Design of Security System Based on Pyroelectric Infrared

introduction

With the increase of people's economic income, how to effectively protect property security has become a very important issue. According to statistics, the security measures of most families are still relatively weak. Even the property parks with property management are inevitably negligent and loopholes, giving criminals an opportunity to take advantage of them; in some small and medium-sized supermarkets and shops. Because of the cost, many of them do not have any preventive measures, and burglary has occurred. In order to protect people's property safety, it is necessary to have security equipment with low cost, easy installation and stable performance.

In this paper, we will introduce a pyroelectric infrared sensor as a detection source, which is sent to the CPU after signal conditioning, and emits an audible and visual alarm signal under abnormal conditions, and automatically blocks the exit security equipment. The system has four working modes: “Auto”, “Shopping Mall”, “Gallery” and “Family” for users to choose.

When someone enters the room, the sensor will detect the infrared signal radiated by the human body. After being amplified and shaped by the conditioning circuit, it will be sent to the CPU. The CPU will judge according to the user's set working mode and decide which circuits will be activated.

2 system design

2.1 Introduction to each module of the system

As shown in Figure 1, the system core control part is a single-chip microcomputer, and the periphery is composed of a signal detection part, a manual setting part (keyboard), a clock timing part, a display and an audible and visual alarm part, and an outlet blocking part.

Figure 1 system overall module diagram

2.1.1 Single Chip Microcomputer

As a kind of CPU which is often used in the control system, the single chip microcomputer has the characteristics of low price, simple programming and stable operation, and thus has a broad market space. This design uses AT89S52, which is responsible for collecting the signal from the detection unit and the time read from the clock chip. According to the working mode set by the user, the corresponding sound and light alarm circuit, the outlet blocking circuit, the counting circuit and the voice playing circuit are activated. At the same time, the output control of the display module is completed.

2.1.2 Signal Detection Section

(1) Core detector - pyroelectric infrared sensor According to scientists, any object whose temperature is higher than absolute zero (-273 °C) always radiates infrared rays to the outside, the size of the radiant energy and the temperature of the object and infrared The wavelength of the radiation is related and satisfies λ m × T = μm × K ≈ 3000. Detectors using the infrared principle generally operate in three bands of 2 to 2.6 μm, 3 to 5 μm, and 8 to 14 μm.

At present, infrared detection is increasingly used in communications, military, aerospace, medical and other scientific fields.

This design uses RE200B, working voltage 3-10V, mainly detects 8 ~ 14 μ m infrared signal, output 1mV, 0.1 ~ 10Hz (depending on the speed of human body movement) weak sine wave signal. The human body radiates infrared rays of about 10 μm due to the constant body temperature, which is just in the middle region of the detection band, so the sensitivity is high. The two internal thermoelectric elements with the same characteristics are connected in series or connected into a differential balance circuit, which can suppress the interference caused by the temperature change. The introduced N-channel junction field effect transistor is connected in a common leakage form to complete the impedance transformation and convert the charge signal into a voltage signal. as shown in picture 2. Adding a Fresnel lens in front of the sensor, when someone walks in front of the lens, the infrared rays emitted by the human body alternately enter the "high-sensitivity zone" from the "blind zone", so that the received infrared signal is pulsed with strong and weak Form input effectively increases the detection range.

Figure 2 RE200B internal structure and Fresnel lens diagram

(2) Bandpass filter

Since the useful signal output by the detector is concentrated in the range of 0.1 to 10 Hz, it is necessary to design a filter to intercept the pulse signal in the frequency range and filter out the out-of-band noise to reduce the false alarm rate of the system. In this design, an RC low-pass filter and a high-pass filter are used to form a first-order band-pass filter of 0.16 to 16 Hz. The circuit design is shown in Fig. 3. Among them, R10 and C3 form a low-pass filter, and C 1 and R 1 1 form a high-pass filter. According to the formula 12fpRC=, the upper and lower limits of the bandpass filter are fL=0.16Hz, fH=16Hz.

Figure 3 0.16 ~ 16Hz bandpass filter and differential amplifier circuit

(3) Differential amplification

Although the out-of-band noise can be suppressed by the above-mentioned band-pass filter, the pulse signal output by the sensor is only 1 mV, and in the useful signal (differential mode signal), the common mode signal interference which is much larger than the useful signal is inevitably mixed. The useful signal is submerged. If these signals are not further removed, they are directly amplified and sent to the MCU for judgment. When the differential mode signal is amplified, the common mode interference is also amplified synchronously, which cannot be distinguished. Therefore, it is necessary to first suppress the common mode signal and amplify the differential mode signal through the differential circuit to ensure correct detection of the subsequent circuit. The system consists of the inexpensive LM324, as shown by the dashed box in Figure 3. According to the superposition theorem, it is not difficult to obtain. When R 1= R 5 and R 2=R 6 , the output of the differential amplifier is:

In the above formula, Vo is only related to the signal difference of the input terminal and R2/R1. The amplification factor depends on the value of R2/R1. In this system, the first stage is amplified by 20 times.

(4) Secondary amplification

In the first stage, the common mode signal is mainly suppressed, so it is impossible to have a large amplification factor. In this system, the amplifier needs to be converted from 1 m V to 4 V, and needs to be amplified by 4000 times. In the first stage, it has been enlarged by 20 times, and this level needs to be enlarged by 200 times, which is determined by the value of R4/R3. The circuit is shown in Figure 4.

Figure 4 secondary amplification and impedance conversion circuit

Among them, the first stage op amp achieves 200 times amplification, and the second stage is voltage follower to realize impedance transformation.

(5) Threshold comparison

When the human body walks in front of the sensor, it will output a pulse signal with strong and weak changes according to the angle with the sensor. Therefore, it is necessary to set a threshold value, and when the signal is greater than the threshold value, a pulse signal is output. The system uses a double threshold circuit, as shown in Figure 5. The upper and lower thresholds are set to 3.75V and 1.25V, respectively. Two diodes provide a stable output.

Figure 5 threshold comparator

2.1.3 Clock and keyboard section

This design uses the D130AS low-power real-time clock chip DS1302 to provide the current time. It can time the year, month, day, week, hour, minute and second, and has several functions such as leap year compensation. Two power supplies can be used to realize that the power-down data is not lost. It is connected to the microcontroller using the SPI bus.

When the user selects the working mode, the system completes the real-time monitoring and alarm function with the help of the clock chip.

2.1.4 Display and Alarm Section

The display circuit has two functions: when applied to the "field" working mode, the total number of personnel in the venue is displayed; when working in other modes, the current time is displayed to determine whether the time of arrival is reached, and whether the burglar alarm device is activated. This design uses LED display digital visually eye-catching, and it communicates with the microcontroller through the driver 74LS245.

The alarm circuit also has two functions: when working in the "shopping" mode and during business hours, play "Hello, welcome!"; when working in "home" mode or other mode, the time reaches the user set time, A voice alarm is issued and a flashing signal is sent through a special LED to alert the criminal and remind the owner. This design is implemented using the ISD4004 voice chip and power amplifier circuit.

2.1.5 Export Blocking Section

After the above analysis, when an abnormal situation occurs, if only the sound and light alarm is issued, once the owner is not at home, the property cannot be guaranteed, and measures must be taken to prevent the criminals from escaping. An outlet blocking device is added to the design, which is composed of an optocoupler TLP521, a relay group and an electromagnet device. Once the MCU receives the abnormal signal, it sends a command to the relay group through the optocoupler, and the relay group turns on to turn on the electromagnet device to block each outlet.

2.2 Detailed description of various modes of the system

1) "Shopping mall" working mode. If during business hours, play “Hello, welcome”; if it is not within the business hours, activate the sound and light alarm and export blocking circuit to prevent theft.

2) "Venue" working mode. If it is during working hours, the number of people entering and leaving is counted and displayed. If the upper limit of the number of people is exceeded, an audible and visual alarm signal is issued; if it is not in working hours, an anti-theft alarm signal is issued and the exit is locked.

3) "Family" work mode. This mode is divided into manual and automatic modes. In the manual mode, when the user goes out, press the start button to start the anti-theft alarm mode. Before the stop button is pressed, anyone entering the alarm will start the anti-theft alarm; the automatic mode is determined by the MCU according to the time read from the time chip. , Start the anti-theft alarm in a certain period of time, stop the alarm in other time periods, and the time period can be set by itself.

4) "Automatic" mode is the same as the automatic mode in "Home" mode

3 system software process

3.1 System main process

As shown in Figure 6, after the system is initialized, when a human body signal is detected, the MCU starts the corresponding circuit according to the current working mode and the set time period; if there is no sensor signal input, it is in the waiting state.

Figure 6 system main flow chart

3.2 DS1302 Control Process

When the system is powered on, the user needs to set the initial time, otherwise the clock chip will not go. During the normal travel time, the user can modify the current time at any time, and the MCU can read the time data from the time chip at any time.

Figure 7 DS1302 program flow chart

4 Conclusion

1) This design uses 51 series single-chip microcomputer as CPU, which is low in price, simple in programming and stable in operation;

2) Using the pyroelectric infrared sensor RE200B instead of the ordinary photoelectric sensor, it is only sensitive to the human body, with high detection accuracy and low false alarm rate;

3) Utilize the keyboard and clock chip to realize human-computer interaction, which is flexible for users to choose from, and has a wide range of applications;

4) On the basis of the alarm, the design increases the function of blocking the exit under abnormal conditions, and more effectively protects the user's property safety;

5) The prototype test operation status is stable, and for specific users, the mass production can be considered after improvement.

references:

[1]. R10 datasheet http://+_1193166.html.
[2]. LM324 datasheet http://+_1054816.html.
[3]. DS1302 datasheet http://+_1055954.html.
[4]. 74LS245 datasheet http://+_1054125.html.
[5]. ISD4004 datasheet http://+_761303.html.
[6]. TLP521 datasheet http://

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