There is no single best presence sensor. There is a best sensor for a budget, a room and a job. The three technologies in this guide sit at different points on one trade-off: how much hardware you add versus how much the sensor can tell you. Understand that line and the choice is easy.
01The three approaches at a glance
PIR (passive infrared) is the classic motion sensor behind most automatic lights. It is cheap, tiny and sips power, but it only sees movement. mmWave radar is an active millimetre-wave radar that detects true presence, down to the micro-motion of breathing, and often reports distance too, at a higher price. Wi-Fi CSI reads Channel State Information from an ordinary Wi-Fi link and turns the radio itself into a sensor, so it adds no dedicated detector at all — at the cost of tuning and a baseline of the empty room.
We cover the first two head to head in mmWave radar vs PIR; here we add the third option and compare all three. If CSI is new to you, the complete guide to Wi-Fi CSI explains the basics.
02How each one senses
PIR hides behind a faceted plastic lens that splits its view into zones. Two pyroelectric elements watch for a warm body crossing from one zone to the next. "Passive" means it emits nothing and only listens for heat change, which is why it is so cheap and low power. The downside is baked into the physics: it reacts to change, so a person who stops moving soon reads as an empty room.
mmWave radar transmits a millimetre-wave signal, commonly 24 GHz or 60 GHz, and listens to the echo. By comparing echoes over time it notices movements as small as a chest rising and falling, which is what lets it report presence rather than just motion. Many modules also give distance, and multi-antenna parts can estimate position and track several targets.
Wi-Fi CSI does not add a sensor. Modern Wi-Fi uses OFDM, which splits a channel into dozens of subcarriers. The receiver measures how each subcarrier was changed by the room — its amplitude and phase — for every packet. Move in the room and that fingerprint changes. Read it many times a second and you can see presence, motion and even breathing on a single link, using hardware whose first job is simply carrying your data.
03Side by side
| Property | Wi-Fi CSI | mmWave radar | PIR |
|---|---|---|---|
| Principle | Reads the Wi-Fi channel (amplitude + phase) | Active radar echo | Passive infrared heat change |
| Still person | Yes, with tuning (breathing band) | Yes, reliably | No |
| Typical range | Room-scale, link dependent | About 6–10 m indoors | About 5–12 m |
| Resolution | Coarse: presence, motion, breathing | Fine: micro-motion, distance, position | Motion only, no distance |
| Added hardware | None beyond a Wi-Fi board | Dedicated radar module | Cheap PIR module |
| Cost | Board you may already own | Several times a PIR | Very low (~$1–3) |
| Power | Wi-Fi-level; prefers wired | Tens of mA; prefers wired | Microamps; great on battery |
| Privacy | Camera-free, no image | Camera-free, no image | Camera-free, no image |
| Setup effort | High: baseline + filtering + tuning | Medium: sensitivity and zone tuning | Low: mount and go |
| False triggers from | Fans, moving curtains, Wi-Fi changes | Fans, curtains, through-wall motion | Warm/cold draughts, sunlight, pets |
04The still-person test
If you remember one thing, remember this: the hardest job for a presence sensor is a person who is not moving. It is also the job that separates these three.
A PIR simply fails it. Once movement stops, the heat change stops, and within seconds the sensor decides the room is empty. This is the famous "lights off while I am still here" problem, and no amount of tuning fixes the physics.
mmWave radar passes it cleanly. Breathing is enough micro-motion for the radar to keep reporting presence, which is why it is the default for living rooms, offices and reading nooks where people sit still.
Wi-Fi CSI can pass it, but you have to earn it. The chest motion of breathing shows up in the slow 0.1 to 0.5 Hz band of the CSI signal, so with a clean baseline and the right filtering a CSI system can hold onto a still person. That takes more engineering than bolting on a radar module, which is the honest trade: CSI adds no hardware but asks more of your software.
05A quick "pick this when" guide
- Pick PIR when targets are always moving, the budget is tight, or the sensor runs on a battery — hallways, stairs, cupboards and outdoor motion alerts.
- Pick mmWave when people sit still and detection must not drop, or you need distance and position — living rooms, offices, bathrooms and occupancy-driven climate control.
- Pick Wi-Fi CSI when you already run a Wi-Fi device in the space and want sensing with no extra sensor — a learning project, whole-room motion, or presence on an ESP32 you own, and you are willing to tune it.
- Use two together when reliability matters: a PIR for an instant, low-power "someone entered", backed by mmWave or CSI to confirm and hold presence.
For a camera-free occupancy sensor where the still-person case is central, mmWave is the safe default. For the lowest cost and effort on moving targets, PIR wins. For the most elegant answer when a Wi-Fi board is already there, CSI is hard to beat, as long as you accept the tuning work. See Wi-Fi CSI presence detection for a focused walk-through of that case.
06Combining them
These technologies are not rivals so much as a toolkit, and the strongest occupancy products often use more than one. A common pattern pairs a PIR for instant, low-power motion with a radar or CSI layer to confirm and sustain presence. The PIR wakes the system the moment someone enters; the presence layer keeps the state alive while they sit still, and lets it drop only when the room is genuinely empty.
Fusion also cuts false alarms. A fan or a swaying curtain can fool a radar or a CSI baseline, and a warm draught can fool a PIR, but they rarely fool two different physics at once. Requiring agreement between sensors, or using one to gate the other, trades a little hardware for a lot of reliability. Wiring is easy: a PIR gives a simple high or low on one pin, most mmWave modules stream data over a serial link, and CSI runs in firmware on the Wi-Fi board itself, so a single ESP32 can host and fuse all three signals in one small project.
07FAQ
Which sensor is best for detecting a person sitting still?
mmWave radar is the most reliable, because it resolves the micro-motion of breathing. Wi-Fi CSI can also hold onto a still person if it is tuned to watch the breathing band, though it takes more care. A PIR cannot, because it only reacts to a moving heat source.
Does Wi-Fi CSI need extra hardware?
Not much. CSI reuses the Wi-Fi radio you already have, so on a board such as an ESP32 you need no extra sensor at all, just firmware. mmWave needs a dedicated radar module, and PIR needs a cheap PIR module, so both add a part that CSI does not.
Which option is the most privacy-friendly?
All three are camera-free and none captures an image, so all are far more private than a camera. Wi-Fi CSI reuses a signal already present in the room, and like the others it reports coarse motion and presence rather than identity, so it is a strong choice when you want sensing without pictures.
Which presence sensor is cheapest?
PIR is by far the cheapest, often a dollar or two, and it runs for years on a battery. Wi-Fi CSI costs only the board you may already own. mmWave radar modules cost several times a PIR and usually prefer wired power, so they are the priciest of the three.