Using public telephone network to build community emergency help system

Abstract: Introduce a residential community emergency help system that uses the public telephone network PSTN (Public Switch TelecommunicaTIon Network) as the transmission medium. The system takes the single chip microcomputer as the core and uses dual tone multi-frequency (DTMF, Dual Tone MulTIple Frequency) as the communication method. The system provides emergency help services for residents in the community, and has the advantages of high reliability, low cost, and easy popularity. The system schematic diagram, main hardware circuit and program block diagram are given in the article. Finally, the application prospect of the system is predicted.

introduction

With the continuous improvement of people's living standards, home security services are gradually attracting people's attention. Due to the work of children, many elderly people, especially sick patients, cannot be well taken care of. When a dangerous situation occurs, it cannot be dealt with in a timely manner, and there are great potential safety hazards. Therefore, a networked emergency help system is set up in the residential area so that the residential property management center can deal with the danger of the residents in a timely manner, which is what people expect. With the large-scale popularization of telephone equipment, it has become simple and easy to use the public telephone network for emergency assistance. Here, this article will introduce a social emergency help system based on the public telephone network. At this time, improving the living environment and realizing building automation will be of great help.

1 Working principle of the system

The system is based on a single-chip microcomputer and is composed of four parts: DTMF dialing circuit, remote control transmitting and receiving circuit, telephone number storage circuit and hands-free talking circuit. When a dangerous situation occurs, press the emergency button on the pager or the remote control. After detecting this signal, the single-chip microcomputer sends out the stored circuit number, and the intercom can be realized after being connected. In addition, since the phone number in the system is stored in an E2PROM chip with an I2C bus interface, the number can be modified at any time, which is extremely convenient. The system block diagram is shown in Figure 1.

figure 2

2 Main hardware circuit

2.1 Interface principle between DTMF dial chip and single chip microcomputer

Dual-tone multi-frequency signal (DTMF), usually referred to as dual-tone signal. It is a combination of two specific tone signals superimposed to represent a certain number or symbol. The frequencies of the two tones are different, and the numbers or functions they represent are also different. In a dual-tone multi-frequency telephone, there are 16 keys, including 10 numeric keys (0-9) and 6 function keys (*, #, A, B, C, D). Thus, according to the principle of combination, it must have 8 different monophonic signals. According to the CCITT (International Telegraph and Telephone Consultative Committee) proposal for frequency combination of dual tone dialing, 4 low frequency frequencies (697Hz, 770Hs, 852H2, 941Hz) and 4 high frequency frequencies (1209Hz, 1336Hz, 1477Hz, 1633Hz) are used internationally One frequency is randomly selected for combination, and there are 16 different combinations, representing 16 different numbers or symbols, as shown in Table 1.

Table 1 Dial code for dual tone multi-frequency

Low frequency group Number or symbol High frequency group 1209Hz 1336Hz 1477Hz
697Hz 1 2 3
770Hz 4 5 6
852Hz 7 8 9
941Hz * 0 ##

In this system, the dual-tone multi-frequency signal is generated by the DTMF signal generator special chip UM95087. The input terminals R1 ~ R4 (row) and C1 ~ C4 (column) can be connected to ordinary telephone keyboards or used for level input. The two input audio signals are linearly superimposed together, and the generated DTMF signal is output by pin 16 of UM95087. One audio signal is selected from the low frequency group by the input terminals R1 to R4, and the other audio signal is selected from the high frequency group by the input terminals C1 to C4. When the (Ri) input is low, select one of its low-frequency groups; when the (Ci) input is level, select one of its high-frequency groups. The output audio single tone amplitude is 400 ~ 500mV, the dual-frequency output harmonic distortion and audio distortion are less than 10%, the frequency error is ± 10%, the accuracy is less than 0.75%, and the high frequency is 2dB heavier than the low frequency. Pin 15 of UM95087 is used to prevent the generation of a single tone. When it is low, the output will not produce a single tone output; Pin 10 is the squelch output terminal; Pin 2 is the transmitter switch control terminal; Pin 7 is the on-chip oscillator Input end; 8 feet are the on-chip oscillator output end. In application, connect 3.57954MHz crystal between pin 7 and pin 8. At present, the single-chip microcomputer AT89C51 is used to control the DTMF generator UM95087 to realize channel coding, and its interface circuit is shown in comparison 2.

According to the combination principle of dual tone signal (DTMF) and the above interface circuit, the corresponding relationship between DTMF signal and binary coded signal can be obtained, as shown in Table 2.

Table 2 Correspondence between DTMF signal and binary code

Key number R1R2R3R4 C1C2C3C4 Hex Low frequency / Hz High frequency / Hz
1 0111 1000 78H 697 1209
2 0111 0100 74H 697 1336
3 0111 0010 72H 697 1477
4 1011 1000 B8H 770 1209
5 1011 0100 B4H 770 1336
6 1011 0010 B2H 770 1447
7 1101 1000 D8H 852 1209
8 1101 0100 D4H 852 1336
9 1101 0010 D1H 852 1477
0 1110 0100 E4H 971 1366

2.2 Hands-free talking circuit

The hands-free communication circuit requires clear and loud voice without interfering with each other due to receiving and sending calls. Therefore, the technical requirements for the hands-free talk circuit are high, the sensitivity of the sending amplifier is high; the output power of the receiving amplifier is large; it can effectively suppress the sidetone effect caused by the receiving call. This system uses a special hands-free calling chip MC34018 to form a hands-free calling circuit. MC34018 is a widely used, high-quality hands-free call integrated chip, which uses voice-controlled half-duplex hands-free calling, including a transmission amplifier, speaker power amplifier, attenuation control system, background noise monitoring system, transmission and reception attenuation Device. The attenuation control system is not only controlled by the corresponding sending and receiving levels, but also by the background noise level. The hands-free call circuit formed by MC34018 is shown in Figure 3.

3 System software design

The software design includes three parts: single-chip microcomputer help signal collection, circuit number reading and sending, and telephone number setting. Among them, the reading and setting of phone numbers is the focus of software design. Because the telephone number is stored in a piece of E2PROM with I2C interface, but the one-chip computer that this system adopts does not have I2C interface, so imitate the sequence of I2C bus through software. On the MCU, an analog I2C bus interface is extended to realize the communication between the MCU and the E2PROM chip.

After the reset and initialization of the MCU is completed, when there is a help signal input, the MCU takes out the stored phone number and sends it to the UM95087 chip to make the phone connected and complete the intercom function. The program flow block is shown in Figure 4.

Conclusion

In recent years, China has invested more and more in the construction of residential quarters. Taking 1999 as an example, the state's total investment in residential quarters was 170-180 billion yuan. According to the Ministry of Construction's division of real estate, medium and high grades in residential quarters, the district's intelligence is 1% to 3% of the total investment in the residential quarter. Calculated at this ratio, the demand for smart residential areas in 1999 alone amounted to RMB 1.7-5.4 billion. With the continuous deepening of the reform of the housing system and the continuous improvement of people's demand for the housing environment, this value will also increase significantly.

The community help system described in this article has strong practicability, low cost, and easy promotion. It has passed comprehensive experiments and has been put into production. The continuous promotion of the emergency help system in residential quarters will definitely have a positive impact on the construction and development of the intelligent building system and the improvement of people's living standards.

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