PIR sensors are devices that can detect motion by measuring the infrared (IR) radiation emitted or reflected by objects in their field of view. PIR stands for passive infrared, meaning that the sensors do not emit any IR radiation themselves, but only receive it from the environment. PIR sensors are widely used in security systems, automatic lighting, and smart home applications. We will explain the basic principles of how PIR sensors work and some of their advantages and limitations.

The PIR Sensor

A typical PIR sensor consists of a pyroelectric material, a lens, and a circuit. The pyroelectric material is a substance that generates an electric charge when it is exposed to heat. The lens is a Fresnel lens that focuses the IR radiation onto the pyroelectric material. The circuit amplifies and processes the electric signal from the pyroelectric material and triggers an output when a motion is detected.

The pyroelectric material is divided into two halves, each connected to a different terminal of the circuit. When the sensor is idle, both halves receive the same amount of IR radiation from the ambient temperature, and the circuit has zero voltage. When an object, such as a person or an animal, moves in front of the sensor, it changes the amount of IR radiation received by each half of the pyroelectric material. This creates a positive or negative differential voltage between the two terminals, depending on the direction of the motion. The circuit detects this voltage change and sends an output signal to indicate that a motion has occurred.

The Lens

The lens plays an important role in determining the range, sensitivity, and detection pattern of the PIR sensor. The lens is composed of multiple segments that create a series of small detection zones. Each segment acts like a mini-lens that focuses the IR radiation from a specific area onto the pyroelectric material. When an object moves from one zone to another, it causes a voltage change in the circuit. The size and shape of the lens segments affect the resolution and coverage of the sensor. A larger number of smaller segments can provide a higher resolution and a wider coverage, but also a higher power consumption and a higher false alarm rate.

The lens also filters out the visible and ultraviolet light wavelengths, allowing only the IR radiation to pass through. This reduces the interference from other light sources and improves the performance of the sensor. The typical wavelength range of the IR radiation that the PIR sensor can detect is between 5 and 14 micrometers, which corresponds to the thermal radiation emitted by humans and animals.

The Advantages and Limitations of PIR Sensors

PIR sensors have several advantages over other types of motion sensors, such as ultrasonic, microwave, or radar sensors. Some of these advantages are

> Low power consumption: PIR sensors consume very little power, as they only operate when a motion is detected. This makes them suitable for battery-powered or solar-powered applications.

> Low cost: PIR sensors are relatively cheap and easy to manufacture, as they use simple and widely available materials and components.

> High reliability: PIR sensors are immune to noise, dust, humidity, and other environmental factors that may affect other types of sensors. They also have a long lifespan and require minimal maintenance.

> Privacy protection: PIR sensors do not capture or store any images or sounds of the objects they detect, unlike cameras or microphones. This protects the privacy of the users and the subjects.

However, PIR sensors also have some limitations that need to be considered, such as

> Sensitivity to temperature: PIR sensors rely on the temperature difference between the object and the background to detect motion. If the temperature of the object is similar to the temperature of the background, the sensor may not be able to detect it. For example, if the object is wearing clothes that match the room temperature, or if the room temperature changes rapidly due to heating or cooling systems, the sensor may fail to sense the motion.

> Sensitivity to speed: PIR sensors are more sensitive to fast-moving objects than slow-moving objects. If the object moves too slowly, the sensor may not register a significant voltage change and miss the motion. For example, if the object is crawling or walking very slowly, the sensor may not detect it.

> Sensitivity to direction: PIR sensors are more sensitive to objects moving across their field of view than objects moving towards or away from them. This is because the objects moving across the sensor create a larger voltage change than the objects moving along the sensor. For example, if the object is moving directly towards or away from the sensor, the sensor may not sense the motion.

> False alarms: PIR sensors may generate false alarms due to various factors, such as sudden changes in lighting, reflections, shadows, insects, pets, or other sources of heat. These factors may cause the sensor to mistake them for a real motion and trigger an output. For example, if the sun shines through a window, or a car passes by, the sensor may detect a motion and activate the alarm or the light.

PIR sensors