Design of Embedded Remote Controller Based on Telephone Network

1 Introduction

With the improvement of industrial automation level, remote control has become an increasingly used control method. Common remote control methods include Ethernet, GPRS, GSM short message, telephone network, etc. The telephone network is the most widely covered network, and the cost of telephone communication is low, which makes it possible to use the telephone network for remote control. This controller uses the cost-effective single-chip microcomputer AT89C51 as the central control CPU to control MT8870 to decode the dual-tone multi-frequency signal of the telephone key to achieve the control of the remote controlled object by operating the telephone key, and play the prompt voice through the recordable voice chip ISD1110. The design realizes a simple operation, easy installation, and low-cost embedded remote controller. The controller can detect the telephone ringing signal and automatically simulate off-hook. The operator can easily query the operating status of the controlled object according to the voice prompt, and the operating status of the controlled object can be changed by operating the telephone key.

2 System Function and Structure

The main problem solved by this system is how to use the telephone line to transmit control information. The telephone line transmits dual-tone multi-frequency signals (DTMF signals). Here, the DTMF signals transmitted by the telephone line are directly used to transmit control information to realize the control operation of the remote control object. The main function of the system is to decode the DTMF signal, decode the decoding result through the single-chip microcomputer, send out the corresponding control signal according to the decoding result, and drive the control circuit to perform the specified control operation. The system must also be able to identify the telephone ringing signal, and if the specified number of rings (5 times) is detected within the specified time, the phone is connected and the prompt voice is played. The controlled object is remotely controlled by operating the telephone buttons.

This system mainly sets up circuits such as ring detection, analog off-hook, analog on-hook, DTMF decoding, voice, relay drive, etc. The system structure block diagram is shown in Figure 1:

System working process:

The ring detection circuit is used to detect the ring signal. When a ring signal is detected, the ring is counted. If the number of rings is less than 5 times, the controller will not act. If the number of rings is greater than 5 times, it means that control is required. The microcontroller outputs a signal to the analog off-hook/on-hook circuit to simulate off-hook.

At this time, the controller is connected to the control center's phone. The controller is set with a password protection function to prevent misoperation. When the control center is connected to the controller by dialing a phone, the microcontroller outputs a signal to the voice circuit to play the password prompt voice. The controller can enter the password through the phone keys. The control system receives the DTMF signal transmitted by the phone line, and the MT8870 decodes the DTMF signal of the phone keys. If the password is correct, the operator can complete the status query or control action according to the voice prompt.

3. Hardware circuit composition

The system hardware mainly consists of ring detection circuit, analog off-hook and on-hook circuit, DTMF signal decoding circuit, voice circuit and output drive circuit.

3.1 Ring detection and analog off-hook circuit

The ring detection and simulated off-hook circuit is shown in Figure 2. The ring detection circuit is composed of optocoupler TLP521-1 and 74LS123. When there is an incoming call, the 25HZ, 90V AC ring signal transmitted on the telephone line is isolated by C1 and C2 and then rectified by the rectifier bridge. The rectified DC voltage value is relatively high. After passing through the optoelectronic isolator U1, a TTL pulse signal is output. The pulse is rectified into a large square wave signal by 74LS123. The square wave signal is sent to the P3.5 pin of the microcontroller for counting. When the count value reaches the preset value, the P1.0 pin of the microcontroller outputs a high level, and the transistor Q1 is turned on, and the relay K1 is activated, and the load resistor R5 (330Ω) is connected to the circuit to realize simulated off-hook. The simulated off-hook here means that after R5 is connected to the circuit, a current greater than 10mA will appear on the telephone line. After the exchange center detects this current, it will no longer output a ring signal but will switch to connecting the phone. The working process when people manually pick up the phone to connect the phone is the same, so it is called simulated off-hook. If the ringing signal disappears before reaching the preset value, the count value of the microcontroller is reset to zero and the controller does not operate.

Figure 2 Ring detection and analog off-hook circuit

3.2 DTMF signal decoding circuit

DTMF (Dural Tone Multiple Frequency) Table 1 Telephone key DTMF frequency correspondence table

Mainly used in telephone exchange systems, it is a composite signal formed by the superposition of two audio signals of different frequencies. These audio signals do not have any harmonic relationship and are divided into a high-pitched group and a low-pitched group. Each button on the telephone corresponds to a group of DTMF signals. The corresponding relationship is shown in Table 1.

This system uses MT8870 as the decoding chip for DTMF signals. The structure of MT8870 is shown in Figure 3:

MT8870 is a highly integrated and widely used communication integrated circuit chip produced by Canada Mitel. MT8870 can be easily interfaced with a single-chip microcomputer. Its main function is to complete the reception and recognition of dual-tone multi-frequency (DTMF) signals. It can be used for wired

Figure 3 MT8870 internal structure

The terminal equipment of telephone network, wireless mobile communication network and computer communication network. The DTMF signal is input into MT8870 through IN-terminal, and the GS pin is connected to the feedback resistor to amplify the input DTMF signal. A 3.5795MH _Z crystal oscillator is connected between OSC1 and OSC2 pins to generate the comparison signal of each single tone in the DTMF signal double tone pair. The signal is filtered, amplified, high and low frequency separated in MT8870, and then converted into the binary corresponding to the DTMF signal through digital processing.

Figure 4 MT8870 and MCU interface

The DTMF signal is decoded into a 4-bit binary code and directly output by Q1~Q4. If you press the "1" button on the phone, there will be a high-frequency 1209 Hz _and a low-frequency 697 _Hz DTMF signal on the phone line.

Table 2 MT8870 decoding table

The signal enters MT8870 for decoding, and the decoding results are output by Q1, Q2, Q3, Q4. The corresponding relationship between the MT8870 output results and the telephone buttons is shown in Table 2. The STD pin of the chip provides DTMF signal detection output. When MT8870 receives the DTMF signal and completes the decoding, the pin is high level. Usually, the pin is low level. The signal can be interrupted to the microcontroller after inversion. The TOE pin is the output enable terminal. When TOE is high level, the decoding results can be output from Q1~Q4. When TOE is low level, the Q1~Q4 pins are in high impedance state. The interface circuit between MT8870 and the microcontroller is shown in Figure 4.

3.3 Voice Circuit

ISD1110 chip is used in voice control and voice recording circuit to make digital recording device. This voice chip is one of the ISD series single-chip voice recording integrated circuits of ISD company in the United States. It uses direct analog storage technology to store each sample value directly in the fast flash memory on the chip, which can better retain the effective components in the analog quantity and has good sound quality. The device is manufactured using CMOS technology and contains clock, microphone amplifier, automatic gain control, noise filter, smoothing filter and speaker amplifier on the chip. The smallest voice recording system consists of only a microphone, a speaker, several resistors, capacitors and buttons. It is currently widely used in voice recording design.

The pin arrangement of ISD1110 is shown in Figure 5. The functions of each pin are as follows:

A0~A7 address input/mode control;

VSSA, VSSD analog ground and digital ground;

SP+, SP- are the positive and negative terminals of the speaker output;

VCCA, VCCD analog and digital power positive terminals;

MIC microphone input terminal;

MIC REF microphone input reference terminal;

AGC automatic gain control;

ANA IN, ANA OUT analog input and output;

/REC recording low level is effective;

/PLAYL level triggers playback;

/PLAYE edge triggers playback;

XCLK external clock;

/RECLED Recording indicator.

The system program is written in C51 and uses high-level language to develop the single-chip microcomputer system, which has the advantages of short development cycle and strong software portability. The system program design is mainly divided into two parts. One part is that the system counts the ringing signal, and when the count value reaches the preset value, it outputs a control signal to simulate the call connection; the other part is to decode the telephone buttons and realize the corresponding operations.

The system program flow chart is shown in Figure 6

5 Conclusion

As a relatively new topic, embedded telephone remote control shows great advantages compared with conventional remote control methods. It does not require special wiring, does not occupy radio frequency resources, and can use the existing mature telephone network to achieve remote control across provinces and cities.

The embedded remote controller based on telephone network introduced in this paper has reliable operation, low manufacturing cost, unlimited application objects, and has wide practicality and promotion value.

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