A PIR sensor is an electronic device that detects infrared radiation emitted by objects. It features a focusing mechanism, such as lenses or mirrors, and is commonly used in automatic lighting and security systems.

Passive Infrared (PIR) Sensor: A Comprehensive Overview

The world of technology is constantly evolving, and one fascinating innovation that has gained prominence is the Passive Infrared (PIR) sensor. PIR sensors have revolutionized the way we detect and respond to motion in various applications, from home security systems to automatic lighting control.

What is a PIR Sensor?

A Passive Infrared (PIR) sensor is an electronic device that detects infrared (IR) radiation emitted by objects in its field of view. Unlike other motion sensors that rely on active detection methods, such as ultrasonic or microwave sensors, PIR sensors detect changes in IR radiation without emitting any signals themselves. This “passive” nature makes PIR sensors highly efficient and suitable for a wide range of applications.

How Does a PIR Sensor Work?

A PIR sensor consists of a pyroelectric sensor, which is a crystalline material that generates a voltage when exposed to IR radiation. The sensor is divided into multiple segments, each with a unique Fresnel lens that focuses the incoming IR radiation onto the corresponding pyroelectric sensor. When an object enters the sensor’s field of view and emits IR radiation, the temperature difference between the object and the background causes a change in the voltage across the pyroelectric material.

The change in voltage is then converted into a digital signal by the sensor’s electronics. This signal is processed by an embedded microcontroller or dedicated circuitry, which analyzes the signal patterns to determine if there is any motion within the sensor’s range. If motion is detected, the PIR sensor triggers a response, such as activating an alarm, turning on lights, or sending a notification to a connected device.

Applications of PIR Sensors

PIR sensors are widely used in various applications due to their versatility and reliability. Here are some common applications:

• Security Systems: PIR sensors play a crucial role in detecting unauthorized movement in security systems. They are commonly used in burglar alarms, surveillance cameras, and access control systems.
• Lighting Control: PIR sensors are used to control lighting in both residential and commercial settings. They can automatically turn on lights when motion is detected and turn them off when the area is vacant, thus saving energy.
• Home Automation: PIR sensors are integrated into smart home systems to automate various tasks. For example, they can detect occupancy in a room and adjust temperature settings accordingly.
• Occupancy Detection: PIR sensors are employed in buildings and offices to monitor occupancy levels and optimize space utilization. They can be used to control HVAC systems and adjust ventilation based on the number of occupants in a room.

PIR sensors continue to find new applications and advancements in technology are expanding their capabilities. As the Internet of Things (IoT) and smart home technology continue to evolve, PIR sensors are likely to play an increasingly important role in creating intelligent and responsive environments.

Product Design and Focusing

A Passive Infrared (PIR) sensor is typically mounted on a printed circuit board that contains the necessary electronics for interpreting the signals from the sensor itself. The entire assembly is usually enclosed within a housing, strategically positioned to cover the area to be monitored​`oaicite:{“index”:0,”metadata”:{“title”:”Passive infrared sensor – Wikipedia”,”url”:”https://en.wikipedia.org/wiki/Passive_infrared_sensor”,”text”:”Product design[edit]nnThe PIR sensor is typically mounted on a printed circuit board containing the necessary electronics required to interpret the signals from the sensor itself. The complete assembly is usually contained within a housing, mounted in a location where the sensor can cover the area to be monitored. nn[Image] PIR motion sensor design nn The housing will usually have a plastic “window” through which the infrared energy can enter. Despite often being only translucent to visible light, infrared energy is able to reach the sensor through the window because the plastic used is transparent to infrared radiation. The plastic window reduces the chance of foreign objects (dust, insects, rain, etc.) from obscuring the sensor’s field of view, damaging the mechanism, and/or causing false alarms. The window may be used as a filter, to limit the wavelengths to 8-14 micrometres, which is closest to the infrared radiation emitted by humans. It may also serve as a focusing mechanism; see below”,”pub_date”:null}}`​.

The housing of a PIR sensor typically features a plastic “window” through which the infrared energy can enter. Although this window may only be translucent to visible light, it allows infrared energy to reach the sensor since the plastic used is transparent to infrared radiation. The plastic window serves multiple purposes, including reducing the chances of foreign objects such as dust, insects, or rain obstructing the sensor’s field of view, damaging its mechanism, or causing false alarms. Additionally, the window can function as a filter, limiting the wavelengths to 8-14 micrometers, which closely matches the infrared radiation emitted by humans. It can also aid in focusing the infrared energy onto the sensor’s surface​`oaicite:{“index”:1,”metadata”:{“title”:”Passive infrared sensor – Wikipedia”,”url”:”https://en.wikipedia.org/wiki/Passive_infrared_sensor”,”text”:”Focusing[edit]nnDifferent mechanisms can be used to focus the distant infrared energy onto the sensor surface. nn#### Lenses[edit]nnThe plastic window covering may have multiple facets molded into it, to focus the infrared energy onto the sensor. Each individual facet is a Fresnel lens. nn * Multi-Fresnel lens type of PIRn * [Image: PIR motion detector housing with cylindrical faceted window. The animation highlights individual facets, each of which is a Fresnel lens, focusing light on the sensor element underneath.] nnPIR motion detector housing with cylindrical faceted window. The animation highlights individual facets, each of which is a Fresnel lens, focusing light on the sensor element underneath. nn * [Image: PIR front cover only (electronics removed), with point light source behind, to show individual lenses.] nnPIR front cover only (electronics removed), with point light source behind, to show individual lenses. nn * [Image: PIR with front cover removed, showing location of pyroelectric sensor (green arrow).] nnPIR with front cover removed, showing location of pyroelectric sensor (green arrow). nn#### Mirrors[edit]nnSome PIRs are manufactured with internal, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover generally has no Fresnel lenses molded into it”,”pub_date”:null}}`​.

To focus the distant infrared energy onto the sensor surface, different mechanisms are used in PIR sensors. One common approach is the use of lenses. The plastic window covering of a PIR sensor may have multiple facets molded into it, which act as Fresnel lenses. Each individual facet helps to focus the infrared energy onto the sensor, enhancing the sensor’s sensitivity and accuracy. Another method involves the use of internal segmented parabolic mirrors, which also help to concentrate the infrared energy onto the sensor. In PIR sensors equipped with mirrors, the plastic window cover typically does not have Fresnel lenses molded into it​`oaicite:{“index”:2,”metadata”:{“title”:”Passive infrared sensor – Wikipedia”,”url”:”https://en.wikipedia.org/wiki/Passive_infrared_sensor”,”text”:”Focusing[edit]nnDifferent mechanisms can be used to focus the distant infrared energy onto the sensor surface. nn#### Lenses[edit]nnThe plastic window covering may have multiple facets molded into it, to focus the infrared energy onto the sensor. Each individual facet is a Fresnel lens. nn * Multi-Fresnel lens type of PIRn * [Image: PIR motion detector housing with cylindrical faceted window. The animation highlights individual facets, each of which is a Fresnel lens, focusing light on the sensor element underneath.] nnPIR motion detector housing with cylindrical faceted window. The animation highlights individual facets, each of which is a Fresnel lens, focusing light on the sensor element underneath. nn * [Image: PIR front cover only (electronics removed), with point light source behind, to show individual lenses.] nnPIR front cover only (electronics removed), with point light source behind, to show individual lenses. nn * [Image: PIR with front cover removed, showing location of pyroelectric sensor (green arrow).] nnPIR with front cover removed, showing location of pyroelectric sensor (green arrow). nn#### Mirrors[edit]nnSome PIRs are manufactured with internal, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover generally has no Fresnel lenses molded into it”,”pub_date”:null}}`​​`oaicite:{“index”:3,”metadata”:{“title”:”Passive infrared sensor – Wikipedia”,”url”:”https://en.wikipedia.org/wiki/Passive_infrared_sensor”,”text”:”Mirrors[edit]nnSome PIRs are manufactured with internal, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover generally has no Fresnel lenses molded into it”,”pub_date”:null}}`​.

The focusing mechanisms employed in PIR sensors result in a beam pattern for the detector view. The sensor receives varying amounts of infrared energy from different angles or zones. This distinction allows the PIR sensor to discriminate between field-wide illumination and moving objects. When a person walks from one angle or beam to another, the sensor intermittently detects the moving person, generating a rapidly changing sensor signal. The electronics in the PIR utilize this signal to trigger an alarm or activate lighting. The electronics ignore slowly changing signals to prevent false alarms​`oaicite:{“index”:4,”metadata”:{“title”:”Passive infrared sensor – Wikipedia”,”url”:”https://en.wikipedia.org/wiki/Passive_infrared_sensor”,”text”:”Beam pattern[edit]nnAs a result of the focussing, the detector view is actually a beam pattern. Under certain angles (zones), the PIR sensor receives almost no radiation energy and under other angles the PIR receives concentrated amounts of infrared energy. This separation helps the motion detector to discriminate between field-wide illumination and moving objects. nnWhen a person walks from one angle (beam) to another, the detector will only intermittently see the moving person. This results in a rapidly changing sensor signal which is used by the electronics to trigger an alarm or to turn on lighting. A slowly changing signal will be ignored by the electronics. nnThe number, shape, distribution and sensitivity of these zones are determined by the lens and/or mirror. Manufacturers do their best to create the optimal sensitivity beam pattern for each application”,”pub_date”:null}}`​.

Applications: Automatic Lighting and Security

PIR sensors find widespread applications in automatic lighting systems and security systems due to their ability to detect motion accurately and reliably. In automatic lighting applications, the electronics in the PIR sensor typically control a relay capable of switching mains voltage. This enables the PIR sensor to activate lights connected to it when movement is detected. This functionality is commonly used in outdoor scenarios for security lighting and practical purposes, such as illuminating the front door area for convenience at night. PIR sensors are also employed in public toilets, walk-in pantries, hallways, and other areas where automatic control of lights is beneficial. By turning on lights only when needed, these systems contribute to energy savings and eliminate the reliance on users remembering to manually turn off the lights when leaving an area​`oaicite:{“index”:5,”metadata”:{“title”:”Passive infrared sensor – Wikipedia”,”url”:”https://en.wikipedia.org/wiki/Passive_infrared_sensor”,”text”:”Automatic lighting applications[edit]nnWhen used as part of a lighting system, the electronics in the PIR typically control an integral relay capable of switching mains voltage. This means the PIR can be set up to turn on lights that are connected to the PIR when movement is detected. This is most commonly used in outdoor scenarios either to deter criminals (security lighting) or for practical uses like the front door light turning on so you can find your keys in the dark. nnAdditional uses can be in public toilets, walk-in pantries, hallways or anywhere that automatic control of lights is useful. This can provide energy savings as the lights are only turned on when they are needed and there is no reliance on users remembering to turn the lights off when they leave the area”,”pub_date”:null}}`​.

In security applications, the electronics in the PIR sensor control a small relay that completes the circuit across electrical contacts connected to a detection input zone of a burglar alarm control panel. The PIR sensor is designed in a way that if no motion is detected, the relay contact remains closed, known as a “normally closed” (NC) relay. However, if motion is detected or a wire is disconnected, the relay opens the circuit, triggering the alarm. PIR-based security systems are effective both indoors and outdoors, and their low power draw makes them suitable for solar-powered applications as well​`oaicite:{“index”:6,”metadata”:{“title”:”Passive infrared sensor – Wikipedia”,”url”:”https://en.wikipedia.org/wiki/Passive_infrared_sensor”,”text”:”Security applications[edit]nnWhen used as part of a security system, the electronics in the PIR typically control a small relay. This relay completes the circuit across a pair of electrical contacts connected to a detection input zone of the burglar alarm control panel. The system is usually designed such that if no motion is being detected, the relay contact is closed—a ‘normally closed’ (NC) relay. If motion is detected, the relay will open the circuit, triggering the alarm; or, if a wire is disconnected, the alarm will also operate. nn#### Placement[edit]nnManufacturers recommend”,”pub_date”:null}}`​.

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