With the development of science and technology and the improvement of people’s living standards, the visual intercom system has been widely used in the residential buildings of new residential areas in modern cities. It can better improve the safety of the living environment of modern residents. But for those old communities, the original construction did not consider the setting of access control system, how to better protect the good environment and safety of the residents of these communities has become an urgent problem.

At present, most of the popular access control systems use 485 bus for data transmission, and a few manufacturers use industrial control bus (such as can) with strong anti-interference ability. The biggest problem of these bus modes is that they must be rewired, which brings a lot of trouble to the installation work, and the cost is relatively high. This design uses the existing power lines for data information transmission, and realizes a set of multi-functional home access control system with fingerprint identification, intelligent unlocking, visual intercom, voice message and intelligent alarm.

Brief introduction of 1st7537hs1 modem

The home automation modem ST7537HS1 is a kind of home special modem which is made of Analog CMOS technology and uses the carrier frequency of 123.45khz to transmit and receive data in half duplex mode. Its speed can reach 1200b / s. Because the amplitude shift key is sensitive to noise and spread spectrum, the circuit is complex and expensive, so st7537 uses frequency shift key to transmit. In addition to this integrated circuit, external devices only need wired drivers, transformers, and microcontrollers to prepare and interpret data information. Transmission data and rated internal information data enter FSK modulator asynchronously at 1200b / s. In the modulator, the data is transformed into two frequencies (133.05khz for “0” and 131.85khz for “1”), which are generated by a cheap crystal oscillator with a frequency of 11.0592Mhz.

In the transmission part, the modulated signal from FSK modulator is filtered by switched capacitor band-pass filter to limit the output spectrum and reduce the output level of harmonic devices. At the end of the transmission channel, an operational amplifier is needed to send a feedback signal from the power amplifier.

In the receiving part, after the incoming signal enters the rai input port, the signal is filtered by the switched capacitor band-pass filter for the first time. The filter is carrier centered and has a bandpass range of about 12 kHz. The output signal of the filter is amplified in a 20dB gain stage, and the amplified signal enters the mixer for frequency reduction and conversion. Finally, the if band-pass filter with a center frequency of 5.4khz improves the signal-to-noise ratio before the signal enters the FSK demodulator. The output port of the IF filter is connected to the inlet of the FSK demodulator through an external capacitor which can cancel the offset of the receiving channel. In addition, st7537 has two important functions: carrier detection and monitoring.

2 system overview

2.1 system structure

The visual intercom access control system is a network based on multi MCU communication. The structure diagram of the system is shown in Figure 1.

Using ST7537HS1 modem to realize the design of multifunctional household access control system

It can be seen from the system diagram that the system is mainly composed of door machine and indoor machine, and the communication between door machine and indoor machine is through power line. When the device is installed, the power line interface module only needs to be inserted into the power grid to be used, which is simple and convenient without rewiring.

2.2 working process of the system

The working process of the system is as follows: when the visitor presses the resident’s room number key through the outdoor unit at the entrance of the building, the controller will convert it into ID code and send it to the indoor unit. The indoor unit will enter the working state after receiving the same ID code, control the electronic doorbell to generate a ring, open the visual intercom at the same time, and timing the video signal and audio signal for 30s. If the host is not at home, start the analog pick-up and hang-up circuit after 30s and play a prompt tone to prompt visitors to leave a message; Turn off the video and audio signals if you can identify the person you are not meeting through the video; If you decide to meet someone, pick up the phone. If the indoor unit detects that it has been off hook, it will terminate for 30s and enter the call state regularly. The host’s voice is transmitted to the call control module of the door machine through the microphone of the indoor machine for processing and then sent to the loudspeaker; The voice of the visitor is transmitted to the call control module of the indoor unit through the microphone of the door machine, and then sent to the loudspeaker to complete the call. If the host receives a visitor, press the unlock key. After the indoor unit detects it, it outputs the unlock signal to the unlock module of the door machine. The electronic lock acts and the door is opened. When the host hangs up and the indoor unit detects the hang up signal, it outputs the video and audio signal to turn off and reset to the initial state.

In addition to the above basic functions, the system also has voice message, fingerprint unlocking, intelligent alarm and unmanned state setting functions.

3 system design and Implementation

3.1 hardware design

3.1.1 door machine design

The door machine is installed at the entrance of the building, as shown in Figure 2. It has the functions of fingerprint unlocking, calling button, intercom, video recording and so on. It takes AT89C51 single chip microcomputer as the core, and is composed of fingerprint identification, video acquisition and control, matrix button, power line interface, electronic unlocking and other modules. The system uses fixed-point DSP chip tms320c642 of TI company to complete the collection, preprocessing and identification of fingerprint information. Fps2000 is used as fingerprint sensor. The CCD (Chang coupled device) camera is used for video acquisition. The working voltage is 12V DC. It is powered by the system power supply and installed on the special protective door of the unit building door. Matrix key selection 3 × 4 determinant non coding keyboard, the keyboard scanning method is used to realize the selection control of 12 household keys in a unit, so as to control the closing and breaking of the corresponding relay, and realize the acquisition and transmission of video signal and audio signal. The user can unlock the lock by fingerprint, or by sending unlocking signal from indoor unit.

3.1.2 indoor unit design

The indoor unit is installed in the user’s room, with functions of intercom, image display, alarm, recording and playback, electronic unlocking, etc., as shown in Figure 3. It is mainly composed of video display module, electronic unlocking, voice channel, voice recording and playing, analog hanging and ringing circuit, alarm module and so on. 7 “color LCD is used for video display. ISD25120 is used for voice recording and playback to complete voice prompt and message information storage. When there is no one at home, the system can simulate picking up and hanging up and automatically play voice to prompt visitors to leave messages. When the system is hung up, the alarm will sound when the passenger stays at the door for a long time.

3.1.3 design of power line interface module

The power line interface module consists of line driver and line interface

(1) In transmission mode, the transmission signal (ATO) sent by st7537 is amplified and filtered;

(2) In the receiving mode, the receiving signal is provided to the receiving port of st7537 from the power line;

(3) There is a protection circuit to resist peak pulse and overload.

The block diagram of power line interface module is shown in Figure 4.

In Figure 4, the line driver amplifies the output signal (ATO) of st7537. In order to make the line driver suitable for power line, the line interface is used. In the online interface, the transformer is used

(1) Separate other circuits from power lines;

(2) Send the transmission signal to the power line;

(3) The received signal is extracted from the power line;

(4) Filter out the harmonic in the transmission signal.

The circuit diagram of power line interface module is shown in Figure 5.

The composite transistors Q6, Q4, Q3 and Q1 in the figure constitute a push-pull amplifier. Resistance R1 and R8 can make the amplifier get the best performance. When Rx / TX = 0 (receiving mode), st7537 output signals PABC = 1 and PABC = 0 make bipolar transistors Q5 and Q6 cut off, cut off the power supply of the power amplifier, and the power amplifier does not work.

The transformer consists of one main winding and two secondary windings. The winding ratio is 4:1:1, and the parameter is L1 winding 9.4 μ H; L2 winding 140 μ H; C4 = 7.2nf (6.8nf is the closest value in practical work). In order to prevent non-linear distortion, the linearity of C11 must be very good, C12 filters out the 50 / 60Hz signal from the power line, and has short-circuit protection function. When the phase is unknown, the additional capacitor C13 is added to C12 to form a discharge circuit to avoid the risk of electric shock.

In order to avoid the damage of peak signal to the circuit, a bidirectional voltage regulator is used. When the voltage value is equal to or greater than the voltage of the voltage regulator, the voltage regulator will be short circuited to the ground, so as to protect the components of the interface circuit from being burnt out.

The main power supply of the system is 220 V AC power supply. After the rectifier and voltage stabilizing circuit is composed of transformer, diode and other devices, 12 V / 2A DC power is output to supply power to the user’s extension and door machine. At the same time, it is also equipped with uninterruptible power supply, which can ensure the normal use of visual intercom in case of short-term power failure.

3.2 system software design

The system adopts the assembly language of single-chip microcomputer for program design, which is divided into indoor unit and door machine. It is composed of the processing and switching of different states of the system. It mainly realizes the functions of standby, signal acquisition, unlocking control, voice recording and playing, alarm and so on. Fingerprint identification is completed by special DSP chip. The program flow chart of door machine and indoor machine is shown in Figure 6.

The visual intercom access control system uses st7537 home automation modem to realize the transmission of signal in the power line. It does not need to lay additional communication lines and does not occupy the frequency band. It is a simple and effective method to realize the visual intercom access control under the existing conditions.

Editor in charge: GT

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