What is a pyroelectric infrared sensor for the human body?
A passive infrared detector made of pyroelectric infrared sensor is used to control automatic doors, automatic lamps and advanced photoelectric toys. The pyroelectric human infrared sensors are composed of differential balance structures, which are composed of sensitive elements, field effect transistors, high value resistors and so on.
(1) sensitive element
The sensitive element is made of pyroelectric human infrared material (usually aluminum zirconate titanate). First, the pyroelectric material is made into small thin slices, and then the electrodes are plated on both sides of the thin plate. Two series sensitive elements are formed on the same chip. This is to suppress the pyroelectric signal interference due to the change of the environment and its own temperature.
When the pyroelectric human infrared sensor is used in practice, the lens is installed in front of the sensor. The external infrared radiation of the lens will only be on one one sensitive element, and the signal produced is not counterbalanced. The characteristic of the pyroelectric human infrared sensor is that it gives a corresponding electrical signal only when it causes its temperature change due to external radiation. When the temperature changes tend to be stable, there is no signal output, so it is said that the pyroelectric signal is proportional to the change rate of its own temperature, or the heat release red external transmission sense. The device is only sensitive to the human body in motion, and is applied to the detection circuit of human motion nowadays.
(2) field effect transistors and high resistance resistance Rg
Generally, the material resistance of the sensitive element is up to 10130. Therefore, the impedance transformation of FET can be used in practice. The role of high resistance resistor Rg is to release the gate charge and make the field-effect transistor work normally. Generally, the source voltage is 0.4~1.0V when the source output is connected. Through field effect transistor, the output signal of the sensor can be processed by ordinary amplifier.
(3) Filter window
The sensing element in pyroelectric infrared sensor is a broad spectrum material, which can detect all kinds of wavelength radiation. In order to make the sensor most sensitive to the human body and have anti-interference to the sun and electric light, the sensor uses the filter as the window, that is, the filter window. The filter is made of a multilayer coating on a S substrate. Each object emits infrared radiation, and its strongest radiation wavelength satisfies the Wien displacement law: m*T=2989 (μm). (m is the longest wavelength, and T is the absolute temperature.)
The body temperature is 36 degrees C~ 37C, that is, 309~310k, and its radiation wavelength is m= 2989/309 ~310=9.67~9.64 m. It can be seen that the strongest wavelength of infrared radiation of human body is at the center of the response wavelength 7.5~14mm of the filter. Therefore, the filter window can effectively let the infrared radiation of human body pass through, and prevent the infrared ray passing through visible light such as sunlight, light and so on, so as to avoid F disturbance. So pyroelectric infrared sensors are sensitive to human body and animals similar to human body temperature.
(4) Finel lens
When the pyroelectric human infrared sensor is used with Finel lens to play the most important role without Finel lens, the detection radius of the sensor may be less than 2M. and the Finel lens can be up to 10m. or even farther. The Finel lens is made of universal polyethylene and is mounted on the front of the sensor. The horizontal direction of the lens is divided into 3 parts. Each part is divided into several different regions in the vertical direction, so the Finel lens is actually a lens group. When the light passes through the lens unit, the visible and blind areas are formed on the opposite side. Each lens unit has only one small field of view, the visible field is visible, and the blind area is outside. The field of view of two adjacent unit lenses is neither continuous nor overlapping, but they are separated by a blind area. When the human body moves in this surveillance range, the human body goes into the field of view of a unit lens and goes out of the field of view. The pyroelectric human sensor will see the movement of the human body for a moment, and then can not see it for a moment, and then see again after a while again, and the infrared radiation of the human body is constantly changing the pyroelectric body. The temperature makes it output one signal after another. The frequency of the output signal is 0.1~10Hz, which is determined by the characteristics of Finel lens, human motion speed and pyroelectric infrared sensor itself.
Finel lens can not only form visible and breeding areas, but also focus. The focal length is usually about 5cm, and the lens matched with different sensors is also different. Lenses are generally fixed at the front of the sensor 1~5cm. The Finel lens is circular and the focal length of the lens is precisely aligned with the center of the sensor sensitive element.