Abstract This paper introduces the working principle of pyroelectric infrared sensors and presents the structural principle and application circuit of a passive pyroelectric infrared alarm. This circuit effectively utilizes the concealment of infrared light in the alarm system, thereby realizing the anti-theft alarm function and achieving the purpose of security protection. Keywords: Passive infrared alarm; Pyroelectric infrared (PIR) sensor; Double-precision monostable multi-frequency oscillator [b]1 Overview[/b] With the continuous progress of the times, people have put forward higher requirements for the security of their environment, especially in terms of home security, and must always be wary of uninvited guests. Now many communities have installed intelligent alarm systems, which greatly improves the security level of the community and effectively protects the personal and property safety of residents. Since infrared light is invisible and has strong concealment and confidentiality, it has been widely used in anti-theft, alarm and other security devices. In addition, in the fields of electronic anti-theft and human body detection, passive pyroelectric infrared detectors are also welcomed by users and professionals for their low price and stable technical performance. At present, various anti-theft and security alarms used in China are basically based on ultrasonic, active infrared transmitting/receiving and microwave technologies. The passive infrared alarm designed here uses a pyroelectric infrared sensor, an American sensing element. This pyroelectric infrared sensor can detect infrared radiation emitted by the human body in a non-contact manner and convert it into a voltage signal. It can also distinguish between moving living beings and other inanimate objects. Pyroelectric infrared sensors can be used in burglar alarm devices, as well as in automatic control, proximity switches, telemetry, and other fields. Burglar alarms made with this sensor have the following characteristics compared to many burglar alarm devices currently on the market: ● No need for infrared or electromagnetic wave emission sources. ● High sensitivity and large control range. ● Good concealment and can be installed in various locations. [b]2. Principle and Characteristics of Pyroelectric Infrared Sensors[/b] Pyroelectric infrared sensors and thermocouples are both thermoelectric infrared sensors based on the thermoelectric effect. The difference is that the thermoelectric coefficient of pyroelectric infrared sensors is much higher than that of thermocouples. The internal thermoelectric element is composed of high thermoelectric coefficient lead-mercury iron titanate ceramics, lithium tantalate, ferric sulfate, etc., combined with a filter window. Its polarization changes with temperature. To suppress interference caused by its own temperature changes, the sensor uses two thermoelectric elements with identical characteristics connected in reverse series or in a differential balanced circuit. This allows for non-contact detection of changes in infrared energy emitted by an object and its conversion into an electrical signal output. The purpose of introducing a field-effect transistor (FET) into the pyroelectric infrared sensor is to perform impedance transformation. Since the thermoelectric element outputs a charge signal, which cannot be used directly, a resistor is needed to convert it into a voltage form. This resistor has an impedance as high as 10⁴ MΩ. Therefore, the introduced N-channel junction field-effect transistor should be connected in a common-drain configuration, i.e., a source follower, to complete the impedance transformation. The pyroelectric infrared sensor consists of three parts: a sensing element, an interference filter, and a field-effect transistor matching circuit. In the design, a thin sheet of high-thermoelectric material of a certain thickness is made, and metal electrodes are plated on both sides. Then, an electric current is applied to polarize it, thus creating the pyroelectric sensing element. Since the voltage applied to polarize the element is polarized, the polarized sensing element also has positive and negative polarities. A schematic diagram of a dual-sensor pyroelectric infrared sensor is shown. In operation, terminal D is connected to the positive power supply, terminal G to the negative power supply, and terminal S is the signal output. This sensor connects two detector elements with opposite polarities and identical characteristics in series to eliminate interference caused by environmental and internal variations. It compensates for this interference by utilizing the principle that two interference signals of opposite polarities and equal magnitudes cancel each other out internally. For infrared radiation reaching the sensor, the pyroelectric sensor focuses it through a Fresnel lens mounted in front of the sensor and applies it to the two detector elements, thus generating a voltage signal. The highly pyroelectric material used to manufacture the pyroelectric infrared detector elements is a broad-spectrum material with a detection wavelength range of 0.2–20 μm. To achieve high sensitivity to infrared radiation within a specific wavelength range, an interference filter is added to the sensor window. This filter allows infrared radiation within certain wavelength ranges to pass through while blocking light, sunlight, and other infrared radiation. [b]3. Structure and Principle of Passive Infrared Alarm Devices 3.1 Structure[/b] Passive infrared alarm devices mainly consist of an optical system, a pyroelectric infrared sensor, signal filtering and amplification, signal processing, and alarm circuits. The Fresnel lens focuses the infrared radiation emitted by the human body onto the pyroelectric infrared detector element, while also generating alternating high-sensitivity and blind zones of infrared radiation to adapt to the constantly changing signal requirements of the pyroelectric detector element. The pyroelectric infrared sensor is the core component in the alarm device design; it converts the infrared signal from the human body into an electrical signal for use by the signal processing section. Signal processing mainly amplifies, filters, delays, and compares the weak electrical signal output by the sensor, laying the foundation for the alarm function. A schematic diagram illustrating the working principle when the target, Fresnel lens, and pyroelectric infrared sensor are used in combination. [b]3.2 Working Principle[/b] In this detection technology, "passive" means that the detector itself does not emit any form of energy; it only relies on receiving energy or energy changes from nature to complete the detection purpose. Passive infrared alarms are characterized by their ability to respond to changes in infrared radiation caused by an intruder's movement within the protected area, triggering an alarm signal from the monitoring alarm system to complete the alarm function. When infrared radiation radiated by a human body is focused onto the detector element of a pyroelectric infrared sensor through a Fresnel lens, the sensor outputs a voltage signal. This signal is then passed through a bandpass filter composed of C1, C2, R1, and R2, with an upper cutoff frequency of 16 Hz and a lower cutoff frequency of 0.16 Hz. Since the detection signal voltage output by the pyroelectric infrared sensor is very weak (typically only about 1 mV) and is a changing signal, and the Fresnel lens causes the output signal voltage to be in pulse form (the frequency of the pulse voltage is determined by the speed of the measured object, typically around 0.1–10 Hz), the voltage signal output by the pyroelectric infrared sensor needs to be amplified. This design uses an integrated operational amplifier LM324 for two-stage amplification to achieve sufficient gain. When the sensor detects infrared radiation from the human body and amplifies it before sending it to the window comparator, the system outputs a high-level signal if the signal amplitude exceeds the upper and lower limits of the window comparator; otherwise, it outputs a low-level signal. In this comparator, R9, R10, and R11 are used as reference voltages, two operational amplifiers are used for comparison, and the two diodes mainly serve to stabilize the output. The upper and lower limit voltages of the window comparator, i.e., the reference voltages, are 3.8V and 1.2V, respectively. The rising edge of this high-low level signal is used as the trigger signal for the monostable multivibrator HEF4538B, which outputs a high-level signal with a pulse width of approximately 10s. This pulse width signal is then used as the input control signal for the alarm circuit KD9561 to generate a 10s alarm signal. Finally, transistors VT1 and VT2 amplify the electrical signal again to provide a sufficient current to drive the speaker to continuously emit a 10s alarm sound. [b]4 Conclusion[/b] The monitoring and alarm system designed with pyroelectric infrared sensors has the advantages of simple structure and low cost. After multiple tests, the system works stably. Pyroelectric infrared alarms can only be installed indoors, and their 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 far away from places where air and temperature changes are relatively sensitive, such as air conditioners, refrigerators, and stoves. (3) There should be no partitions, furniture, large potted plants, or other barriers within the detection range of the alarm. (4) The alarm should not be directly facing the window, otherwise the disturbance of hot air flow outside the window and the movement of people will cause false alarms. If possible, it is best to draw the curtains. In addition, the alarm should not be installed in places with strong airflow. Alarm System Solution for Pyroelectric Infrared Sensors - Technical Article Currently, the main pyroelectric infrared human body sensors on the market include SD02 and PH5324 from Shanghai, LH1954 and LH1958 from Germany, P2288 from HAMAMATSU (USA), and SCA02-1 and RS02D from NIPPON CERAMIC (Japan). Although their models differ, their structure, appearance, and electrical parameters are largely the same, and most can be used interchangeably. A pyroelectric infrared human body sensor (hereinafter referred to as: sensor) consists of three main parts: a sensitive element, an impedance transducer, and a filter window.