Modular design of wireless sensor network node hardware

As people's requirements for environmental monitoring continue to increase, wireless sensor network technology has been widely used with its low investment cost, convenient installation and high reliability. Since wireless sensor network nodes need to realize multiple functions such as collection, processing and communication, the modular design of hardware can greatly improve the stability and security of network nodes.

1 Introduction to CC2430 chip
CC2430 is a wireless transceiver chip that works on the 2.4 GHz free frequency band and supports the IEEE 802.15.4 standard. The chip has a high degree of integration, small size and low power consumption. The ZigBee radio frequency (RF) front end, memory and microcontroller are integrated on a single chip. CC2430 has an 8-bit MCU (8051), 8 KB of RAM, 32 KB, 64 KB or 128 KB of Flash, and also includes an analog-to-digital converter (ADC), 4 timers, AES128 coprocessor, watchdog timer, sleep mode timer with 32.768 kHz crystal oscillator, power-on-reset circuit, brown-out detection, and 21 programmable I/O interfaces.
The CC2430 chip is produced using 0.18μm CMOS technology, and the current consumption is 27 mA when working; in receiving and transmitting modes, the current consumption is 26.7 mA and 26.9 mA respectively; the current in sleep mode is 0.5 μA. The sleep mode of CC2430 and the ultra-short time of switching to active mode are particularly suitable for applications that require very long battery life.

2 Wireless sensor network system structure
The entire wireless sensor network consists of several collection nodes, 1 aggregation node, 1 relay, and 1 host control center. The system structure is shown in Figure 1. The wireless sensor network collection node completes data collection, preprocessing and communication; the aggregation node is responsible for the initiation and maintenance of the network, collects and uploads data, and notifies the collection node of the command issued by the relay; the relay is responsible for uploading the collected data and passing the command information issued by the control center to the aggregation node; the control center is responsible for processing the final uploaded data, and the user can issue network operation commands.

The acquisition node and the aggregation node are controlled by CC2430. The acquisition node can collect and transmit data, and the aggregation node is responsible for collecting the data collected by all acquisition nodes. The transfer device uses an ARM processor as the control core, and uses serial communication with the aggregation node, and interacts with the host computer control center in GPRS communication mode. The host computer control center realizes human-computer interaction, can process
and display the uploaded data, and can directly issue network action execution commands from the customer.

3 Node modular design
The aggregation node and the acquisition node are basically the same in hardware configuration, and the modular design makes the design more versatile.
Each node is mainly composed of four parts: control module, wireless module, acquisition module, and power module. The modular design structure diagram of the network node is shown in Figure 2.

3.1 Control module
The control module is mainly composed of CC2430 and its peripheral circuits, which complete the processing, storage, and transmission and reception of collected data, and manage the power module. The chip CC2430 includes 21 programmable I/O ports, including 8 A/D interfaces, which can meet the collection and processing requirements of multiple sensors. As shown in Figure 3, CC2430 has a reset interface, and an external reset button can realize hardware initialization of the system. The 32 MHz crystal oscillator provides the system clock, and the 32.768 kHz crystal oscillator is used when the system is dormant.

The node uses the chip FM25L256 as the storage device. This is a 256 Kb ferroelectric memory with an SPI interface frequency of up to 25 MHz. Low power consumption and 10-year data retention ensure low cost and reliability of node data storage. The memory peripheral circuit connection is shown in Figure 4.

3.2 Wireless module
The wireless module is responsible for the transmission of data and commands between nodes. Therefore, the reasonable design of the wireless module is an important guarantee for the stable and efficient communication of the nodes.
TI provides a microstrip balun circuit suitable for CC2430. This design converts the impedance of the differential signal of the radio RF pin to a single-ended 50 Ω. Since this circuit directly affects the communication quality of the node, it must be simulated and verified before use. ADS simulation software was used for simulation in the design, and the joint simulation method of the layout and schematic diagram was adopted. The simulation circuit diagram is shown in Figure 5. The microstrip circuit is a microstrip balun circuit provided by TI, and the discrete components are all selected from the models in the component library of Murata, which strictly guarantees the authenticity and reliability of the simulation data. The simulation results are shown in Figure 6. It can be seen from the figure that the signal transmission characteristics of the balun circuit are efficient and stable within the working frequency band (2.400~2.4835 GHz).

3.3 Acquisition module
The acquisition module is responsible for collecting data and conditioning data signals. In this design, the temperature and humidity data of the soil are monitored, and the sensors used are the PTWD-3A soil temperature sensor and the TDR-3 soil moisture sensor.
The PTWD-3A soil temperature sensor uses a precision platinum resistor as a sensing component, and its resistance changes with temperature. In order to accurately measure, the four-wire method is used to measure the resistance principle, and the resistance signal is conditioned into a voltage signal that can be sampled by the A/D channel of the CC2430 chip. In Figure 7, a 10 mA constant current source is composed of a P354 operational amplifier, a high-precision precision chip resistor, and a 2.5 V power supply. The 10 mA current loop passes through the sensor resistors R1 and R2 to convert the resistance signal into a voltage signal, and the differential amplifier LT1991 with a one-time gain converts the signal into a single-ended output and sends it to the ADC channel of the CC2430 chip for sampling.

The output signal of the TDR-3 soil moisture sensor is a voltage signal, and its conditioning circuit is shown in Figure 8. The sensor output signal is sent to the ADC channel of the CC2430 chip through the P354 operational amplifier for sampling.

3.4 Power module
The power module is responsible for regulating voltage and distributing energy. It is divided into three modules: charging management module, dual power switching management module, and voltage conversion module. This design uses a lead-acid battery with a rated voltage of 12 V and a capacity of 3 Ah for power supply.
As a wireless sensor network application for environmental monitoring, the node needs to work in the wild without supervision. Energy replenishment is an important guarantee for the continuous operation of the system. This design uses solar panels to replenish electricity for the nodes when they work in the wild, and the charging management module charges the lead-acid battery reasonably and effectively according to the sunshine conditions and battery energy status. As shown in Figure 9, the optocoupler TLP521-100 and the field effect tube Q together constitute the switching circuit of the charging module, which can be easily controlled by the I/O port of the CC2430 chip.

When the solar panel is charging the battery, the battery cannot power the system. Therefore, the design adopts a dual power supply mode to maintain the working state of "one charging and one supply". The dual power switching management module is responsible for the safe and fast switching of the power supply. As shown in Figure 10, two switch circuits are used to switch the two power supplies.

When switching power supplies, always turn on the idle power supply first, and then turn off the power supply that is powering the system. Therefore, there will be two power supplies to the system at the same time for a short period of time. This is to prevent the system from losing power.
The power module needs to provide multiple power supplies such as 5 V, 3.3 V, and 2.5 V to meet the energy supply requirements of each module in the node. Since there are many power supply groups in the system, the voltage conversion module uses a variety of voltage conversion chips such as switching buck regulators and low-dropout linear regulators to convert the power supply voltage, while ensuring the high efficiency of the power supply of the power module.

Conclusion
The design of the node is crucial to the entire wireless sensor network system. This design uses the powerful RF chip CC2430 as the core management chip, which can better complete multiple functions such as data collection, analysis, and transmission. The modular design of the hardware greatly enhances the stability, reliability, and versatility of the node. The wireless sensor network system can perform long-term and stable environmental monitoring in the field without human supervision.