When the temperature changes when someone or a moving target passes by, the detector detects the temperature difference and outputs the alarm signal to the alarm host.

 

When no one or moving target passes by and the temperature changes little, the detector detects that the temperature difference is small, and does not output alarm signal to the alarm host.

 

So why would a sensor detect a temperature difference?

Let's go into more detail.

 

1. Pyroelectric sensor

 

In the far infrared detector, we need a kind of important parts called "pyroelectric sensor" which can detect human body indispensable. So what exactly is a "pyroelectric sensor"?

 

All objects in the world generate heat (far infrared). For example, the floor or glass, animals or automatic doors, their own heat.

 

A pyroelectric sensor is a component that detects an object's heat, or far-infrared radiation, and converts it into an electrical signal. This kind of temperature sensitive sensor is composed of ceramic oxide or piezoelectric crystal element. The two surfaces of the element are made of electrodes. When the temperature within the monitoring range of the sensor changes, the pyroelectric effect will generate charges on the two electrodes, that is, a weak voltage will be generated between the two electrodes. Pyroelectric sensor is mainly composed of shell, filter, pyroelectric element PZT, FET and so on. Wherein, the filter is arranged at the window, forming the window through which the infrared ray passes. The filter is a multilayer interference filter, which can filter the short wavelength of sunlight and fluorescent light (about 5mm below) very well. The pyroelectric element PZT converts the weak change of infrared signal with wavelength between 8mm~12mm into electrical signal, which is analyzed and processed by the detector chip and other circuits. Finally, the relay is pushed to work, and then the relay outputs the switch to the alarm host through the wire, thus realizing the alarm function.

 

Besides the pyroelectric sensor, Fresnel lens is the key element

 

2. Fresnel lens

 

FRESNEL lens is based on the French light physicist Fresnel invention. Fresnel lens is the infrared probe of the "glasses", it is like the human glasses, properly used or not directly affects the efficacy of use, improper use of misaction and missing action, causing users or developers to lose confidence in it. Properly used to give full play to the role of human body induction, so that its application field continues to expand.

 

Principle using electroplating mold process and PE (polyethylene) materials pressed. The surface of the lens (0.5mm thick) is burned in concentric circles from small to large and outwards from shallow to deep, which looks like a sawtooth from the section. Many ring lines and dense induction Angle, focal length; The deep feeling of the ring line burning should be far away and the focal length is close. The closer the infrared rays are to the concentric rings the more concentrated and intense they are. Several concentric rings in the same row form a vertical induction zone, and a horizontal induction zone is formed between concentric rings. The more vertical induction area, the greater the vertical induction Angle; The longer the lens the more sensing segments the greater the horizontal sensing Angle. If the number of sections is large, the motion amplitude of the human body will be small. If the number of sections is small, the motion amplitude of the human body will be large. The concentric circles in different zones cross each other to reduce the blind areas between sections. Blind areas are formed between regions, between segments and between segments, that is, the detection area is divided into a number of bright areas and dark areas, so that the moving objects entering the detection area can produce changes in the form of temperature changes on the pyroelectric human infrared sensor (PIR), the focusing effect of Fresnel lens, also greatly increase the sensitivity of pyroelectric human infrared sensor (PIR). This allows the PIR to generate electrical signals that change. Then the thermoelectric element is connected with the appropriate resistance, when the element is heated, there is a current flowing on the resistance, the voltage signal is obtained at both ends.