Design of wireless emergency rescue system

In modern society, when an elderly person is alone at home and encounters an emergency, he or she cannot make a phone call or call for help. This article designs a wireless emergency call system, so that when an elderly person at home encounters an accident, he or she can automatically connect to the phone of his or her children with one button on the handheld terminal, and talk to the children in time through the handheld terminal. The system is composed of a single-chip microcomputer AT89C51 as the main control part, which performs the main information processing and receives external operation instructions to form various control signals.

1 Hardware design of the system

When an emergency occurs, press the emergency button of the wireless emergency call controller, and the wireless emergency call device sends an emergency call signal. This call signal is used as a trigger switch signal of the emergency call device, which enables the main control end of the wireless emergency call system connected to the home telephone line to start working. After the wireless emergency call system is triggered to work, the single-chip microcomputer will call out the telephone numbers stored in it one by one, and convert these digital numbers into dual-tone multi-frequency signals through the MT8880 dual-tone multi-frequency circuit, and finally send this dual-tone multi-frequency signal to the telephone network, so that the wireless emergency call system can talk to another user in the telephone network through the telephone network to report the emergency. If the first number or the first few numbers sent by the wireless emergency call system are not answered or cannot be connected, the wireless emergency call system will continue to dial the phone number stored in the single-chip microcomputer according to the set rules, and the cycle will continue until someone in the telephone network answers the call.

The system is based on the single-chip microcomputer and consists of four parts: DTMF dialing circuit, remote control transmission and receiving circuit, phone number storage circuit and hands-free call circuit. When a dangerous situation occurs, press the emergency button on the caller or remote control. After the single-chip microcomputer detects the signal, it will send the phone number or mobile phone number stored in the single-chip microcomputer. After the call is connected, the caller and the phone can be connected hands-free. In addition, since the system is stored in an E2PROM chip with an IC bus interface, the number can be modified at any time.

1.1 Handheld end of the wireless emergency call system

The wireless emergency call system consists of two parts: wireless transmission circuit (Figure 1 (a)) and wireless receiving circuit (Figure 1 (b)). As shown in Figure 1(a), when the S2 button is pressed, a pulse signal is generated at the Imod end of the MC13175/13176, which is modulated by the MC13175/13176 and then transmitted to trigger the phone end of the wireless emergency rescue system to realize wireless emergency rescue. Its maximum output power can reach +10 dBm. When making an emergency call through the wireless emergency rescue system, the MCROPHONE in this circuit can be used to pick up and transmit the sound, and a wireless call can be realized with the phone end of the wireless emergency rescue system. The voice signal sent by the phone end is received by the receiving circuit RX3400 and converted into sound by the SPEADKER.



1.2 The landline end of the wireless emergency rescue system

The phone end of the wireless emergency rescue system consists of three parts: the wireless transmitting part, the wireless receiving part and the single-chip control part. When the antenna of the wireless receiving part receives the trigger signal sent by the handheld end of the wireless emergency rescue system, it is processed and sent to the single-chip microcomputer. The single-chip microcomputer calls out the phone number stored in it and sends it to the MT8880 dual-tone multi-frequency chip of the single-chip microcomputer control part. Through the telephone network system, it can talk with the user's phone or mobile phone. During this call, the wireless receiving part transmits the voice signal sent by the telephone network to realize wireless communication with the handheld end. The single-chip microcomputer is connected to 4 buttons, of which the first button is a function key for modifying the first number of the number stored in the single-chip microcomputer, the second button is a function key for modifying the second number of the number stored in the single-chip microcomputer, the third button is a function key for modifying the third number of the number stored in the single-chip microcomputer, and the fourth button is the reset button of the single-chip microcomputer to reset the single-chip microcomputer. 1.3 MT8880 receiving circuit

When MT8880 is used as a DTMF receiver, the DTMF signal is input via IN+ and IN-. The signal received by the on-chip operational amplifier of the MT8880 receiving part is amplified and then passes through the high-pass filter, low-pass filter and 350-440 Hz notch circuit, and enters the high-frequency group decoding circuit and the low-frequency group decoding circuit respectively. When the two decoding circuits detect a valid dual-tone pair, the EST pin outputs a high level, thereby starting the receiving delay circuit to charge the capacitor C2. When the voltage of the St/GT pin is charged to the threshold voltage VTST, the St/GT pin is also driven to a high level. At the same time, the bit code corresponding to the valid dual-tone pair is latched into the receiving flag position in the receiving data register and the Receive Data Register status register. The frequency of the DTMF signal is detected by digital counting and translated into a 4-bit binary code by the decoder. The 4-bit binary code is latched in the receiving data register. At this time, the delay control identification bit in the status register is reset, and the receiving data register full flag bit in the status register is set. For the outside, when the delay control identification bit in the register is reset, IRQ/CP changes from a high level to a low level.

1.4 MT8880 Transmitting Circuit

The transmitting part uses a switched capacitor D/A converter, and can select two modes: burst mode and continuous mode. In burst mode, as long as the audio output is enabled, the dual audio signal corresponding to the data register is sent from the TONE pin, lasting for 511 ms, and then paused for 511 ms. At this time, a data is sent, and the send flag position of the status register indicates that the next data can be sent. In this mode, only DTMF signals can be sent, but DTMF signals cannot be received.

2 System Software Design

The software adopts structured and modular design. Its main modules are: main program, number modification module, data receiving module, and data sending module. The main program of the software includes the main program and number modification, and its flow chart is shown in Figure 2 and Figure 3 respectively. The corresponding serial port sending program is used in the data receiving program design of MT8880 and the data receiving program of MT8880.


3 Conclusion

This paper makes a preliminary discussion on wireless emergency call technology through the application of single-chip microcomputer (AT89C51) and dual-tone multi-frequency (MT8880) technology, making full use of the advantages and application characteristics of single-chip microcomputer and dual-tone multi-frequency technology. AT89C51 and MT8880 integrate the sending and receiving functions of DTMF signals to complete the processing and transmission of data and voice signals. This design greatly simplifies the complexity of the design, reduces costs and reduces volume.