to an external 32MHz crystal oscillator, and the station address coding data is dynamically set through the 8-bit DIP switch SW. After being processed by the impedance matching network, it is sent out through the antenna, and the coverage range can reach tens of meters.

The hardware design method has the characteristics of simple circuit, stable signal, low cost and easy installation.

2.2 On-board automatic station announcement device

The hardware design of the on-board automatic station announcer is shown in Figure 4 (except the keyboard and display module).

Figure 4 Hardware schematic diagram of the on-board automatic station announcement device

The single-chip microcomputer uses TI's MSP430F149 chip, and its power consumption current is μA level. The MSP430 core is a 16-bit CPU with an efficient RISC instruction system and unified interrupt management. It has a precision hardware multiplier, two 16-bit timers, a 14-channel 12-bit analog-to-digital converter, a watchdog, six parallel ports, two USART communication ports, a comparator, two external clocks and 60kB flash memory, of which two communication ports can work in UART and SPI mode.

The ZigBee receiver module uses the CC2420 chip, which has a fully integrated voltage-controlled oscillator and only requires a few peripheral circuits such as an antenna and a 16MHz crystal oscillator to work in the 2.4GHz frequency band. The CC2420 only provides an SPI interface to connect to the microprocessor, through which the settings and data transmission and reception are completed. The MSP430F149 integrates an SPI controller and can be easily used with the CC2420.

The clock chip uses DS1302, which is a high-performance, low-power serial clock calendar chip developed by DALLAS based on the I2C bus. It has RAM and leap year compensation function. It can generate time stamp data such as year, month, day, week, hour, minute, and second and save them in the corresponding registers. The microcontroller can read these data directly.

The temperature sensor uses DS18B20, which is a one-wire single-bus intelligent digital temperature sensor produced by DALLAS. It has the advantages of miniaturization, low power consumption, high performance, and strong anti-interference ability. The temperature measurement range is -55℃~+125℃, and the measurement accuracy is 0.5℃ at -10℃~+80℃. After the microcontroller issues a temperature conversion instruction, DS18B20 saves the measured temperature value in two units, MSB (high 8 bits) and LSB (low 8 bits), which can be directly read by the microcontroller.

The main parts of the LED dot matrix display module are the display array and the row and column drive circuit. The display array of this design is a 16-row × 64-column monochrome dot matrix. The row drive circuit is composed of MC74HC138 chip, TC74HC04 chip and transistor NPN9013, and the column drive circuit is composed of M74HC595 chip. It adopts a dynamic scanning drive mode, which can dynamically and real-time display the site, current temperature, date and time and other information.

The USB interface chip is PDIUSBDI2, which is one of the most used chips by Philips in the USB1.1 protocol device end. It meets the specifications of most device classifications and can be used on USB flash drives. PDIUSBDI2 integrates many features, including programmable clock output, low-frequency crystal oscillator, endpoint register, analog transceiver, voltage regulator, bit clock reset, serial interface engine, memory management unit, SoftConnect, GoodLink and integrated RAM. It contains 3 endpoints, each of which has two endpoints: input and output. Among them, the index 0 and 1 of endpoint 0 are control input and output endpoints, which are used to transmit USB control commands and complete USB enumeration; the index 2 and 3 of endpoint 1 are ordinary input and output endpoints, and the index 4 and 5 of endpoint 2 can be configured as ordinary input and output endpoints, synchronous input endpoints, synchronous output endpoints or synchronous input and output endpoints. The buffer size of endpoints 0 and 1 is 32kB, and the buffer size of endpoint 2 is 128kB, and it is double buffered. The transmission of USB data is carried out through these endpoints.

The voice amplifier module includes voice chip, power amplifier, speaker, etc. The voice chip uses ISD4003 integrated chip, which has few voice signal connections and a single-chip recording and playback time of 4 to 8 minutes. The chip uses multi-level direct analog storage technology, and each sample value is directly stored in the on-chip flash memory, which can reproduce the voice signal truly and naturally. The power amplifier chip uses TDA2822 dual-channel audio power amplifier circuit, which has a wide operating voltage range and is suitable for working under low power supply voltage, with small crossover distortion and low static current, and the maximum output power can reach 3W.

3 Software Design

The software flow of the vehicle part is shown in Figure 5, which mainly includes: system initialization program, keyboard interrupt service program (manual station announcement part), ISD4003 voice playback program, U disk management program, LED display program and subprograms for reading DS1302, DS18B20, CC2420, etc. The software flow of the platform part is relatively simple and will not be described here.

Figure 5: Software flow of the on-board automatic station announcer

4 Conclusion

1) The station part is designed with CC2430 chip to realize the automatic transmission of station address coding information. It has few peripheral circuits and can be powered by batteries for a long time. It is low in cost, reliable in operation, and easy to install and maintain.

2) The onboard part can conveniently switch bus routes through the route information stored in the U disk. The LED dot matrix display has good effect. While automatically announcing the station, it can scroll and display the station name, calendar, time, week and temperature information in real time, which increases practicality and makes the bus service more humane.

3) In view of the characteristics of bus systems in small and medium-sized cities and the actual operating environment, a single-chip microcomputer automatic station announcement system based on ZigBee technology was designed taking into account performance-price ratio and practicality. It has strong scalability, low cost, and is very suitable for large-scale promotion and use.