Infrared Pyroelectric Detectors: Working Principles and Applications

This page describes the basic operation of an Infrared Pyroelectric Detector. It covers the function and applications of this detector, which is commonly used in the optical domain.

Understanding Infrared Light

The wavelength of visible light ranges from 380nm to 700nm. Infrared (IR) light lies above this range, with wavelengths between 700nm and 1mm. Infrared transmission is optical in nature, carried by a beam of light that is invisible to the naked eye. To utilize infrared waves emitted by ourselves and other objects, we need to detect wavelengths above 1 micrometer, specifically around 10 micrometers.

Traditionally, thermistors, thermopiles, and thin-film thermocouples have been used for mid-range infrared detection. However, Pyroelectric detectors have become a popular choice for broadband infrared detection due to their signal strength, speed of response, and minimal need for complex interfacing circuits.

How Pyroelectric Detectors Work

In its simplest form, a Pyroelectric detector or sensor is a combination of a capacitor and a charge generator. It acts as a charge generator because when infrared light strikes the detector’s surface, it’s absorbed as heat, creating polarization. These devices don’t require any cooling or biasing to function properly.

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

Operation Explained

Every object acts as a transmitter by emitting infrared light, and the infrared detector acts as a receiver, responding to this emission. When an intruder enters a room equipped with a Pyroelectric sensor, the sensor responds to the change in light caused by the intruder’s presence and generates a signal.

Responsivity Formula

The responsivity of a Pyroelectric detector can be represented by the following equation:

$V\_{responsivity} = Current \ Responsivity \times Effective \ Impedance$

$V\_{responsivity} = I \times \frac{R}{\sqrt{1 + (2 \pi f R C)^2}}$

Where:

• I = 0.5 to 1 microamp per watt (typical current)
• R = Load resistor or feedback resistor
• C = Detector capacitance for voltage mode (approx. 30 pF) or stray feedback capacitance for current mode (approx. 0.03 pF)
• f = frequency

Circuit Configurations

Pyroelectric detectors are usually followed by either a source follower or a current-to-voltage converter.

Source Follower Configuration:

In a source follower configuration, the voltage output is expressed by the following equation:

$R*V = R_i \times Z*{eff} \times A_o$

Where:

• $R_V$ is the voltage response in V/W
• $R_i$ is the current responsivity
• $Z_{eff}$ is the lumped impedance of the crystal and stray capacitance at the input
• $A_o$ is the follower gain (typically about 0.8)

Current-to-Voltage Converter Configuration:

In a current-to-voltage converter configuration, the voltage output is given by:

$R_V = R_i \times Z_F$

Where:

• $Z_F$ is the lumped impedance of the feedback loop, including $R_F$ (feedback resistance) and $C_S$ (stray capacitance).

Applications of Infrared Pyroelectric Detectors

Here are some common applications:

• Security Systems: Sensor circuits used for security to prevent unauthorized entry into homes or offices.
• Lighting Control: Used in automatic lighting systems that turn on when motion is detected.
• Household Appliances: Found in various appliances like microwave ovens and automatic faucets.