Design of intelligent telephone alarm system based on single chip microcomputer

1 Introduction

As people's living standards continue to improve, family safety is gradually attracting attention. Installing anti-theft and emergency alarm systems in family homes so that the owner can promptly grasp the alarm situation in the home is exactly what people expect. The intelligent telephone alarm system is based on a single-chip microcomputer and uses the public communication network as a transmission medium to monitor the safety of the home. It has the advantages of high reliability, low cost, and easy popularization.

2 System Functions

The main functions of the smart phone alarm system are as follows: When an alarm occurs indoors (such as someone illegally entering the room, gas leakage, fire, etc.), the smart phone alarm system can promptly obtain the alarm through various sensors, determine the type of alarm, and immediately and automatically dial the preset 1 to 9 groups of telephones in sequence. After dialing, first determine whether the other party has picked up the phone. If so, play the corresponding recorded voice alarm content according to different alarm situations. Otherwise, hang up and dial the next group of pre-set telephones until all 1 to 9 groups of telephones are connected.

3 Hardware Composition and Principle

This system mainly uses AT89C51 as the core. HT9170 and HT9200A are used as dual-tone multi-frequency (DTMF) signal receiver and generator.

ISD1420 is used as the recording and playback component. The system structure is shown in Figure 1.

3.1 AT89C51 Microcontroller

AT89C51 has 4 KB internal read-only memory. It uses CHMOS process technology and is compatible with MCS-51 products. It has a general 8-bit CPU and Flash RAM. It has a wide range of applications and good performance, and can be used to solve complex control problems. The I/O port of AT89C51 is used to monitor sensor signals in real time, remotely control the HT9200A dial, and control the voice playback of the ISD1420 voice device.

3.2 Dial-up system

The dialing system uses HT9170 and HT9200A as dual-tone multi-frequency (DTMF) signal receiver and generator respectively. HT9200A is a serial DTMF signal generator with good temperature adaptability. Its operating temperature range is -20℃～+70℃. It uses 8-pin DIP or SOP package. The pin arrangement is shown in Figure 2. The function of each pin is as follows:

CE: Chip select signal input terminal, low level is valid;

X1, X2: 3.579 545 MHz crystal oscillator access port;

VDD, VSS: positive and negative input terminals of the power supply, the normal operating voltage range is 2 V to 5.5 V, and the operating current is about 2 mA;

CLK: Synchronous signal input terminal of serial data, fCLK is about 100kHz;

DATA: serial data input terminal;

DTMF: DTMF signal output terminal. [page]

The HT9170 series is a dual-audio DTMF receiver that integrates digital decoder and multi-band filter functions. It can work in power-down mode and suppression mode. The HT9170 series devices all use digital calculation methods to identify and decode the 16-fold DTMF audio into a 4-bit code output. The high-precision conversion capacitor filter separates the audio DTMF signal into low-frequency and high-frequency signals. The built-in dial audio blocking circuit can eliminate the blocking circuit required for the pre-filter. The pin arrangement of the HT9170 is shown in Figure 3.

The schematic diagram of the dialing circuit is shown in Figure 4 and Figure 5.

3.3 Voice Circuit

ISD1420 is a product launched by ISD Corporation of the United States. The single-chip recording and playback time is 8 s to 20 s, and the sound quality is good. ISD1420 adopts CMOS technology and contains an oscillator, microphone preamplifier, automatic gain control, anti-aliasing filter, smoothing filter, speaker driver and EEPROM array. The smallest recording and playback system only requires a microphone, a speaker, two buttons, a power supply and a small amount of resistors and capacitors. After the recording and playback operation is completed, the device automatically enters a low-power saving mode with a power consumption of only 0.5μA.

The ISD1420 series has two playback controls: unique recording control and edge/level triggering. When not segmented, the peripheral circuit is the simplest, and it can also be segmented arbitrarily according to the minimum segment length, providing several operation modes, which greatly improves the flexibility of control. The ISD1420 adopts patented multi-level direct analog storage technology. Each sample is directly stored in a single EEPROM unit on the chip, so it can reproduce the voice, music, and tone effects very realistically and naturally, avoiding the quantization noise and "metallic sound" caused by quantization and compression of general solid recording circuits. The sampling frequency is 5.3kHz, 6.4 kHz to 8.0 kHz, which has only a slight impact on the sound quality. The information on the chip can be stored for 100 years (no backup power supply is required), and the EEP-ROM chip can record repeatedly 100,000 times. The voice circuit schematic is shown in Figure 6.

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4 System Software Design

In order to make the program clear and readable and run at a high speed, this system software adopts a modular structure design, which consists of a main program, an interrupt service program and related subroutines (such as a delay subroutine, a voice prompt subroutine, a sound playing subroutine, a dialing subroutine, etc.). The main program flow chart is shown in Figure 7. The dialing alarm subroutine flow chart is shown in Figure 8.

5 Conclusion

The system design uses a single chip microcomputer to realize the alarm function, and the control is flexible; using the telephone network as the transmission medium, it has the characteristics of fast alarm, accuracy and reliability. The system design has low cost, is safe and convenient for home use, and has a wide range of application prospects.