The alarm network adopts a star topology and consists of a central node and a wireless alarm network node. The central node is a wireless communication module that is connected to a computer and can communicate with any of the network nodes in the network. The wireless alarm network node is the basic unit that constitutes the intelligent home alarm system, and is the basic platform that constitutes the intelligent home alarm system. It consists of a microprocessor (AT89C2051), a wireless communication module (nRF2401), an environmental monitoring sensor (a magnetic switch, an infrared sensor, a fire smoke sensor, a gas leak sensor, a glass break alarm, etc.) and some peripheral devices, which are responsible for the home. Environmental monitoring and delivery of alarm information.
This kind of star topology can be used to expand the combination well, and it is easy to increase the network nodes to meet the uncertainty of the distribution of network nodes in the home. A node can be placed in the room and isolation to avoid wireless interference and alarm area uncertainty. The problem is that the central node performs comprehensive processing on the data transmitted by the plurality of network nodes, and analyzes whether an alarm signal is issued. To some extent, reduce the false alarm of the alarm system and improve the reliability of the wireless alarm system. In the alarm system network, the main function of the network node is to collect data, and transmit the data to the network central node in a wireless transmission manner, and the central node performs comprehensive analysis and processing on the data to determine whether to give an alarm.
Wireless alarm network node hardware design
The wireless alarm network node is mainly composed of a microprocessor AT89C2051, an nRF2401 wireless communication chip, a peripheral device and a sensor of a digital input interface. The microprocessor AT89C2051 is the core control component of the hardware system of the entire network node. It is responsible for receiving the switch alarm signal triggered by the sensor, transmitting the nRF2401 communication command, and transmitting the control information. The single-chip RF transceiver chip nRF2401 is a key device for wireless data transmission. The nRF2401 operates in the 2.4~2.5GHz ISM band. The chip has built-in frequency synthesizer, power amplifier, crystal oscillator and modulator. The output power and communication channel can be configured through the program. The power consumption of the chip is very low, and the power is transmitted at -5dBm. When the working current is only 10.5mA, the working current is only 18mA when receiving, a variety of low-power working modes, energy-saving design is more convenient. The network node uses nRF2401 for data transmission to realize wireless transmission of alarm information. The network node senses abnormal changes in the external environment through the connected sensors. The circuit schematic of the network node is shown in Figure 2.
Figure 2 Network node circuit schematic
In the network node, the power source is the energy center, which plays a very important role in the node. Especially in the wireless communication system, the power source is not only the energy supplier, but also directly affects the quality of the communication. The device is usually limited to the voltage applied to the input or output pin. These pins are connected to Vcc by diodes or separate components. If the voltage applied is too high, current will flow to the power supply through the diode or separate components. The current voltage output module is mostly 5V, so it is necessary to design a 5V to 3V voltage regulation module with stable output. In this system, the LM1117 is designed to provide a conversion voltage for the communication control chip, which has the advantages of low power consumption and small size. And because the voltage contains many sources of high frequency interference, these high frequency components can easily enter the communication system through the power supply. In addition, the transmission frequency of the system itself will also be interfered by the power supply feedback to the communication system. Therefore, a 220 μH inductor can be added to the power supply circuit, and a filter circuit composed of a plurality of capacitors having different capacitance values ​​can be used to suppress various high-frequency signals. Enables the node to obtain a stable, reliable and low-interference power supply to ensure reliable operation of the node. The power supply circuit is shown in Figure 3.
Figure 3 power circuit
The main problems and solutions encountered in the design process of the network node are: nRF2401 RF circuit works in the high frequency working band of 2.4GHz~2.5 GHz, and the anti-interference design is directly related to the RF performance and the operation of the entire network node. When wiring in the wireless communication part, reasonable layout and wiring and the use of multi-layer boards are both necessary for wiring and an effective means to reduce electromagnetic interference and improve anti-interference ability. The following points should be noted when wiring: First, the area of ​​the wireless communication part that is not used for wiring needs to be filled with copper and connected to the ground to provide RF shielding to achieve effective anti-interference; second, the bottom of the nRF2401 chip should be grounded; Reduce delay, reduce crosstalk, and ensure high-frequency signal transmission. Use multiple ground vias to connect the bottom of the nRF2401 chip to the ground plane. Third, reduce crosstalk as much as possible and reduce the influence of distribution parameters. The device should be closely distributed in the nRF2401. All around.
According to the characteristics of wireless network technology and smart home, the composition scheme of alarm system based on wireless network technology is proposed. The design of the system adopts modular design, which makes it have good portability and scalability. The alarm system based on wireless network technology has high communication efficiency and good stability.
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