Part 1 - Sensor technology and practical applications in KNX Projects: Passive Infrared (PIR) sensors

In the field of smart homes and building automation, sensors (detectors) are likened to the "eyes and ears" of a KNX system. They collect important environmental information, allowing the system to make intelligent control decisions, providing convenience, safety, and energy optimization.

If you are new to KNX, or are an engineer, a technical specialist looking to optimize your solutions, this article series will be helpful to you. In Part 1, we will explore the most common type of sensor: the Passive Infrared (PIR) sensor.

KNX sensors play a crucial role in smart home and building automation projects. (Image source: 1home.io)

Passive Infrared (PIR) sensors: Effective motion detection

In the world of KNX smart homes and building automation, Passive Infrared (PIR) sensors are among the most common and fundamental types of sensors. Although not a new technology, their effectiveness and flexibility still make PIR a top choice for many applications.

Working principle

Passive: PIR sensors operate on the principle of detecting changes in the average temperature within a specific area. PIR sensors do not emit any energy (such as sound waves or electromagnetic waves) but rather passively "listen" or "read" natural infrared energy from the environment.

Infrared: All objects with a temperature greater than absolute zero (approximately -273.15°C) emit a certain amount of infrared radiation. Humans, animals, and other warm objects emit measurable infrared radiation. When a warm object moves into or out of the sensor's field of view, the change in infrared energy is detected by the sensor, thereby triggering an output signal.

Operating mechanism:

1. Fresnel lens: The most recognizable feature of a PIR sensor is the textured or opaque surface of its lens. This is a Fresnel lens. Its function is to divide the sensor's field of view into many small areas, called "sensing zones" or "facets." These zones alternate between infrared-sensitive areas and obstructed areas.

2. Pyroelectric sensor: Inside the PIR sensor is one (or more) pyroelectric sensors. Pyroelectric materials have the unique property of generating a small electrical charge when they absorb infrared radiation and their temperature changes.

3. Motion detection:

   • When there is no motion, the pyroelectric sensor receives a stable amount of infrared energy from the surrounding environment.

   • When a warm object (like a person) moves into one of the sensing zones, it creates a sudden change in the amount of infrared radiation received by the sensor.

   • As the object moves from one sensing zone to another, the continuous change in infrared radiation creates a time-varying electrical voltage signal at the pyroelectric sensor.

   • The microprocessor inside the PIR sensor analyzes this signal. If the signal is large enough and changes in a specific pattern (indicating the movement of a warm object), the sensor will trigger its output.

4. Most sensitive response: PIR sensors respond best when an object moves across the sensor's field (from one sensing zone to another). Moving directly towards or away from the sensor will cause less signal change and may be harder to detect.

PIR sensors respond best when an object moves across the sensor's field (image source: steinel.de)

Outstanding technical features

Detection range and angle: Determined by the design of the Fresnel lens. Can be 360 degrees (ceiling mounted) or 90-120 degrees (wall mounted), with a range from a few meters to tens of meters depending on the type.

Adjustable sensitivity: Most PIR KNX sensors allow sensitivity adjustment via ETS software. This helps prevent false triggers by unwanted small movements (e.g., pets, curtains swaying in the wind).

Independence from ambient light: PIR only reacts to infrared radiation and is not affected by ambient light intensity (day or night). However, many PIR KNX sensors integrate additional light sensors to optimize lighting control based on natural light conditions.

Delay time: After detecting motion and triggering, the sensor maintains the "detection" state for a certain period (delay time) to prevent devices from turning off immediately if the user stops moving. This time can be configured in ETS.

Advantages:

1. Affordable price: Compared to other technologies like ultrasonic or camera sensors, PIR generally has a lower cost.

2. Low power consumption: Passive operation helps PIR consume very little energy.

3. Easy to install and configure: Relatively simple to deploy in a KNX system.

Disadvantages:

1. Prone to false triggers: Can be triggered by unwanted moving heat sources (pets, hot/cold air currents from air conditioners, direct sunlight, heaters).

2. Limited presence detection: Not effective at detecting people who are sitting still with little movement. Therefore, they are primarily referred to as "motion sensors" rather than true "presence sensors."

PIR motion sensors are commonly applied in KNX smart homes. (Image source: knx.org)

Applications of PIR sensors in KNX projects

Thanks to these characteristics, PIR KNX sensors are ideal for specific applications in smart home and building automation systems:

Lighting automation:

1. Corridors, stairwells, restrooms, storage rooms: Automatically turn on lights when people enter and turn them off after a period of no detected motion. This is the most common and effective application for energy saving.

2. Building entrances, parking lots: Turn on security lights or streetlights when people/vehicles are detected.

Basic HVAC Control: Automatically turn off air conditioning or ventilation fans in less frequently used areas like meeting rooms after work hours, or empty offices, to avoid energy waste when unoccupied.

Security and Surveillance Systems: Detect sudden motion in restricted areas or during off-hours, triggering alarms or sending notifications to owners/management systems.

Curtain/Blind Control: In some simple scenarios, PIR can be used to automatically open curtains when someone enters a room (combined with a light sensor to prevent opening in the dark).

Notes on using PIR in KNX

To optimize the effectiveness of PIR KNX sensors, engineers should note:

1. Installation position: Avoid pointing directly at heat sources (windows with direct sunlight, AC vents) or areas with large obstructions.

2. Delay time configuration: Set an appropriate delay time to prevent lights/devices from turning off too quickly, causing inconvenience to users.

3. Integration with light sensors: For more effective lighting automation, combine with a light sensor. Only turn on lights when there is motion AND natural light is insufficient.

4. Consider combining technologies: For spaces requiring more accurate presence detection (people sitting still), consider using ultrasonic sensors or multi-technology sensors (PIR + ultrasonic/microwave).

Conclusion

The PIR sensor is a basic yet important component, offering high efficiency in automation and energy savings for KNX projects. With its reliable motion detection capabilities, PIR is ideal for automatic lighting applications in corridors, restrooms, or storage areas.

However, as analyzed, PIR has certain limitations, especially in detecting stationary people or when penetration through obstacles is required. This raises the question: Is there a more flexible solution for complex spaces that demand more accurate presence detection?

In the next part of the "Sensor Technology and Practical Applications in KNX Projects" series, we will explore the Microwave/High-frequency sensor – an active technology that offers enhanced capabilities...