Sensor Sense: Passive Infrared-Motion Sensors

Unlike infrared optical sensors that use an LED transmitter and IR receiver, the PIR emits nothing. As its “passive” name suggests, it responds only to infrared energy radiated by the object being sensed. The most common object a PIR sensor detects is the human body, so these sensors find use in automatic light switches, alarm systems, and door openers.

Any object with a temperature above absolute zero emits infrared energy through black-body radiation. This energy is invisible to the human eye, but not to the pyroelectric material at the core of a PIR sensor. When subjected to infrared radiation, pyroelectric materials generate a small electric charge similar to the electric charge created when visible light strikes a solar cell. Pyroelectric materials used in these sensors include galliumnitride, caesium nitrate, and lithium tantalate.

Skin temperature is approximately 34°C, typically higher than background temperatures. As a person walks past the sensor, their higher skin temperature creates a greater charge in the pyroelectric material. An amplifier circuit boosts the small signal generated by the added infrared energy and feeds it to a differential comparator. The comparator looks for a difference in the signal from prior readings to trigger the output. However, this simple arrangement can also be tripped by any source of rapidly changing heat, such as the flash of bright lights or reflections off objects during hot, sunny days.

Several techniques reduce these false activations. First, the human body emits infrared energy at a wavelength of 9 to 10 μm. So an infrared filter that passes wavelengths from 8 to 14 μm is placed in front of the sensor to boost sensitivity to the infrared energy given off by people.

Second, a Fresnel lens positioned in front of the sensor performs two functions. It concentrates the IR energy emitted over a wider area onto the sensor and it divides the area into hot and cold zones of sensitivity. As a person walks across the zones, the sensor sees a changing IR value that produces a varying output signal from the sensor indicating motion. The comparator looks for and responds to this changing signal. Hot items that don’t move, like heaters and lights, do not produce output variations. The comparator ignores these constant IR sources.

<em>The pyroelectric material in a PIR sensor generates an electric charge proportional to the amount of thermal energy striking it. As a person walks past the sensor, their higher body temperature boosts sensor output. The change is amplified and sent to a comparator that detects the difference from prior readings.