Occupancy Sensors: Why, and What are Your Best Options?

As technology continues to improve, more and more people are creating a “smart home” type of environment. As with anything in life, we need to step back and consider what are the best choices, and why. We are seeing an increase in IoT (internet of things) technology being used in residential applications, whereas IoT has primarily been used in corporate and industrial environments. Let’s discuss one application of these “smart technology” type of devices: sensors for lighting.

Two Types of Detectors

There are 2 types of detectors that are generally used: Motion (also called Presence) and Absence. Motion detectors trigger an action based on assumed presence in a given area (e.g. turn lights ON when you enter the room.) Absence detectors trigger an action based on assumed lack of presence in a given area (e.g. turn OFF lights when you leave the room.)

Which of these you decide to use is really dependent upon the function you’re looking for. The reason most people use these smart devices is to save on energy/electricity. One thing to consider when looking at either motion or absence detectors, is absence detectors do not have any effect on turning the lights ON. That’s up to you when you walk into a room. So, if you’re the type that often forgets to turn the light off, this may be the best choice for you.

4 Types of Sensor Technology

When it comes to occupancy sensors, there are 4 different types of sensor technology that you can choose from: passive infrared (PIR), microwave, ultrasonic, and smart. Here are explanations for each, and then my recommendations on which you should (or shouldn’t!) use if you do decide to use them in your home. If you are a regular reader here, you probably already know which I will or will not recommend!

Passive Infrared (PIR)

pir-passive-infrared-sensor-detectorPassive infrared (PIR) sensors function within the infrared spectrum, which we as humans emit as heat. As the passive part of the name suggests, PIR sensors don’t actually do anything. It is a passive device that receives thermal signals from the surrounding environment. These types of detectors have a matrix of sensors that recognize the movement of a heat signature between sensors. These are interpreted as a person moving within the area, which triggers the lights to turn on.

How well any particular PIR sensor works is obviously based on the sensitivity of the sensors. The PIR sensor provided the original basis of occupancy detection and were motion detectors, pure and simple. Developments in technology have improved the sensitivity of PIR sensors by increasing the matrix of sensors, which of course makes them more capable of working effectively as presence detectors.



microwave-sensor-detectorMicrowave sensors are the opposite of passive. They regularly shoot microwave pulses into the environment and then measure the reflected signals. When a person enters the field of the sensor, it triggers the lighting.

While infrared detectors rely on line-of-sight to be effective, microwave detection can see through most building materials (though microwaves cannot penetrate metal.) This can cause problems, of course, since detection of movement in an adjacent room isn’t usually what’s wanted. To prevent accidental triggering, the sensors need to be properly calibrated and adjusted.

Obviously, we have concerns about microwave radiation. Most manufacturers of microwave sensors claim that the output from these devices is minimal, enough to not warrant concern. They admit that research is always ongoing as to the possible effects of exposure to EMF radiation, yet claim there is no proof that microwave exposure at these levels leads to a higher incidence of cancer. This, is in spite of the fact that even the International Agency for Research on Cancer (IARC) classifies radiofrequency radiation as possibly carcinogenic. The real concern is the continually increasing exposure to RF sources, and whether we should be worried about the accumulating effects of microwaves in our homes.

Worldwide you will find some companies prefer to not use the word microwave when describing this type of sensor. Of course, they are worried that the public will associate a microwave sensor with the microwave oven. They are instead often described as “high frequency (HF) sensors” – as if that doesn’t still trigger concern for the observant health-conscious consumer.


Ultrasonic sensors work in similar ways as radar: a signal is radiated out into a space and the reflected audio signal is then immediately analyzed. The sensor recognizes the steady state, or background signal, and if that changes it assumes something has moved into the environment. Ultimately, the ultrasonic occupancy detector comprises a tiny speaker that transmits the signal, and a microphone that picks up the reflected signal.

The sound waves are transmitted at around 40kHz, which is well above human’s ability to hear. It’s worth noting that dogs can hear up to 60kHz (and cetaceans – dolphins, whales – and bats can hear above 100kHz.) While it’s not likely that any dolphins will be troubled by your ultrasonic occupancy sensor, dogs and bats may be affected.



Smart sensors are similar to PIR in that they are passive devices. However instead of reading infrared signals, they read visible light within the electromagnetic spectrum.

Smart sensors are progressively becoming more and more valuable in data harvesting: gathering all manner of data for the purpose of auditing and analytics. This raises obvious concerns regarding data protection and privacy. Manufacturers ensure consumers that their sensors are not sensitive enough to identify individual people – they’re basically cameras producing unprocessed, raw images – but personal privacy is still a growing issue. The ultimate gatekeeper for the data that is collected is the software that is interpreting it (which most often comes from the manufacturer.) It’s important that we continually fight for limits to be placed on character recognition technology with these manufacturers.

Summary: Which Sensor to Use?

I think it’s obvious that I would not recommend any type of sensor or electronic device to continually send signals into your home. No matter what the manufacturers say, or the federal government’s bought-and-paid-for entities, there is always a risk when it comes to using frequencies around our body. You can learn more about EMFs and the health effects from them by searching our website.

Though the Smart sensor is passive and does not send signals out into your home environment continually, there is still the matter of internal processing and privacy/data control. In my opinion, the best option would be to use a passive infrared (PIR) sensor in any detector in your home.

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