Abstract: This paper introduces an intelligent telephone alarm system consisting of an embedded alarm host and wireless sensor network nodes. The system's functions and components are described, along with the wireless communication protocol and some software flow. Wireless sensor network technology is used to create a wireless network of security sensor nodes. Dual-tone multi-frequency (DTMF) encoding and decoding technology enables dialing, voice alarm, and simple remote telephone control, making it a novel alarm system that is easy to operate and offers good real-time performance.
Keywords: Wireless sensor network telephone alarm, Dual-tone multi-frequency, Telephone control
Modern residential layouts are making family life increasingly enclosed, making security a paramount concern. Current security and alarm systems are crucial for ensuring the safety of homes and residents, and are also an essential component of the digital home. In emergencies such as robbery, fire, or gas leaks, it is essential to promptly call for help via telephone.
This paper introduces an intelligent telephone alarm system consisting of an embedded alarm host and wireless sensor network nodes. Wireless sensor network technology is used to create a wireless network of home security sensors; dual-tone multi-frequency (DTMF) encoding and decoding technology is used to achieve active dialing, voice alarm, and simple telephone control; and an embedded development platform with the S3C2410 as the core processor is used to build a new type of embedded alarm system that is easy to configure, powerful, and has a wide coverage.
1. System hardware structure and working principle
1.1 System Hardware Structure
The system consists of an alarm control panel and wireless sensor network nodes. Figure 1 is a simplified diagram of the overall system structure; the wireless communication modules in the figure are identical in structure. The alarm control panel comprises a development platform based on a 32-bit embedded microcontroller (S3C2410) and a wireless communication module. The alarm control panel and the wireless sensor network nodes form a star-topology wireless network.
Using the IIS interface of S3C2410 itself, an audio processing module is formed by expanding UDA1341[1], realizing two channels of audio input and one channel of audio output. The alarm host exchanges audio data through the IIS interface. After the audio power is amplified, the audio signal is coupled into the telephone line by the audio transformer to minimize the impact on the dual-tone multi-frequency (DTMF) signal.
The ring detection module [2] first rectifies and filters the telephone line signal and sends it to the external interrupt input of the S3C2410. The S3C2410 controls the telephone interface circuit to perform different operations according to different ring tone signals, and identifies the type and number of signals and provides them to the upper application. When the pick-up and hang-up control module receives the pick-up and hang-up control signal sent by the CPU, the relay in the module connects or disconnects a 300Ω resistor to realize automatic pick-up and hang-up [3].
In this system, the DTMF encoding/decoding circuit uses the MT88L89 as the core device for encoding and decoding DTMF signals. The DTMF signal generated by the remote user's telephone keypad is blocked from DC by a coupling capacitor, then received and decoded by the MT88L89. After decoding, an interrupt signal is sent from the interrupt control terminal to notify the CPU that the data is ready for reading. The four-bit parallel binary data output by the MT88L89 is connected to the CPU via the Local Bus, at which point the CPU can obtain the key value information. The CPU then sends the phone numbers to be dialed sequentially to the MT88L89 via the Local Bus, which converts them into DTMF signals to realize the dialing function.
1.2 System Working Principle
When safety hazards such as theft, fire, or gas leaks occur, the sensors transmit digitally encoded alarm signals to the alarm control panel via a wireless sensor network. The alarm control panel immediately sounds an siren and simultaneously dials the user's pre-set phone number, notifying the user of the alarm via voice. The voice prompts the user to confirm receipt of the alarm information by pressing keys; the smart telephone alarm recognizes the key values pressed by the user and determines the next action. The system also has remote arming/disarming capabilities, meaning that after authentication, the owner can remotely arm/disarm the system via telephone. It also features a call blocking function; if no one is home, the phone line is busy, or someone calls from outside attempting to monopolize the alarm line for theft, the system will automatically block the line and dial the alarm.
2. Wireless Sensor Network Node Design
The composition of wireless sensor network nodes varies in different applications, but they generally consist of four parts: data acquisition, data processing, data transmission, and power supply [4]. The form of the monitored physical signal determines the type of sensor. The processor is usually an embedded CPU, such as Motorola's 68HC16 or C51 series microcontrollers. The data transmission unit is mainly composed of low-power, short-range wireless communication modules, such as Xuntong's PTR8000 or ZigBee wireless modules. Figure 2 illustrates the composition of the node, where the direction of the solid arrows indicates the direction of data flow in the node.
2.1 Processor
The AT89C2051 chip was selected as the processor for the wireless sensor network nodes. Furthermore, it uses I/O ports to simulate SPI interfaces for communication with the nRF905, and the memory interface configuration also uses I/O ports for simulation.
2.2 Sensors
For home security, it is essential to ensure that monitored areas (balconies, doors and windows, corridors, vaults, etc.) are within the sensor's sensitive area. Many sensor products are available, such as infrared pyroelectric probes, microwave Doppler effect probes, and microwave-infrared multiplexed probes, which are highly sensitive for detecting intruders. Ionization smoke detectors and semiconductor gas sensors are used to monitor fire alarms and gas leaks.
2.3 Wireless Communication Module
The external MCU configures the PTR8000's internal registers and data transmission and reception via the SPI bus [5]. The PTR8000's SPI bus includes four pins: CSN (SPI enable), SCK (SPI clock), MISO (master input, slave output), and MOSI (master output, slave input). Each operation of the SPI bus must begin on the falling edge of the enable pin CSN. CSN is active low, and data on the bus is active on the rising edge of the clock. The MCU operates the SPI bus in two ways: read and write. When performing a read operation, CSN is first set low, and then a byte representing the read command is output on the MOSI data line. At the same time, the PTR8000 will output 1 byte of data representing status information on the MISO data line, followed by 1 byte of address, and then valid data. When performing a write operation, it is simpler. The MCU first pulls CSN low, and then outputs the write command byte and data byte on the MOSI line.
2.4 Power Supply
In this system, the nRF905 is a low-voltage device requiring 3.3V. To interface with it, the AT89C2051 also operates at 3.3V (the AT89C2051 is a wide-voltage device). The system is powered by a +5V supply, which is converted to 3.3V via the onboard LT1117.
3. Design of the alarm control panel
In this system, the alarm control panel acts as a protocol conversion gateway connecting the Public Switched Telephone Network (PSTN) and the wireless sensor network. Its hardware utilizes the Youlong FS2410 core board, with peripheral boards redesigned as needed. The FS2410 core board integrates an ARM9 2410 processor, 64MB SDRAM, 64MB NAND FLASH, 2MB NOR FLASH, and an Ethernet controller, providing ample space for software development.
The peripheral board provides one 10M/100M Ethernet interface, three RS-232 serial ports, two USB interfaces, and one RJ11 interface. It also includes an expansion module for wireless communication, a telephone alarm module, and a power supply module. The Ethernet interface on the peripheral board can be directly connected to the Internet, and the RS-232 serial ports connect to the PTR8000 wireless transmission module via a MAX232 microcontroller.
The alarm host uses the ARM Linux operating system as its basis, and the embedded database SQLite has been ported to it. Intelligent telephone alarm software has been developed, which has functions such as active voice alarm, remote disarming/arming, and remote setting of alarm telephone numbers.
4. System Software Design
The entire system software design includes the software design of the wireless sensor network nodes (lower-level machines) and the alarm host system (upper-level machines). Due to space limitations, only the data wireless communication strategy and the main program of the wireless sensor network node system will be introduced here.
4.1 Data Communication Strategy Between Host Computer and Slave Computer
In this system, data is transmitted wirelessly between the alarm control panel and each wireless sensor network node. The communication protocol uses the frame format shown in Table 1.
In Table 1, the preamble character can be 0xAA, 0xAA, 0xAA, 0xAA, 0xFF, or 0x00, totaling 6 bytes. The first 4 bytes are for synchronization signals, and the last 2 bytes are for the start of the frame. The total length of the frame cannot and will not exceed 256 bytes. If it does, it will be divided into multiple 256-byte frames for transmission. The frame type field is used to define the frame format for commands and responses. This field is 8 bits. Protocol frames are divided into various types such as initialization frames, acknowledgment frames, data frames, and control command frames. The address indicates the destination address of the data frame. The checksum is a CRC16 checksum.
In the system, the wireless sensor network nodes and the alarm host form a single-hop star topology network. Each wireless sensor network node has a unique node number, assigned by the alarm host and addressable by it. After powering on, the wireless sensor network nodes enter standby mode. The alarm host encapsulates a query command according to the aforementioned protocol and sends it to the lower-level device. Upon receiving the data, the lower-level device first checks the preamble to determine if it is valid, and then performs verification. If the verification is successful, it checks if the address is its own address; if so, it decapsulates the packet and performs the corresponding operation; if it is a query command, it encapsulates alarm information according to the aforementioned protocol and returns it to the alarm host; otherwise, the lower-level device discards the data packet. After receiving the alarm protocol frame, the alarm host actively dials a pre-set phone number and broadcasts the alarm via voice. The alarm host continues to query the lower-level device in the same way until it finds the desired lower-level device. The entire network timing is centrally controlled by the master station to ensure that no access conflicts occur.
4.2 Wireless Sensor Network Node System Programming
The wireless sensor network node system program consists of a main program, data acquisition subroutines, and subroutines for communication with the host computer. This section mainly introduces the design of the main program.
The main program is the core of the lower-level computer monitoring and control system. It primarily calls various subroutines to collect, store, and send alarm data to the upper-level computer. When the lower-level computer receives an instruction from the upper-level computer, it executes the corresponding operation according to the instruction. The flowchart of the main program is shown in Figure 3.
The system employs wireless sensor network technology, connecting home security sensors into a star-topology wireless sensor network. When a smoke alarm or infrared alarm signal is detected, it can proactively call the homeowner, allowing them to be aware of potential security risks immediately. Users can also remotely set new alarm phone numbers, arm and disarm the system via telephone, achieving simple control of the alarm control panel. Adding control functions to the system enables remote telephone control of home appliances, significantly promoting the networking, digitalization, and intelligentization of homes.
