Ever wonder what that blinking red light on the restaurant bathroom light switch is doing and how the lights know that you’re in there?

Of course you do! It’s magical.

Occupancy sensors, small devices designed to detect the presence of a person are both mysterious and ubiquitous. They are in bathrooms, supermarket aisles, warehouses, hallways, storage rooms, conference rooms, porches, and garages. They can be found in nearly any space with intermittent usage.

They are so popular largely because they are extremely cost-effective tools for reducing energy waste. If human beings were faultless about turning off lights, these sensors would be less important. But if someone forgets to turn off the lights in a large hallway over the weekend, they could be wasting up to 25 pounds of CO2 emissions and $10. Occupancy sensors, meanwhile, start at only $30. That’s a three weekend payback.

Occupancy sensors come in two main flavors. Let’s look at each of them.

The first type is a passive infrared (IR) sensor. These are the most common and are typically the kind blinking at you in a restaurant bathroom. The blinking usually indicates they are detecting an object. Namely, you. The sensors use passive infrared sensing technology, which is a fancy-sounding way of saying they sense heat. Upon detecting heat, they send an electrical signal to a circuit to turn a light on.

The basic building block of this heat-sensing technology is the pyroelectric effect. Literally “fire” and “electricity,” pyroelectricity, takes advantage of the crystal structure of atoms. When a certain crystal is warmed, atoms move slightly, changing the atoms polarity and giving rise to a voltage across the crystal (a related concept is the easily confused piezoelectricity in which a mechanical force, i.e. pushing, on a crystal generates a voltage; also very cool).

The infrared energy emitted by human beings is warmer than the background room temperature, unless you artificially up the room temperature to 98.6 (perhaps, in order to break into Cosmo’s office). The crystals within the passive infrared sensor sense this change in heat, generating a voltage, telling the lights to turn on.

So those occupancy sensors on the conference room wall are winking at you: they “see” you and think you’re hot.

But don’t take it too personally. They think everyone is hot.

Passive infrared sensors, however, are not go-getters. They just sit there, waiting for a change to come to them. As a result, if you are sitting behind an office partition, that wall blocks your heat from the sensor. So the sensor can’t “see” you. To turn on, infrared sensors must have a direct line of sight to the person. But a direct line of sight is not always an option. Furthermore, IR sensors work by sensing the transition of heat from one area to another: in the picture to the right for example, the street lamp would only turn on when a car passed from one box to the next.

If you do not have a direct line of sight to a sensor, or you are going to be sitting in a chair working and not passing from zone to zone, ultrasonic occupancy sensors can be useful. These detectors, the second main flavor of occupancy sensors, are best in open offices or in restrooms with stall partitions.

Ultrasonic sensors work using the Doppler effect. This is the same principle dolphins use to echolocate. You’ve seen it in action, too. Ever notice how an approaching car seems to make a high-pitched noise that switches to a lower pitch after is passes you? It doesn’t just seem that way. That’s what’s happening. Ultrasonic detectors use the same principle. They actively emit ultrasonic waves.

When the waves bounce back around a still room, they will return to the sensor at the same pitch with which they were emitted. When they bounce off a person behind a cubicle wall walking towards the sensor, the pitch shifts higher. That change in pitch triggers an electrical signal to tell the lights to turn on.

How does this work? See here for good visuals, but the basics are governed by the equation:

F2 = F1 * C / (C + V)

Where:

• F1 = The emitted frequency. What the ultrasonic sensor sends out
• F2 = The received frequency. What the ultrasonic sensor observes/receives
• C = The speed of sound at sea level, 330 m/s
• V = The speed of the person approaching the ultrasonic detector; it is positive if the person is moving away from the detector

So if someone is moving away from the detector, V is positive and C / (C + V) < 1, then F2 < F1, and the frequency shifts down. Ff someone approaches the detector then V is negative and C / (C + V) > 1, then F2 > F1, the frequency shifts up. Just as we experience with an approaching and departing car.

The important fact for an occupancy sensor is that that F2 is different from F1; it doesn’t matter if it shifts up or down. Whenever there is a change in frequency, it sends a signal to turn the light on.

Using this principle, ultrasonic detectors allows for effective occupancy based control in areas where there may not be a direct line of sight to the individuals in the room, like office cubicles. If you are behind an office cubicle wall, an infrared detector cannot “see you” (“sense your radiation”). But if you lean in towards your monitor, an ultrasonic detector can “hear you.” That is, it hears a change in pitch associated with your movement.

There are also dual-technology sensors, that have both ultrasonic and occupancy sensors built into one unit. These are more expensive, on the order of $150, but also the most reliable. Only when both the infrared and ultrasonic detectors pick up a signal do the lights turn off, and only when both detectors no longer receive a signal do they turn off. This protects against false positives (turning on unnecessarily) as well as false negatives (turning off prematurely).

You may have noticed that it seems that lights on an occupancy sensor stay on for a long time after you depart and other times they seem to turn off two minutes later. This time delay is usually adjustable and 15-minutes is commonly found to be to most occupants’ satisfaction. Occupancy sensor’s sensitivities can also be adjusted so they do not see or hear too far beyond the room. Both of these adjustments may be as simple as twisting a small screw on the face of the sensor.

There’s lots of energy to be saved through the installation of occupancy sensors . Hopefully now you understand their magic a little better. When selected, installed, and programmed properly, these little blinking wizards should never leave you in the dark.