Design of wireless sensor network for water quality monitoring in Poyang Lake

Poyang Lake in Jiangxi is the largest freshwater lake in China and one of the lakes with the best water quality in the country, with a drainage area of ​​162,200 square kilometers. However, in recent years, the water quality of Poyang Lake has also been declining. Monitoring by the water environment monitoring department of Jiangxi Province shows that in 2002, the proportion of water quality of Poyang Lake above Class III accounted for 99.7%, while in 2006 it dropped to 82.1%, showing a downward trend year by year.
As the water area of ​​Poyang Lake is wide, including the Ganjiang River (Waizhou Station), Fuhe River (Lijiadu Station), Xinjiang River (Meigang Station), Changjiang River (Dufengkeng Station), Le'an River (Shizhenjie Station), Xiuhe River (Yongxiu Station), Xihe River (Shimenjie Station) and Boyang River (Zifang Station), the current water quality testing method is to send people to various areas to take water samples, and then put them in a special laboratory for testing. This cannot achieve continuous testing of lake water and is not efficient. This paper proposes a wireless sensor network design scheme for automatic water quality detection, which can realize automatic detection of water quality, and the detected data can be sent to the computer at the receiving point designated by the environmental protection department through the GPRS network. The detected data can be displayed in real time using the software of the computer there, and data sharing can be achieved through the Internet network to realize multi-level data management.
1 System overall structure
The water quality wireless sensor network detection system adopts a two-level system. The first level is the computer (host computer) designated by the environmental protection department to receive water quality detection data, and the second level is the water quality monitoring station in the water area. The host computer is responsible for monitoring, managing and controlling the water quality monitoring station; the water quality monitoring station is responsible for collecting and transmitting water quality data. GPRS wireless transmission is used for data communication between the host computer and the water quality monitoring station. Since GPRS communication is a data packet communication network based on IP address, the host computer is configured with a fixed IP address, and each water quality data collection point uses a unified SIM card of a mobile communication company. At the same time, the system database written in the host computer is used to save relevant water quality parameter data, perform statistical processing on the information, and generate various report outputs. It supports 24-hour real-time online, and enables the information collection point to transmit the collected water quality information data every 20 minutes, and can display the data graphically to realize the mapping and visualization of water quality data. The structure of the single-point detection point and the host computer system is shown in Figure 1.

2 Hardware Design
2.1 CPU Module Design
The importance of water quality protection in Poyang Lake has put forward high requirements for water quality detection with fast speed and accurate measurement. The CPU is the "heart" and "brain" of the entire water quality detection. As the center of the entire system, it receives all signals and data from water quality sensors, processes each data, and finally sends it to the GPRS module. The use of high-performance CPU chips can greatly improve the efficiency of water quality detection. This design uses a Samsung ARM core chip S3C44B0, which has a working efficiency of 4 to 5 times that of ordinary 8-bit microcontrollers and is very suitable for water quality parameter processing. S3C44B0 is a CPU based on the ARM7TDMI-S core. The 32-bit memory interface and unique acceleration structure enable the code to run at the maximum clock rate. From the overall performance point of view, the reason for using the S3C44B0 chip design is that it is fast, easy to debug, and stable. The signal lines connecting S3C44B0 and other external devices are shown in Figure 2.

2.2 Design of water quality sensor unit
The quality of water is determined by parameters such as pH value and turbidity, among which pH value is a very important parameter. The pH meter circuit is shown in Figure 3.

The core of this pH meter circuit is an inexpensive silver or silver chloride probe connected to the input of an ultra-low current amplifier. The typical impedance of the signal output from the pH meter probe is 10 to 1000 MΩ. Because of the high impedance, the input current of the amplifier is very low, which is very important. V1 is the LMC6001 amplifier, which has an input current of less than 25 fA, which is exactly what a pH meter needs. The theoretical output voltage of a standard silver or silver chloride probe at room temperature is 59.16 mV/pH at 25 °C, and the output voltage is 0 V at pH 7. The ultra-low input current amplifier LM6001 amplifies the output signal of the probe to +/-100 mV/pH relative to pH 7. The overall gain of the pH meter can be adjusted by the adjustable potentiometer R2. V2 provides reverse bias for the micropower amplifier LMC6041, so that the output voltage remains linear with pH over the entire measuring range of the probe.
2.3 GPRS unit design
GPRS is the "wireless" transmission part of the wireless sensor network. The GPRS module used in the system is WISMOQ2403 from WAVECOM. This module has complete functions, stable performance, small size, and is very suitable for embedded applications.
Its circuit design is shown in Figure 4. Most of the pins of this module do not need to be connected. You only need to pay attention to the SIM card signal and the serial port signal. As long as these two parts of the circuit are connected well, the GPRS hardware module can work normally.

3 Software Design
The online water quality monitoring system software consists of two parts: the lower computer program and the upper computer program. The design process is shown in Figure 5(a) and Figure 5(b) respectively.

4 System Experimental Results
Figure 6 is the crystal oscillation diagram of the CPU minimum system test. Figure 6 (a) is the main crystal oscillation diagram, the oscillation frequency is 10 MHz, and Figure 6 (b) is the RTC oscillation diagram, the oscillation frequency is 32.76 kHz. The main crystal oscillator starts to oscillate, proving that the CPU minimum system has started to work. The RTC mainly provides clock signals for the timing module.

Figure 7 is a pH value display chart, which can display the current pH situation in real time. Environmental monitoring personnel can not only see the data in real time, but also process the data in a graphical form, which is more intuitive.