Knowledge about Infrared Pyroelectric Sensor

Pyroelectric sensor is a sensor that can detect infrared radiation emitted by humans or animals and output an electrical signal. Objects exist in the natural world, such as the human body, flame, ice will emit infrared radiation, with varies of wavelength. The human body temperature is 36-37 ℃, the infrared wavelength emitted is 9-10 um, belongs to the far-infrared region; 400-700 ℃ heat emitter, the infrared wavelength emitted is 3-5 um, belongs to the mid-infrared region. Pyroelectric Infrared Sensors are not affected by day and night, and can be used monitoring all time, widely used in security systems, burglar alarms, visitor identification, light switch control and smart toys, etc. 

Principle of Infrared Pyroelectric Sensors

Pyroelectric Infrared Sensors are generally composed of major parts such as caps, holders, infrared filters, sensitive elements, field effect tubes or digital chips.

Using the spontaneous polarization strength of pyroelectric material with the change of external temperature and the characteristic of charge movement, the change of infrared energy radiated by the human body is detected in a non-contact way, which is converted into an electrical signal and output in the form of voltage or current, and the signal is amplified by a power amplifier to achieve the purpose of controlling the circuit.

Spectral Response of Window Materials:

    Typical average transmition curve of 5.5μm pass IR filter is figured, which is vacuumed on silicon filter

Categorization of Infrared Pyroelectric Sensors

According to different signal processing circuits, Pyroelectric Infrared Sensors can be divided into two categories: 

Analog Pyroelectric Infrared Sensors and Digital Pyroelectric Infrared Sensors

Signal processing circuit of Analog Pyroelectric Infrared sensor is field effect tube, while signal processing circuit of Digital Pyroelectric Infrared sensor is digital chip.

Characters of Infrared Pyroelectric Sensors

Pyroelectric infrared sensors are only effective when used with Fresnel lens. Without Fresnel lens, the sensor may have a detection radius of less than 2 meters, and with a Fresnel lens it can reach 10 meters or more. The Fresnel lens is made of a common polyethylene and is mounted on the front of the sensor.

Principle of Fresnel lens is shown below:

When the light passes through the lens unit, the visible and blind areas are formed on the opposite side of the light and dark, each lens unit has only a small field of view, within the field of view for the visible area, outside for the blind area. Through the human body in the dark and light areas of the infrared radiation constantly changing changes in the temperature of the pyroelectric sensitive element, so that it outputs one after another corresponding weak electrical signals. The frequency of the output signal is about 0.1~10HZ, this frequency range is determined by the Fresnel lens, the speed of human body movement and the characteristics of the pyroelectric human body infrared sensor itself.

Applications of Infrared Pyroelectric Sensors

Pyroelectric infrared sensors are now mainly used in home security, smart office, smart lamps and lanterns, smart switches, smart home, smart doorbell, burglar detection, smart toys, smart home appliances, automation control and other fields.