Both mmWave and PIR sensors detect presence, but they work on completely different principles, and the difference matters for how reliably they perform.
PIR (Passive Infrared) sensors detect changes in infrared heat radiation, essentially noticing when a warm body moves across their field of view. They're cheap and reliable for basic motion triggers, but they only detect movement. A person sitting still at a desk can fall out of a PIR sensor's detection and the lights will turn off.
mmWave (millimeter wave) sensors send out radio waves and measure the reflections, which lets them detect presence even when a person isn't moving, including breathing and micro-movements. This makes them far better for applications like keeping lights on in a room where someone is sitting still, reading, or sleeping.
The tradeoff is cost and complexity: mmWave sensors are more expensive and can occasionally be more sensitive to interference than a simple PIR unit.
FAQ
Which one should I use for a bathroom or study where people sit still?
mmWave. PIR sensors will time out and turn lights off on someone who isn't moving.
Is mmWave overkill for a hallway or entryway?
For simple pass-through detection, PIR is usually sufficient and cheaper. mmWave earns its cost in rooms where stillness matters.
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