The working principle of pyroelectric infrared sensor is introduced, and the structure principle and application circuit of a passive pyroelectric infrared alarm are given. The circuit applies the concealment of infrared rays to the alarm system, so as to realize the function of anti-theft alarm and achieve the purpose of safety protection.
Keywords: passive infrared alarm; Pyroelectric infrared sensor; Double precision monostable multi-frequency oscillator
1 overview
With the continuous progress of the times, people put forward higher requirements for the safety of their own environment, especially in home safety, and have to pay attention to those uninvited guests at all times. Nowadays, many residential quarters have installed intelligent alarm systems, which greatly improve the security of residential quarters and effectively ensure the personal and property safety of residents. Because infrared ray is invisible light, it has strong concealment and confidentiality, so it is widely used in security equipment such as anti-theft and vigilance. In addition, passive pyroelectric infrared detectors are also welcomed by users and professionals in the fields of electronic anti-theft and human detection because of their low price and stable technical performance.
At present, all kinds of anti-theft and safety alarms used in China are basically based on ultrasonic wave, active infrared emission/reception and microwave technology. The passive infrared alarm designed here adopts the sensitive element of the United States-pyroelectric infrared sensor. This pyroelectric infrared sensor can detect infrared rays radiated by human body in a non-contact way and convert them into voltage signals. At the same time, it can also distinguish between moving creatures and other abiotic creatures. Pyroelectric infrared sensor can be used in both anti-theft alarm devices, automatic control, proximity switch, telemetry and other fields. Compared with many anti-theft alarm materials sold in the market at present, the anti-theft alarm made of it has the following characteristics:
● No emission sources such as infrared rays or electromagnetic waves are needed.
High sensitivity and wide control range.
Low concealment, and can be installed in flowing water.
Principle and characteristics of pyroelectric infrared sensor
Pyroelectric infrared sensor and thermocouple are both pyroelectric infrared sensors based on the principle of pyroelectric effect. The difference is that the pyroelectric coefficient of pyroelectric infrared sensor is much higher than that of thermocouple. The pyroelectric element in pyroelectric infrared sensor is composed of lead-mercury ferrite titanate ceramics, lithium tantalate, triethyl sulfate and other matched filter windows with high heat release coefficient, and its polarization changes with temperature. In order to suppress the interference caused by its own temperature change, the sensor connects two thermoelectric elements with the same characteristics in reverse series or in a differential balance circuit, so it can detect the infrared energy change emitted by the object in a non-contact way and convert it into an electrical signal for output. The purpose of introducing field effect transistor into pyroelectric infrared sensor structure is to complete impedance transformation. Because the output of thermoelectric element is a charge signal, it can't be used directly, and it needs to be converted into voltage form through resistance. The resistance impedance is as high as 104 mω, so the introduced N-channel junction field effect transistor should be connected in the form of * * drain, that is, the source follower, to complete the impedance transformation. The pyroelectric infrared sensor consists of a sensor detector, an interference filter and a field effect tube matcher. In the design, the pyroelectric material should be made into a thin sheet with a certain thickness, and then the pyroelectric detector can be made by plating metal electrodes on both sides, and then electrifying polarization. Because the polarization voltage is polar, the polarization detector also has positive and negative polarities.
Structure of pyroelectric infrared sensor with double detectors. 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 detectors with opposite polarity and consistent characteristics in series to eliminate the interference caused by environment and self-change. It uses the principle that two interference signals with opposite polarity and equal size cancel each other internally to compensate the sensor. For the infrared radiation radiated to the sensor, the pyroelectric sensor focuses it through the Fresnel lens installed in front of the sensor and adds it to two detection elements, so that the sensor outputs voltage signals.
The pyroelectric material for making pyroelectric infrared detector is a broad-spectrum material, and its detection wavelength range is 0.2 ~ 20μ m. In order to have high sensitivity to infrared radiation in a certain wavelength range, the sensor is equipped with an interference filter on the window. This kind of filter can not only let infrared radiation in a certain wavelength range pass through, but also block infrared radiation such as light and sunlight.
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. Fresnel lens can focus the infrared radiation from human body on pyroelectric infrared detector, and at the same time, it also produces alternating infrared radiation high-sensitive areas and blind areas to adapt to the characteristics that pyroelectric detector requires constantly changing signals. Pyroelectric infrared sensor is the core device in the design of alarm, which can convert infrared signals of human body into electrical signals for signal processing. Signal processing is mainly to amplify, filter, delay and compare the weak electrical signal output by the sensor, which lays the foundation for the realization of alarm function.
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 receives natural energy or energy changes to complete the detection purpose. The characteristic of passive infrared alarm is that it can respond to the change of infrared radiation caused by the activities of intruders in the protected area, so that the monitoring alarm can generate alarm signals, thus completing the alarm function.
When the sensor detects the infrared signal radiated by human body, it is amplified and sent to the window comparator. 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 abnormality, a low level signal is output. In this comparator, R9, R 10 and R 1 1 are used as reference voltages, and two operational amplifiers are used for comparison. The main function of these two diodes is to make the output more stable. The upper limit voltage of the window comparator is 3.8V, and the lower limit voltage is1.2v.. The rising edge signal of the high-low level change signal is used as the trigger signal of the monostable circuit HEF4538B, allowing the output of a high-level signal with a pulse width of about 10s. This pulse width signal is used as the input control signal of the alarm circuit KD956 1, which makes the circuit generate the alarm signal of 10s. Finally, transistors VT 1 and VT2 are used to amplify the electrical signal again, so that it has enough current to drive the horn to continuously give out the alarm sound of 10s.
4 conclusion
The monitoring and alarm system designed with pyroelectric infrared sensor has the advantages of simple structure and low cost. After many tests, the system works stably.
Pyroelectric infrared alarm can only be installed indoors, and its false alarm rate has a great relationship with the location and mode of installation. Proper installation shall meet the following conditions:
(1) The alarm shall be 2.0 ~ 2.2m above the ground.
(2) The alarm should be far away from air conditioners, refrigerators, stoves and other places where 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) Don't aim the alarm directly at the window, otherwise the hot air disturbance and pedestrians outside the window will cause false alarm. If possible, you'd better close the curtains. In addition, the alarm should not be installed in a place with strong airflow activity.
Pyroelectric infrared sensor alarm system scheme-technical article pyroelectric infrared sensor alarm system scheme
At present, the pyroelectric infrared human body sensors on the market mainly include SD02 and PH5324 produced in Shanghai, LH 1954 and LH 1958 produced in Germany, P2288 produced by Hamamatsu Company in the United States, SCA02- 1 and RS02D produced by Japan Ceramics Company, etc. Although their models are different, their structure, appearance and electrical parameters are roughly the same, and most of them can be used interchangeably.