Realization of Intelligent Remote Control System Based on Telephone Network

0 Introduction
The intelligent remote control system based on the telephone network allows users to remotely control electrical equipment using fixed telephones or mobile phones at a remote end. The system can make full use of the existing public telephone network and mobile communication network, does not require special wiring, does not occupy radio frequency resources, can save investment and facilitate promotion and use. At the same time, since the telephone lines are connected to the network in various places, remote control can be easily achieved. In addition, the telephone is a duplex communication, and the operator can instantly understand the relevant information of the controlled object through various prompt sounds, so as to perform relevant operations and realize the interactivity and intelligence of the product.
This system adopts single-chip microcomputer intelligent control and is connected in parallel to both ends of the telephone, which will not affect the normal use of the telephone. When remote control is required, the user dials the telephone number connected to this device. When the telephone ringing recognition circuit detects that no one answers the call after five rings (the number can be changed by software), the controller automatically picks up the phone and enters the password under the prompt of the voice circuit. If the password is incorrect, the system automatically hangs up the phone. When the password is correct, just press the voice prompt to select the controlled electrical appliance for remote control. After completing the operation, enter the "#" key to automatically hang up the phone.

1 Overall system design
The system consists of a single chip as the main control part, which performs the main information processing, receives external operation instructions to form various control signals, and completes the recording of various information. This system includes ring recognition circuit, off-hook circuit, DTMY dual audio decoding circuit, voice prompt circuit and electrical control circuit, etc. The system principle block diagram is shown in Figure 1.

Various signal lines and control lines are connected to the main control chip AT89S51, which will output corresponding control signals according to different input signals. The system uses CM8870 signals to decode the dual audio signals of the telephone, selects the voice recognition chip RSC4128 for password detection and voice prompts, and uses thyristor circuits to realize control operations on the control objects, ensuring reliable and stable operation of the system.

2 Implementation of each unit circuit of the system
2.1 Ring recognition circuit
The function of the ring recognition circuit is to detect the ring current signal on the telephone line so as to provide the microcontroller with the number of times the phone rings. The ring recognition circuit is shown in Figure 2.

Since capacitor C1 cannot pass DC voltage, no current flows through the circuit in standby mode. When there is a ring, the ring current voltage turns on the phototransistor inside G1, and the voltage at point P3.5 (connected to the P3.5 port of the microcontroller) drops to 0 V; when there is no ring current signal, the voltage at point P3.5 is high level VCC. It can be seen that the pulse at point P3.5 appears with the appearance of the ring current signal. Therefore, as long as a low-level pulse is detected at point P3.5, it means that there is a ring current signal on the line, and the number of pulses at point P3.5 per unit time represents the length of the ring time. By accumulating the number of pulses at point P3.5, the length of the ring time and the number of rings can be determined.
2.2 Analog off-hook and on-hook circuits
The off-hook and on-hook circuits are actually electronic switches that control the connection between the circuit board and the telephone line. Normally, this switch should be in the off state to avoid busy telephone lines; when remote control is required, if the ringing rings five times and no one answers, the circuit board and telephone line need to be connected, that is, the off-hook action is completed. The analog off-hook and on-hook circuit is shown in Figure 3. V1 is an electronic switch, and the conduction of the switch is controlled by the P1.4 port of the microcontroller.

If the off-hook and on-hook circuit is designed with relays, the circuit is simpler, but it is found in practice that the power consumption is large. The 5 V relay pull-in current is as high as 30 μA, which is nearly 3 times the static current of AT89S51. The volume and weight are also relatively large. In addition, the relay is also prone to spark interference. The use of transistor off-hook and on-hook circuits overcomes these problems.
2.3 DTMF decoding circuit
Dual-tone multi-frequency signal is a combination signal composed of a high-frequency signal and a low-frequency signal. Dual-tone multi-frequency signal decoding is the main component of the control system. Correctly decoding the dual-tone multi-frequency signal sent by the switch through the telephone line is the key to the entire system. Its working condition directly determines the reliability of the system. This system uses the dual-audio decoding integrated chip CM8870 to complete this function. CM8870 integrates a frequency band separation filter and a digital decoder, which can convert the received DTMF signal into 8421 code.
The dual-audio decoding circuit is shown in Figure 4. The dual-audio signal input point is connected to the collector of the transistor V1 in Figure 3. When V1 is turned on, the dual-audio signal sent from the telephone line enters CM8870. If the CM8870 receives a valid DTMF signal, it decodes the corresponding 8421 code and outputs it from the data output terminals Q1 to Q4. The data enters the microcontroller P1.0 to P1.3 ports to complete data collection, judgment and processing. In addition, the status signal from the 15th pin of the CM8870 enters the P1.5 port of the microcontroller to notify the microcontroller to read the data.

2.4 Voice password detection circuit design
This system uses specific person voice recognition technology to achieve reliable password detection function. The voice signal processing chip RSC4128 is a highly integrated language and analog input/output composite signal processor with speaker confirmation function. After the speaker trains a special word or phrase on the chip, the chip can identify whether a specific word is uttered by the original speaker. RSC4128 can store 10 speaker confirmation (SV) templates in the chip, or use external programmable memory to store more than 10. As the user password of the user's remote control answering machine, the voice password has strong security, and is simpler than the general dial password, has strong confidentiality and high reliability. If there are multiple owners at home, each person's voice can be recorded as a training template, and only speakers who meet the requirements can control the system. The voice recognition system is shown in Figure 5.

The voice recognition module is connected to the host through three wires: SHS, DATA, and MHS, using a three-wire synchronous serial communication method. For the voice recognition module, SHS is the output, MHS is the input, and DATA is the bidirectional input/output line. In addition, RSC4128 also has a voice recording and playback function. In this mode, the system can record and play a voice on the spot and play back the recorded sound.
2.5 Electrical control circuit
Since the bidirectional thyristor can realize the function of low-power DC controlling high-power AC, this system uses it to control the switch of household appliances. The electrical control circuit is shown in Figure 6.

3 Software Design
The software design of this system adopts modular design. The main unit function modules include ring detection counting, control of on-hook and off-hook, password verification, dual audio signal analysis and processing, control of electrical appliances, signal tone prompts, etc. The main program flow chart is shown in Figure 7.

4 Conclusion
The intelligent remote control system based on the telephone network has the advantages of not occupying radio resources, perfect network functions and wide coverage, low investment, etc., which can fully meet the actual life and production needs. Tests have proved that the circuit of this system is simple and practical, the control is reliable, and it does not affect the normal use of the telephone. Based on this system, it is very convenient to expand functions, such as message circuits, and it can also be used in the field of production automation.