mmWave sensor vs PIR: how presence and motion detection differ

An mmWave sensor is a small, low-power radar that detects people by the movement they make, down to the rise and fall of the chest while breathing, so it can tell that someone is in a room even when they are sitting perfectly still. A PIR sensor, the ordinary motion sensor, reacts only when a warm body moves across its field of view. That single difference explains why lights on a PIR sensor switch off on someone reading in an armchair, and why presence sensors built on radar have become common. This article explains how each one works, where each gets it wrong, what they draw in power, why many smart home sensors now combine both, and where to put them. For the wider family of sensors these sit in, see the guide to IoT sensors.

What a PIR sensor is

PIR stands for passive infrared. Everything warm gives off infrared radiation, and a person at body temperature gives off a lot of it in the long-wave infrared band, around ten micrometers. A PIR sensor contains a pyroelectric element, a material that produces a small electrical charge when the amount of infrared falling on it changes. It is passive because it emits nothing; it only watches for that change.

Most PIR sensors have two or four sensing elements wired in opposition, so that a change seen equally by all of them cancels out. A slow rise in room temperature therefore produces no signal, while a warm body moving from the view of one element to the view of the next produces a strong one. In front of the elements sits a Fresnel lens: the white faceted dome or window on the front of the sensor. It is a thin plastic lens divided into segments, and each segment focuses a separate slice of the room onto the elements. The result is a fan of sensitive zones with blind gaps between them. A person walking through the room crosses from zone to gap to zone, the infrared on the elements keeps changing, and the output fires.

Three behaviors follow from that design. A PIR sensor sees movement across its zones much better than movement straight toward it, because walking toward the sensor keeps a person in the same zone. It cannot see through glass, because ordinary window glass blocks long-wave infrared. And it cannot see a person who is sitting still: once they stop moving, the infrared reaching the elements stops changing and the signal disappears. Motion sensors hide this with a hold time, keeping their output on for a set period after the last movement, which is why a room goes dark a few minutes after someone settles down to read.

What an mmWave sensor is

An mmWave presence sensor is a radar module on a small circuit board. It transmits a very low-power radio signal and listens for the reflections from everything in its field. The common bands are 24 GHz and 60 GHz, in slices of spectrum set aside for this kind of short-range device, with the exact rules varying by region.

The name comes from the wavelength. At 60 GHz a radio wave is about five millimeters long; at 24 GHz it is about twelve and a half millimeters, which strictly sits just below the range conventionally called millimeter wave, though the name is used for both.

Most presence modules use frequency-modulated continuous wave radar: the transmitted frequency sweeps up in a ramp, and the difference between what is being sent and what comes back tells the module how far away each reflection is. Comparing successive sweeps reveals movement. Because the wavelength is only millimeters long, even the small movement of a chest while breathing shifts the reflection measurably, which is how the sensor keeps detecting a person who is otherwise motionless. Many modules report the distance to the person as well as a simple present or absent, and some let you divide the room into zones or track more than one target.

This is what the smart home industry means by a presence sensor: something that answers “is anyone in this room?” continuously, where a motion sensor only reports “something moved just now”.

mmWave vs PIR: the differences that matter

Still presence is the reason radar exists in this market. A PIR sensor with a long hold time can keep a light on through a short pause, but it cannot tell a still person from an empty room. A radar sensor can, which lets lights and heating switch off within seconds of a room emptying instead of after a long safety timeout.

False triggers come from different places. A PIR sensor responds to anything that changes the infrared it sees, so a radiator warming up in its view, a blast of warm air from a vent, or a patch of sunlight moving across the floor can set it off, and so can a cat. A radar sensor responds to anything that moves and reflects radio waves. A ceiling fan, a curtain in a draft, a swaying plant or a robot vacuum can all hold it in the present state. Radio also passes through plasterboard, glass and wooden doors, so a radar can pick up someone in the next room or the hallway. Good radar sensors let you set a maximum range and ignore zones to deal with this, and getting those settings right is most of the work of installing one.

Power is the biggest practical difference. A PIR sensor draws only microamps while it waits, which is why battery motion sensors run for years. A radar module must transmit and process continuously, and it draws far more, so most mmWave sensors are mains powered or run from a USB supply. Battery radar sensors exist, and they get there by sampling less often or by using a PIR to wake the radar, at some cost in how quickly they respond. The guide to powering IoT sensors shows how quickly continuous current empties a battery.

Privacy is similar for both. Neither one makes an image. A PIR reports that something moved; a radar reports that something is present and perhaps how far away it is. That makes both reasonable in bathrooms and bedrooms where a camera would not be. Radar data is richer, and some sensors can estimate breathing rate or detect a fall, so treat the sensor’s network connection with the same care as any other device in the home.

Why many sensors combine PIR and mmWave

Several recent presence sensors contain both, and the two cover each other’s weaknesses. The PIR gives a fast, reliable trigger when someone walks in, so the light comes on at once. The radar then holds the presence state while the person sits still, and reports the room as empty soon after they leave. A radar alone can be the slower of the two to confirm a new arrival, because it filters its readings to avoid false triggers, and a PIR alone would switch the light off mid-evening.

If you already have PIR sensors, a similar result is possible in an automation: turn the light on from the PIR, and only turn it off when a radar sensor in the same room also reports it empty.

Where to put them

For a PIR sensor, place it so people walk across its view rather than toward it, for example on the wall beside a door looking into the room, or in a corner covering the main route. Keep it away from heaters, radiators, air vents and direct sunlight, and do not aim it through a window. A height of around two meters, or the height the maker specifies, gives the lens pattern the spread it was designed for. In a hallway or on stairs, where the only job is to switch a light on quickly, a PIR is usually all you need.

For an mmWave sensor, think about what else in the room moves. Point it away from ceiling fans, curtains, plants and pet beds where you can, and check what lies beyond thin walls and doors in its field. Mount it where the maker says, since ceiling and wall models have different coverage shapes. Use the sensor’s live readout of distance, if it has one, to set the maximum range to the edge of the space you care about and to add ignore zones for anything that moves on its own. Put it where it can see the places people sit still, such as the sofa, the desk or the bed, because that is what you bought it for. Metal shelving or appliances in the way block the radar just as walls block Wi-Fi.

How PIR and mmWave sensors connect

The radio a sensor uses to report its readings is a separate choice from how it senses. Battery PIR sensors mostly use Zigbee, Z-Wave or Thread, low-power mesh protocols built for devices that sleep between messages. Mains-powered radar sensors often use Wi-Fi, since power is not the constraint, and a growing number use Thread through Matter. The comparison of Zigbee, Z-Wave and Wi-Fi covers how those trade range, power and hub requirements.

For a room where people move about, a PIR sensor on a battery is the simple choice. For a room where people sit, work or sleep and you want the house to know they are there, use a mains-powered mmWave sensor, or one that combines both, and spend ten minutes tuning its range and zones once it is on the wall.