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What is Infrared Pyroelectric Detector-Function,operation

This page describes Infrared Pyroelectric Detector basic operation. It mentions function and applications of Infrared Pyroelectric Detector used in optical domain.

Wavelength of visible light ranges from 380nm to 700nm. Infrared lies above this range of wavelength. Infrared has wavelength between 700nm and 1mm.

Infrared transmission is optical in nature carried by beam of light invisible to naked eye. In order to use infrared waves emitted by ourselves and by objects, wavelengths above 1 micrometer and specifically around 10micrometers need to be detected. Thermistors and thermopiles or thin film thermocouples have been used to detect mid-range infrared transmission.

Now-a-days Pyroelectric has become ideal choice for broadband infrared detection. The reason for its popularity is signal strength, speed of response and minimal use of interfacing circuits.


Infrared Pyroelectric Detector

In the simple form, Pyroelectric detector or sensor is combination of capacitor and charge generator. It is referred as charge generator due to the fact that when infrared light strikes on the face of detector, it is absorbed as heat. This will create polarization. The device does not require any kind of cooling or biasing of any sort in order to perform satisfactorily.

The Infrared Pyroelectric sensor is a thin wafer of lithium tantalate with electrodes deposited on both the faces. It has capacitance of value about 30pF and insulation resistance of value about 5 x 1012 Ohms.

The figure depicts operation of infrared Pyroelectric Detector. As mentioned every objest acts as transmitter as they emit infrared light. While infrared detector acts as receiver as it responds to infrared emission.

When an intruder enters the room where Pyroelectric sensor or detector is mounted, it responds to change in light due to presence of intruder. Based on this, detector generates useful signal.

Following equation or formula is used to represent responsivity of Pyroelectric detector.
Vresponsivity = Current Responsivity * Effective Impedance
Vresponsivity = I * { R/(1+(2*π*f*R*C)2)1/2 }

In the above equation,
I - Use 0.5 to 1 microamp per watt
R - Use load resistor or feedback resistor
C - Use detector capacitance for voltage mode(approx. 30 pF)
or use stray feedback capacitance for current mode(approx. 0.03 pF).

Usually Pyroelectric detector is followed by either source follower or current to voltage converter.
In source follower configuration, voltage output is expressed by following equation:
RV = Ri * Zeff * Ao
Where, RV is voltage response in V/W, Ri is current responsivity, Zeff is lumped impedance of crystal and stray capacitance at input, Ao is follower gain typically about 0.8 .

In Current to voltage converter configuration, voltage output is given by following equation.
RV = Ri * ZF
Where, ZF is lumped impedance of feedback loop including RF and CS.

Applications of Infrared Pyroelectric Detector

Following are the applications of Infrared Pyroelectric Detector:
• Sensor circuits used for security to prevent intruder entry into home or office premises.
• Lighting appliances
• Household appliances

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