With the continuous development of intelligent sensing technology, intelligent sensing applications are gradually integrated into people’s life and work, such as our common electronic anti-theft, automatic energy-saving lighting, monitoring, alarm, doorbell system and other fields. In these fields, passive pyroelectric infrared detectors are the most widely used, but PIR signal processing is not simple and requires a lot of time and effort from developers. In order to reduce the production cost and difficulty of manufacturers, Hangzhou Jinghua Microelectronics independently developed a SD4101R chip with PWM output and high-precision ADC, which is used in PIR signal acquisition and processing.
1. Characteristics of Pyroelectric Infrared Sensor (PIR): Any object whose temperature exceeds absolute 0 °C can generate thermal radiation (infrared spectrum), while objects with a temperature lower than 1725 °C, the generated thermal radiation spectrum is mainly concentrated in infrared light. Therefore, all objects in nature can radiate infrared heat outward. Objects with different temperatures emit different wavelengths of infrared light, so the temperature is related to the length of infrared wavelengths. The human body has a constant temperature of about 37°C, and the wavelength of infrared rays emitted is about 10 μm. Pyroelectric sensors are usually covered with a special Fresnel filter, which is a device specially designed for infrared light waves of the human body. effect.
2. Working principle of pyroelectric infrared sensor (PIR): Pyroelectric infrared sensor generally includes two (or more) pyroelectric units connected in series or parallel to each other. Moreover, the polarization directions of the two electrodes are exactly opposite, and the environmental background radiation has almost the same effect on the two pyroelectric units, so that the electric discharge effects generated by them cancel each other out, so the environmental background radiation has no signal output to the detector. . When someone walks in the detection area, the human body radiation is focused by the Fresnel lens and received by the pyroelectric infrared sensor. The heat received by the pyroelectric unit is different and cannot cancel each other out, and the output pin will have a changed signal output for the post-stage circuit to do signal processing in order to achieve different control outputs. In addition, pyroelectric infrared sensors have different window shapes and sizes. The larger the window area, the higher the sensitivity and the higher the corresponding cost. Different models can be selected according to the requirements of the product. The working principle is shown in the figure below.
PIR working principle diagram
3. Fresnel lens: Fresnel lens (Fresnellens), also known as spiral lens, is mostly a thin sheet made of polyolefin material, and it is also made of glass. The surface of the lens is smooth on one side, and the other side is inscribed. From small to large concentric circles, its texture is designed according to light interference, interference radiation, relative sensitivity and receiving angle requirements.
The Fresnel lens has two functions: one is focusing, that is, the pyroelectric infrared signal is refracted (reflected) on the PIR, and the second is to divide the detection area into several bright areas and dark areas, so that the The moving object does not switch back and forth between the bright area and the dark area when it is active, and is refracted to the pyroelectric infrared sensor to form a temperature change, which generates a changing pyroelectric infrared signal on the PIR.
A Fresnel lens, simply put, has equidistant tooth patterns on one side of the lens. Through these tooth patterns, the light bandpass (reflection or refraction) of a specified spectral range can be achieved. Bandpass optical filters are expensive, and Fresnel lenses can greatly reduce costs. A typical example is PIR (Passive Infrared Detector). PIR is widely used in alarms. We can find that every PIR has a small lid. This is a Fresnel lens. The inside of the small cover is engraved with tooth patterns. This Fresnel lens can limit the wavelength of incident light to about 10μm (the wavelength of human infrared rays), and the cost is quite low.
A Fresnel lens can focus light passing through a narrow-band interference filter onto the photosensitive element of a silicon photodiode.
Precautions for use: The Fresnel lens is made of plexiglass and cannot be wiped with any organic solution (such as alcohol, etc.). When removing dust, it can be rinsed with distilled water or ordinary clean water, and then wiped with absorbent cotton.
If the selection of the Fresnel lens is improper, it will have a greater impact on the sensitivity, and the Fresnel lens has a focusing point. Only when the pyroelectric infrared sensor is located at the focusing point can the best focusing effect be achieved. , to maximize the sensitivity. The figure below shows the light condensing diagram of the Fresnel lens and the different dimensions.
Fresnel lens condensing diagram
Fresnel Lenses in Different Form Factors
Measurement circuit schematic
The PIR signal measurement circuit designed in this scheme has a simple peripheral circuit structure. , and then through the processing of the internal software algorithm of the microcontroller, the effective PIR signal can be analyzed. When a valid PIR signal is detected, the OUT end of the microcontroller will output a high-level signal for a certain period of time. Adjust the resistance value of R4 to control the high-level output duration (adjustable within the range of 1~600 seconds), and adjust the resistance value of R6 to change the PIR signal detection sensitivity (the common sensitivity is 65%~90%VDD), R2 and R8 ( The circuit composed of photoresistor) is used to judge day or night (CDS voltage is greater than 1.0V at night, less than 0.9V during daytime), SW terminal can be used to select different working modes (test mode, fixed resistance non-test mode, continuous adjustable mode) In order to be suitable for more applications, its working mode is shown in the table below.
Typical Application Circuit
Output delay time segment adjustment application circuit:
Typical Application Figure 1: Output Delay Time Segment Adjustment
Output delay time stepless adjustment application circuit:
Typical application Figure 2: stepless adjustment of output delay time