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PIR

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Introduction of the scheme

  Passive pyroelectric infrared detectors are widely used in electronic anti-theft, human body detectors, and intelligent lighting. But PIR signal processing is not simple, it requires developers to invest a lot of time and effort. So Hangzhou SDIC MicroelectronicsCo., Ltd. launched an SD4101 IC with PWM output and high-precision ADC for PIR signal acquisition and processing to reduce the production cost and difficulty of manufacturers.


Application Characteristic

2.1.  WorkingPrinciple and Characteristics of PIR

   2.1.1. Characteristics of PIR

           Any object with a temperature above absolute 0℃ can generate thermal radiation (infraredspectrum). If an object with a temperature below 1725℃, the generated thermal radiation     spectrum ismainly concentrated in the infrared region. So all objects in nature canradiate infrared heat outward. Objects with different temperatures, thewavelengths of infrared light are different, so the temperature is related tothe length of the infrared wavelength. The constant temperature of the humanbody is about 37℃, and the wavelength of the infrared raysemitted is about 10 μm. The pyroelectric sensor is usually covered with aspecial Fresnel filter. Fresnel filter is a device designed specifically forthe infrared light wave of the human body and is only very sensitive toinfrared light waves with a wavelength of about 10 μm, while it has asignificant inhibitory effect on infrared components at other wavelengths.

      2.1.2. Working Principle of PIR

            Pyroelectric infrared sensors generally include twoor more pyroelectric units connected in series or parallel to each other andthe two polarized directions are exactly opposite. The  radiation in thesurrounding environment has almost the same effect on the two pyroelectricunits, so that the generated pyroelectric effect cancels each other, so theradiation in the surrounding environment has no signal output to the detector.When someone moves in the detection area, the radiation of the human body isfocused through the Fresnel lens and received by the pyroelectric infraredsensor. Pyroelectric infrared sensors lose their charge balance when theyreceive changes in human body infrared radiation temperature. It is because theheat received by the two pyroelectric units is different and cannot cancel eachother, there will be a varying signal output at the output pin, which is usedfor signal processing by the subsequent stage circuit in order to achievedifferent control outputs. Pyroelectric infrared sensors with different shapesand sizes. The area larger, the sensitivity is higher and the correspondingcost is higher. Different models can be selected according to the requirementsof the customers. The principle and different types of pyroelectric infraredsensors are shown below:

Figure 2.3SpotlightDiagram of Fresnel Lens

Figure 2.3SpotlightDiagram of Fresnel Lens

2.2. Fresnel Lens

       Fresnel lens, also known as spiral lens, is mostlya thin sheet made of polyolefin material and another type is made of glass. Oneside of the lens surface is smooth, and the other side is engraved withconcentric circles from small to large. The texture of the circles are designedaccording to light interference and relative sensitivity and reception anglerequirements.

       The Fresnel lens has two functions: The first one is the focusingeffect, which is to refract (reflect) the pyroelectric infrared signal on thePIR. The secondfunction is to divide the detection area into several bright and dark areas, sothe moving objects that enter the detection area can switch between theseareas, and refract the pyroelectric infrared sensor to form a temperaturevariety and produce a pyroelectric infrared signal that changes on the PIR.

       The Fresnel lens has equidistant toothpatterns on one side. Through these tooth patterns, the effect of lightbandpass (reflection or refraction) on the specified spectral range. Thetraditional band-pass optical filter for polishing optical equipment is veryexpensive, but the Fresnel lens can the cost a lot. A typical example is PIR,which is widely used in alarms. We can find a small lid on each PIR. This isthe Fresnel lens. The inside of the small cover is engraved with toothpatterns. This type of Fresnel lens can limit the wavelength of the incident lightto about 10 μm (the wavelength of the human body's infrared), and the cost isquite low.

       Fresnel lens can focus the light passingthrough the narrow-band interference filter on the photosensitive element ofthe silicon photoelectric secondary detector.

       Precautions: Fresnel lens is made ofplexiglass and cannot be wiped with any organic solution (eg.  alcohol, etc.). You can rinse with distilledwater or ordinary clean water before wiping with cotton wool to clean theFresnel lens.

       If the Fresnel lensis not selected properly, it will affect the sensitivity. The Fresnel lens hasa focus point. Onlywhen the pyroelectric infrared sensor is at the focus point can have the bestfocus effect and the highest sensitivity. The following figure is the Fresnellens spotlight diagram and different dimensions:

Figure 2.3SpotlightDiagram of Fresnel Lens

Figure 2.4Fresnel Lensesof different dimensions

3. Characteristics of SD4101 Signal Processing IC

  3.1. ADC Characteristics

         lHigh precision ADC, 20.6bitsENOB at 250sps. It can be set to 2 differential or 4 single-ended inputs (to VSS).

  3.2. Power Consumption

         8-bit RISC low power MCU. The typical MCU current consumptionis: 300uAat 3Vand 2MHz operating clock rate, 2uA at standby and32kHz clock, and 1uA at sleep

  3.3. Memory

         4k Bytes OTP, 256 Bytes SRAM

  3.4. Clock System

         Internal 4MHz and 32kHz RC oscillator

   3.5. Operating Voltage and Temperature Characteristics

         Operating voltage range:2.4V~ 3.6V

         Operating temperature range:-40℃~85℃

   3.6. Other Features

         1 channel 8bits PWM output,WDT enhances reliability,AllI/O with Schmitt trigger input and pull-up resistor,Powerfailure detection circuit and power-on reset circuit, 1external interrupt I /O.


Application block diagram

Figure 4.1Typical Application I: Delay Time Segment Adjustment Output

Figure 4.2Typical Application I: Delay Time Stepless Adjustment Output


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