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PIR sensors

Pyroelectric Infrared (PIR) sensors contain two slots filled with material that reacts to IR radiation and heat. Typical use cases are security, or warm-body movement. In its simplest form, a Fresnel lens sits on top of the PIR sensor, allowing both slots to form a widening arc outward. These two arcs create detection zones. When a warm body enters one of the arcs, or leaves one, it generates a signal that is sampled. The PIR sensors use a crystalline material that generates current when subjected to IR radiation. A Field Effect Transistor (FET) detects the change in current, and sends the signal to an amplification unit. PIR sensors respond well in the 8 to 14 um range, which is typical of a human body.

The following diagram illustrates two IR regions detecting two zones. While this is fine for some purposes, generally we need to inspect an entire room or area for movement or activity:

PIR Sensor. Two elements responding to a source of IR moving across its field of view.

To scan a larger area with a single sensor requires multiple Fresnel lenses that condense light from regions of the room to create distinct regions on the PIR array. This also has the effect of condensing the infrared energy to discrete FET areas. Typically, such devices allow the architect to control the sensitivity (range) as well as the hold time.

The hold time specifies for how long to output a motion event after an object has been detected moving across the path of the PIR. The shorter the hold time, the more events may be output. Below is a diagram of a typical PIR sensor with a Fresnel lens focusing on the substrate by a fixed focal length:

Left: Fresnel Lens condensing IR region onto PIR sensor. Cypress Microsystems Application Note AN2105.