As we mentioned before, PIR sensor has another name, which is pyroelectric infrared sensor. What is the pyroelectric effect? It describes a phenomena in which crystals undergo temperature fluctuations and produce charges at opposite ends. Simply put, the substances exhibit electrical polarization in response to thermal variations.

The pyroelectric effect plays a fundamental role in the operation of PIR sensors. It allows PIR sensors to detect infrared radiation emitted by people and and convert them into electrical signals.

Is it a little difficult to understand? In order to simplify the concept, it’s important to firstly realize that all objects in nature emit infrared radiation.

Objects with different temperatures emit infrared rays at different wavelengths, so infrared wavelengths are related to the temperature. The human body typically radiates infrared rays of approximately 10μm at a temperature of 37°C.

PIR sensors typically employ pyroelectric elements as their infrared sensing source. When a person enters the detection area of a PIR sensor, the temperature difference between the human body and the ambient environment causes the pyroelectric element to experience a change in infrared radiation temperature. This upsets the charge balance within the element, resulting in the release of charges. These charges are then detected and processed by the subsequent circuit, generating a signal output. The detection wavelength range of PIR is generally in the range of 5-14um, that is, it can convert infrared signal changes of 5~12um wavelength into electrical signals.

It is important to note that PIR sensors detect changes in temperature, rather than fixed-wavelength infrared radiation. If an object with a fixed wavelength stays motionless within the detection range or moves too quickly, the PIR sensor can not detect it. Consequently, if a human body enters the detection area but remains stationary, without any temperature change, the sensor will not produce an output. Therefore, PIR sensors are specifically made to recognize variations in infrared radiation brought on by movement, which allows them to identify human or animal activity.

A Fresnel lens is a specialized type of lens that differs from traditional lenses in its design. It is composed of a series of concentric annular convex mirrors, with their centers all located at the same point. The radius of curvature of these mirrors gradually decreases, creating a lens-like structure. When it comes to PIR sensors, the infrared rays emitted by the human body are enhanced by the Fresnel lens and then concentrated on the pyroelectric element of the infrared sensing source.

Functions of Fresnel Lens on PIR Sensors

Focusing – Improve Efficiency

The Fresnel lens aids in focusing the infrared radiation emitted by the human body onto the pyroelectric element of the PIR sensor’s infrared sensing source. This improves the efficiency of the PIR sensor in detecting human presence.

Zone Division – Increase Sensitivity

The Fresnel lens can divide the detection area into multiple sections, creating alternating bright and dark zones. When a person moves across the lens’ field of view, the infrared radiation emitted by their body transitions from the dark region to the bright region, or vice versa, which results in variations in the PIR sensor’s signal, making the detection of movement possible and enhancing the detection sensitivity and accuracy of the PIR sensor.

In the given diagram, which of the two motion trajectories labeled as ① and ② is easier for a PIR (Passive Infrared) sensor to detect? The answer is ①. The direction in which a person moves in relation to the sensor’s field of view affects the sensitivity of PIR sensors. They are usually more sensitive to movement in the transverse direction (perpendicular to the radius) than to movement in the radial direction because of their dual pyroelectric element construction. Therefore, rather than moving towards or away from the sensor, the PIR sensor’s capacity to detect human movement is most effective when the movement happens across its field of view. Why bring this up? Comprehending this idea will enable you to install your PIR sensors with improved precision.  

The PIR sensor installation position must be carefully chosen in order to maximize detection sensitivity and reduce false alarms. The direction of anticipated human movement, the sensor’s coverage area and range, and any possible sources of interference or false triggers are all important factors to take into account. The sensor can be positioned to minimize false alarms from irrelevant movement or ambient influences and maximize sensitivity when capturing movement in the desired direction.

A PIR sensor mainly consists of a Fresnel lens, two pyroelectric infrared elements, and a signal processor. The core component of a PIR sensor is the pyroelectric element made of materials that exhibit the pyroelectric effect, which causes them to produce electric charges when exposed to temperature fluctuations. There are usually two pyroelectric elements connected in series or parallel within the sensor, and the directions of the heated electrode polarization of the two elements are exactly opposite. When combined with a Fresnel lens, the dual pyroelectric elements create two detection areas in the front.

The pyroelectric infrared elements continues to receive infrared radiation from the background space even in the absence of moving heat sources. However, in this case, no electrical signal output is produced because of the complimentary design of the two elements.

When a warm object moves within the detection area, the potential difference across the dual pyroelectric elements changes accordingly. When the warm object leaves the detection area, the potential difference is reversed. The dual pyroelectric elements lose their complementary balance and produce a signal output in response to this change.

The generated signal is then processed by a signal processor, which analyzes the signal for changes that exceed a predefined threshold. Should the detected alteration exceeds this threshold, the signal processor triggers an alarm or initiates other predetermined actions. This is the basic principle of how PIR sensors work.