Circuit design of scattering atmospheric low visibility instrument based on Si4432

　　Aiming at the problems of power consumption, real-time performance, expansion compatibility and cost in the actual application of visibility detector, this paper proposes a design scheme based on Si4432 scattering atmospheric low visibility instrument. The design scheme is based on the visibility detection principle of measuring the intensity of forward scattered light, and the embedded control system designed with Cortex-M3 and RTOS realizes data reading, processing and system management. The early warning data adopts three output modes. The off-site experiment shows that the functions and performance of the prototype have met the design requirements, the error is controlled within 20%, and it has strong practicality.

　　Hardware implementation of control system based on Cortex-M3 microprocessor

　　The system control and data processing part uses the LPC17xx series microprocessor based on the ARM Cotex-M3 core architecture as the core hardware platform. The LPC17xx series has a high operating speed and rich peripheral interfaces, meeting the system's design requirements of low cost, low power consumption, and high performance. The control system hardware mainly includes the microprocessor and its supporting circuits, real-time clock (RTC), SD card local data storage circuit, data transmission interface circuit, AD sampling circuit, temperature monitoring circuit, etc. AD sampling uses the 16-bit precision AD7705 chip with automatic calibration function, and uses the SPI bus to connect to the processor. The temperature sensor chip uses LM75 and is connected to the processor using the IIC bus. Data transmission is mainly achieved through the USART and SPI data bus interfaces. Make full use of the timer of LPC17xx to realize digital waveform generator and digital phase shifter, output low-frequency square wave signal for modulating the transmitting light source, and phase-shift reference waveform for phase-sensitive detection of the phase-locked amplifier. Use negative feedback comparison measurement method to automatically determine the phase to be adjusted, so as to replace the square wave generating circuit and phase shifting circuit built by traditional analog circuit, which can simplify the hardware design and the debugging process of phase adjustment and help improve stability.

　　Design of early warning data output circuit with multiple transmission modes

　　The data output of the low visibility warning detector supports multiple modes of wired transmission, short-distance wireless data transmission, and public network data transmission to cope with complex installation conditions and diverse application requirements on site, and flexibly and conveniently access the warning monitoring network. Wired transmission uses MAX232 and MAX13082 interface chips, connected to the microprocessor UART, to complete the conversion from TTL level to RS232 and RS485 level standards. Short-distance RF wireless data transmission uses Silicon Labs EZRadioPRO series ISM band wireless transceiver integrated chip SI4432. The latest version of this chip is B1 version. It uses SPI bus for data transmission and reception with microprocessor. The output power can reach +20dBm at 240-960MHz frequency, and the receiving sensitivity is -117dBm, realizing reliable data transmission with variable information signs, road signs and other on-site warning execution equipment within a range of 500 meters. In places where there is no wired communication network, GPRS or CDMA public network data transmission is used to promptly inform the command and monitoring center and relevant on-duty personnel of the warning situation. The data transmission in various modes adopts modular design, and the corresponding transmission modules can be configured and replaced as needed, which helps to reduce the equipment cost. The data transceiver circuit of the low visibility warning detector is shown in Figure 2, where U9 is SI4432. Due to the single antenna form, the RF transceiver conversion switch UPG2214T is used for transceiver switching.

　　The scattered low visibility warning detector is mainly composed of infrared light transmitting and receiving devices, signal conditioning modules, control modules, warning data output modules and power modules. This paper proposes a design scheme based on Si4432 scattered atmospheric low visibility instrument. The control and data processing of the system designed in the scheme adopts a new microprocessor based on the ARMCortex-M3 core, running RTOS real-time operating system, which is conducive to improving operating efficiency and system performance, and is easy to upgrade, maintain and expand. It has good social benefits and broad industrialization prospects.