“With the continuous progress of the times, people have put forward higher requirements for the safety of their environment, especially in terms of home safety, they have to always pay attention to those uninvited guests. Now many communities have installed intelligent alarm systems, which greatly improves the safety of the communities and effectively guarantees the personal and property safety of residents. Because infrared light is invisible light, it has strong concealment and confidentiality, so it has been widely used in security devices such as anti-theft and alert.In addition, in the fields of Electronic anti-theft and human detection, passive pyroelectric infrared detectors are also widely used due to their low price and stable technical performance.

“

1. Overview

With the continuous progress of the times, people have put forward higher requirements for the safety of their environment, especially in terms of home safety, they have to always pay attention to those uninvited guests. Now many communities have installed intelligent alarm systems, which greatly improves the safety of the communities and effectively guarantees the personal and property safety of residents. Because infrared light is invisible light, it has strong concealment and confidentiality, so it has been widely used in security devices such as anti-theft and alert. In addition, in the fields of electronic anti-theft and human body detection, passive pyroelectric infrared detectors are also popular with users and professionals for their low price and stable technical performance.

The various anti-theft and security alarms currently used in China are basically based on technologies such as ultrasonic, active infrared emission/reception, and microwave. The passive infrared alarm designed here uses the American sensor element DD pyroelectric infrared sensor product. This pyroelectric infrared sensor product can detect the infrared radiation radiated by the human body in a non-contact form and convert it into a voltage signal. At the same time, it can also distinguish moving creatures and other non-living creatures. Pyroelectric infrared sensor products can be used not only in anti-theft alarm devices, but also in automatic control, proximity switches, telemetry and other fields. Compared with many anti-theft alarm devices currently on the market, the anti-theft alarm made with it has the following characteristics:

●No need to use infrared or electromagnetic wave and other emission sources.

●High sensitivity and large control range.

●Good concealment and mobile installation.

2. The principle characteristics of pyroelectric infrared sensor

Pyroelectric infrared sensors and thermocouples are both pyroelectric infrared sensors based on the principle of pyroelectric effect. The difference is that the thermoelectric coefficient of the pyroelectric infrared sensor is much higher than that of the thermocouple. The internal thermoelectric element is composed of lead mercury iron titanate ceramics with high thermoelectric coefficient and lithium tantalate, triglyceride sulfate and other filter lens windows. , Its polarization changes with temperature. In order to suppress the interference caused by its own temperature changes? In terms of technology, the sensor connects two thermoelectric elements with the same characteristics in reverse series or in a differential balance circuit mode, so that it can detect the change of infrared energy emitted by the object in a non-contact manner? And convert it into electrical signal output. The purpose of introducing the field effect tube into the structure of the pyroelectric infrared sensor is to complete the impedance transformation. Since the output of the thermoelectric element is a charge signal, it cannot be used directly? So need to use resistance to convert it into voltage form? The resistance of this resistor is as high as 104MΩ, so the N-channel junction FET should be connected in a common drain form? The source follower? To complete the impedance transformation. The pyroelectric infrared sensor is composed of three parts: sensing element, interference filter and field effect tube matcher. In the design, the high pyroelectric material should be made into a thin sheet with a certain thickness, and metal electrodes should be plated on both sides of it, and then the electricity should be applied to polarize it, thus making the pyroelectric detection element. Since the voltage applied for polarization has polarity, the polarized detector element also has positive and negative polarity.

Figure 1 is a schematic diagram of the structure of a dual detection element pyroelectric infrared sensor. When in use, the D terminal is connected to the positive pole of the power supply, the G terminal is connected to the negative pole of the power supply, and the S terminal is the signal output. The sensor connects two detection elements with opposite polarities and the same characteristics in series to eliminate interference caused by changes in the environment and itself. It uses the principle that two interference signals with opposite polarities and equal magnitudes cancel each other internally to compensate the sensor. For the infrared radiation radiated to the sensor, the pyroelectric sensor is focused by a Fresnel lens installed in front of the sensor and then added to the two detection elements, so that the sensor outputs a voltage signal.

The high pyroelectric material used to make the pyroelectric infrared detector is a broad-spectrum material, and its detection wavelength range is 0.2-20 μm. In order to have a higher sensitivity to infrared radiation in a certain wavelength range, the sensor is equipped with an interference filter on the window. In addition to allowing certain wavelength ranges of infrared radiation to pass through, this filter can also block light, sunlight, and other infrared radiation.

3. The structure principle of passive infrared alarm

3.1 Structure

Passive infrared alarm is mainly composed of optical system, pyroelectric infrared sensor, signal filtering and amplification, signal processing and alarm circuit. Its structural block diagram is shown as in Fig. 2. In the picture, the Fresnel lens can focus the infrared radiation radiated by the human body onto the pyroelectric infrared detection element, and at the same time, it also produces alternating high-sensitivity and blind areas of infrared radiation to adapt to the constantly changing characteristics of the pyroelectric detection element. ; Pyroelectric infrared sensor is the core device in the alarm design, it can convert the infrared signal of the human body into an electrical signal for use in the signal processing part; signal processing is mainly to amplify, filter and delay the weak electrical signal output by the sensor , Comparison, lay the foundation for the realization of the alarm function. Figure 3 shows a schematic diagram of the working principle when the target to be measured, Fresnel lens, and pyroelectric infrared sensor are used in combination.

3.2 Working principle

In this detection technology, the so-called “passive” means that the detector itself does not emit any form of energy, but only relies on receiving natural energy or energy changes to complete the detection purpose. The characteristic of the passive infrared alarm is that it can respond to the infrared radiation changes caused by the intruder moving in the protected area, and can make the monitoring alarm generate an alarm signal, thereby completing the alarm function. Figure 4 shows the working circuit schematic diagram of the alarm.

When the infrared radiation radiated by the human body is focused on the detection element of the pyroelectric infrared sensor through the Fresnel lens, the sensor in the circuit will output a voltage signal, and then make the signal pass through a circuit composed of C1, C2, R1, and R2. Band pass filter, the upper cut-off frequency of the filter is 16 Hz, and the lower cut-off frequency is 0.16 Hz. Because the detection signal voltage output by the pyroelectric infrared sensor is very weak (usually only about 1mV), and it is a changing signal, at the same time, the function of the Fresnel lens makes the output signal voltage in the form of pulse (the frequency of the pulse voltage is determined by The moving speed of the measured object is determined, usually about 0.1～10Hz), so the voltage signal output by the pyro-infrared sensor should be amplified. This design uses the integrated operational amplifier LM324 to carry out two-stage amplification to enable it to obtain sufficient gain.

When the sensor detects the infrared signal radiated by the human body and sends it to the window comparator after being amplified, if the signal amplitude exceeds the upper and lower limits of the window comparator, the system will output a high-level signal; when there is no abnormal condition, it will output a low-level signal. In this comparator, R9, R10, R11 are used as reference voltages, and two operational amplifiers are used for comparison. The main function of the two diodes is to make the output more stable. What is the upper and lower limit voltage of the window comparator? That is the reference voltage? They are 3.8V and 1.2V respectively. Will this high and low level change signal? Rising edge signal? As the trigger signal of the monostable circuit HEF4538B, and let it output a high-level signal with a pulse width of about 10s. Then use this pulse width signal as the input control signal of the alarm circuit KD9561 to make the circuit generate a 10s alarm signal. Finally, the electric signal is amplified by the transistor VT1 and VT2 again, so that there is enough current to drive the horn. The alarm sounds for 10s continuously.

4. Conclusion

The monitoring and alarm system designed with pyroelectric infrared sensors has the advantages of simple structure and low cost. After many tests, the system works stably.

The pyroelectric infrared alarm can only be installed indoors, and its false alarm rate is greatly related to the location and method of installation. The correct installation should meet the following conditions:

(1) The alarm should be 2.0 to 2.2 meters above the ground.

(2) The alarm should be kept away from air conditioners, refrigerators, stoves and other places where the air and temperature changes are sensitive.

(3) There shall be no screens, furniture, large bonsai or other partitions within the detection range of the alarm.

(4) The alarm should not be directly facing the window, otherwise the hot air turbulence outside the window and the movement of people will cause false alarms. If possible, it is best to close the curtains. In addition, do not install the alarm in a place with strong air currents.