The effect of the Fresnel lens on the human body, first of all, let's first understand the working principle of the pyroelectric infrared sensor. The pyroelectric infrared sensor is composed of porcelain metal oxide or piezoelectric crystal components. When the temperature of the controller changes, the pyroelectric effect will cause a positive charge on two electrical levels. When the body enters the detection area, because of the difference between the human body temperature and the working temperature, the pyroelectric effect will cause a positive charge on the two electrical levels, that is, the weak working voltage difference between the two electrical levels. This output has a field effect tube in the sensor for impedance conversion. If the human body does not move after entering the detection area, the temperature does not change and the sensor does not output.

This kind of weak voltage difference requires two products to amplify and process the signal. First, the infrared operational amplifier processing circuit. Second, the Fresnel lens. Here, let's talk about the Fresnel lens first.

The simplest understanding of the role of Fresnel lens is: 1. Fresnel lens has the function of filtering interference light. 2. Fresnel has the function of amplifying infrared signals.

When we do not use the Fresnel lens, the detection radius of the pyroelectric infrared sensor at room temperature is less than 2 meters, and it is very unstable and easily interfered by external light. Only when used with a Fresnel lens can it be used to its maximum effect. Use Fresnel lens to detect the half length of the sensor up to 20 meters.

Fresnel spectacle lenses are made of high-density polyethylene (HDPE), generally used for transparent color, starry sky or black, but the fully transparent light wavelength is 7-14μm infrared sensor, only 7-14μm infrared Within the scope. The lens is divided into many parts in the horizontal direction, and each part is divided into many different parts in the vertical direction. Each equal part of the uppermost part is a lens unit, which is composed of concentric circles, and the centers of the concentric circles are in the lens unit. Each equal part of the middle and lower halves is also a separate lens unit, which is also composed of concentric circles, but the center of the concentric circles is not in the lens unit. Multiple lens units are combined into a lens group unit. Theoretically, the more lens units and the denser the sensitivity, the higher the cost of the mold.

When light passes through these lens units, a visible area and a blind area of light and dark are formed. Since each lens unit has only a small angle of view, the inside of the angle of view is the visible area, and the outside of the angle of view is the blind area. Any two adjacent lens units are separated by a blind zone and a visible zone, and they are intermittent and do not overlap or cross. Therefore, when the camera lens is placed in the right part directly in front of the sensor, the body will perform fitness exercises directly in front of the camera lens, and the body will produce a continuously changing black shadow area (shiny area) and light on the sensor according to the infrared radiation caused by the camera lens. Area, change the sensor's ambient temperature, and output electronic signals.

It can also be understood that when the body moves in the detection area, once it leaves the field of view of one camera lens module, it immediately enters the field of view of another camera lens module (because the adjacent camera lens module is very close), a blind spot appears on the controller And the visible area from this causes the temperature transition of the controller and the output of the electronic signal.