PIR Sensors Explained

Passive infrared (PIR) sensors are commonly used for a variety of applications and security purposes. Read on to discover more about these components and how they can benefit your business.

PIR sensors are designed to recognise infrared light emitted by moving objects in the vicinity of the sensor. All objects and people that radiate heat also give off a low level of infrared radiation, relative to their temperature. The hotter an object, the more radiation it emits. These devices simply detect the IR and transmit it into signals that can be used to monitor locations for motion detection and security systems.

As a result, PIR sensors are widely used in thermal sensing applications including motion detection, security alarms, and automatic lighting. They are versatile and low-cost yet can bring a wide range of benefits.

As well as PIR, these devices are sometimes also referred to as passive infrared detectors (PID).

These devices are centred around two balanced strips of a pyroelectric material which are placed parallel to one another, creating a pyroelectric sensor. This detects thermal energy in the surrounding area. An infrared filter is also included to block out all other light wavelengths.

A Fresnel lens helps to focus the infrared energy on the sensor and increases the sensing range. This is achieved by the lens collecting light from multiple angles and focusing it into a single point.

If a change to the signal differential between the two pyroelectric elements is detected, the PIR sensor engages and sends an electronic signal to an embedded computer. Several things could happen in this instance depending on the setup, such as triggering an alarm or activating a security light.

Crucially, a PIR motion detector will not determine the specific location of an object. Instead, it merely tells you that an object has entered the detection area.

The range, or sensor radius, will vary between different models and brands. PIR sensors tend to be fairly straightforward in themselves, so their sensitivity and range are two of the most significant factors to consider when choosing a PIR sensor.

The sensor radius can vary from as little as 30mm right up to 240m for covering significantly larger areas.

Some of the most common PIR sensor range options include:

There are two types of infrared sensors - active and passive. As explained above, passive IR sensors just detect infrared radiation and are used as motion detectors in a range of security applications.

On the flip side, active infrared sensors are more complex devices. The primary difference is that whereas passive sensors only detect radiation, active sensors both emit and detect it. There are two parts to an active IR sensor - a receiver and a light-emitting diode (LED). When an object moves within the sensor's range, the infrared light from the LED will reflect off the object. This will then be detected by the receiver.

Active infrared sensors are often used in robotics and obstacle detection systems.

Infrared motion sensors are typically straightforward to install. Before installation, however, it is important to familiarise yourself with the particular sensor's range. This will help you to plan an effective coverage system and ensure the entire area that needs covering will be monitored.

Common installation locations include ceiling or wall mounting as these tend to provide a clear, unimpeded view of the detection area. They may also be installed alongside wall lights, for example, in automatic lighting applications.

Most manufacturers will provide the necessary fixings needed to mount an adjustable bracket for the sensor.

These tips should help you to decide the best placement for a passive infrared sensor:

• Make sure all key areas are covered, including possible routes of entry, hallways, and corners
• Avoid installing sensors near radiators and air vents, for example, as temperature changes could impact reliability or result in a false alarm
• The ideal height for installation is typically 2-3 metres above ground level
• Avoid placing the sensor in direct sunlight