Design of Intelligent Wireless Burglar Alarm

Introduction
Traditional wired burglar alarms only sound an alarm locally when a burglary is detected. They have no internal controller and are easily damaged and ineffective. They are also inconvenient to install and expand. The wireless burglar alarm designed in this paper is controlled by a single-chip microcomputer. It is powerful and easy to expand into a multi-purpose smart home system.

1 System hardware circuit

1.1 Overall structure

The overall hardware structure of the intelligent alarm system is shown in Figure 1, which mainly includes a central controller, a transmitting and receiving module, a DTMF (dual-tone multi-frequency) module, a voice module, a telephone interface module, etc.

1.2 Central Controller

The AT89C51 microcontroller is selected, and the circuit is shown in Figure 2. P3.2 (INT0) is connected to the anti-theft detector to detect theft. If theft occurs, external interrupt 0 is triggered. P2.1 is connected to the voice circuit to realize voice playback control. P2.3 is connected to the telephone interface chip to realize analog off-hook and on-hook control. P1.0 outputs analog remote control. P1.4 is connected to the alarm buzzer. P0.0~P0.3 are respectively connected to D0~D3 of MT8888 and used as data bus. P2.0 is connected to RS0 of MT8888 to control the selection of MT8888 internal registers. P2.7 and CS of MT8888 control the selection of MT8888. P3.3 (INT1) is connected to pin 24 of the telephone interface chip to detect ringing. P3.6 and P3.7 are respectively connected to WR and RD of MT8888 to control the read and write operations of MT8888.

1.3 Detector and wireless transmitting and receiving circuits

The detector uses a wireless door sensor, which consists of a permanent magnet and a door sensor body (with a normally open reed switch inside). The wireless transmitting circuit is contained in the door sensor body, the receiving part is a super-regenerative module circuit, and the PT2262/PT2272 constitutes an encoding and decoding chip pair. When the permanent magnet leaves the reed switch at a certain distance, the detector immediately transmits a 315 MHz high-frequency radio signal containing an address code and its own identification code (data code). The receiving circuit determines whether it is from the same alarm system by identifying the address code of the radio signal, and then determines which detector is alarming based on its own identification code.

1.4 DTMF transceiver circuit

The alarm circuit shares a telephone line with the user's telephone. The MT8888 DTMF transceiver is selected and combined with the single-chip microcomputer and voice circuit to detect various telephone signal tones and perform automatic dialing; or decode the remote telephone key signal and transmit it to the single-chip microcomputer to achieve remote control. The circuit is shown in Figure 3.

1.5 Voice Circuit

ISD1420 is selected as the basic recording and playback circuit. All address lines are set to 0, and the starting address of playback is 0. When the S3 key is pressed, recording starts, and data is stored from address 0 until the memory is full or the key is released. When the S1 key is pressed, playback starts. The circuit is shown in Figure 4.

1.6 Telephone interface circuit

The telephone interface circuit is shown in Figure 5.

The DTMF transceiver circuit and the voice circuit both need to be connected to the outside world through the telephone interface circuit, and the PH8809 telephone interface chip is selected.

2 System Software

2.1 Main program flow

The system is controlled by a single-chip microcomputer. When there is a theft, the buzzer circuit is activated and a pre-set phone number is automatically dialed to alert the police; or it can accept remote control. The main process is shown in Figure 6.

2.2 Telephone alarm subroutine flow

The telephone alarm process is shown in Figure 7.

The detection subroutine is used to obtain the echo signal after picking up the phone and obtain a count value. The judgment subroutine determines whether the detected echo is a dial tone, a busy tone, or a ringback tone according to the standard of the program-controlled exchange. The dialing subroutine dials the pre-set phone number under the dialing conditions. If the other party is busy or no one answers after the ringing, it will delay for a period of time and wait for the next round of dialing. The playback subroutine plays back the pre-recorded alarm voice after the dialed phone is connected.

2.3 Remote Control Subroutine Flow

The remote control subroutine flow is shown in Figure 8.

If no theft occurs and the homeowner calls back, the system detects a ringing sound. After a set number of rings and no one answers, the system automatically picks up the phone and waits for the homeowner to enter a command through the telephone keypad. The command is transmitted to the host through DTMF and decoded to complete various remote control actions.

3 Conclusion

This intelligent anti-theft alarm system has complete functions, is easy to install and use. By connecting other sensors, such as fire, temperature, and smoke detectors, it can be used as a fire alarm system. In addition, by expanding the external memory, the number of probe interfaces of the host, the display module, the voice circuit, etc., the system can be made more intelligent and more powerful.