How Occupancy Sensors Work

How Occupancy Sensors Work
November 19, 2016 jeff

Those blinking red lights on lights and light switches in bathrooms, hallways, and conference rooms, are (probably) not some government camera spying on you, but rather an occupancy sensor. This post looks at how they work.

Occupancy sensors, small devices designed to detect the presence of a person are ubiquitous. This is largely because they can be extremely cost-effective tools for reducing energy use and costs. If human beings were faultless at turning off lights, these sensors would be less valuable. But human nature is what it is, and as a result we’ve invented occupancy sensors instead.

These sensors come in two main flavors.

The most common type is a passive infrared (IR) sensor. Passive infrared sensing technology simply means that they sense heat. When they detect heat, they send an electrical signal to a circuit to turn a light on.

Their basic building block is a scientific concept called the pyroelectric effect. Literally “fire” and “electricity,” pyroelectricity, is possible because of the crystal structure of atoms. When a crystal is warmed, the atoms move slightly, a voltage across the crystal is generated, and that voltage becomes the electrical signal telling the lights to turn on.

Passive infrared sensors are not go-getters though. They just sit tight, waiting for a change to come to them. So if, for example, you are sitting behind an office partition, the sensor may not “see” you because it doesn’t have a direct line of sight.

This is where the second type of sensors, ultrasonic occupancy sensors, become important.

Ultrasonic sensors can “sense” motion through and around obstacles. They work using a different scientific effect called the Doppler effect. The Doppler effect is something you’ve probably experienced frequently. When an approaching car emits a high-pitched noise then switches to a lower pitch noise after is passes you, that’s the Doppler effect in action. Ultrasonic detectors use the same principle. It’s all about shifts in sound waves.

And unlike passive infrared sensors, ultrasonic sensors are actively emitting ultrasonic waves. These waves bounce back around the room, then return to the sensor at the same pitch with which they were emitted. But, when they bounce off a person behind a cubicle wall who is shifting in her seat, the sound wave pitch shifts higher. The ultrasonic sensor detects this pitch change and sends an electrical signal to turn the lights on.

This shift is sound waves is based on the equation:

F2  = F1 * C / (C + V)

Where:

  • F1 = The sound the ultrasonic sensor sends out
  • F2 = The sound the ultrasonic sensor receives back
  • C = The speed of sound t sea level, about 700 miles per hour
  • V = The speed of the person approaching the ultrasonic sensor. About 3 miles per hour.

The important thing to note is that F2 will be different from F1 if someone is moving, and when F2 is different from F1, the light will turn on.

You can combined both types of sensors, too. There “dual-technology sensors” are more expensive but also more reliable. They will only turn on or off when both detectors pick up the signal, protecting against lights turning on unnecessarily as well as lights turning off prematurely. The result is much less energy waste thanks to two interesting scientific phenomena: the pyroelectric effect and the Doppler effect.

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