mmWave Radar vs PIR: Choosing a Presence Sensor

"The lights went off while I was reading" is the complaint that sells mmWave sensors, and it is not the PIR's fault — the two technologies answer different questions. A PIR answers did something move?; a radar answers is something there? This index documents 4 radar and 3 PIR pairings, so the comparison below links to real hardware rather than generalities.

How each one senses

• PIR (passive infrared) watches for changes in heat across a segmented lens. Cheap, tiny, near-zero power — and blind to anything that holds still. It cannot see through glass and is genuinely passive: it emits nothing.
• mmWave radar transmits at 24 GHz and reads the reflections, detecting movement down to breathing depth. It holds presence on a motionless person, sees through thin partitions and enclosures, and reports distance — some models track multiple targets with coordinates. It costs more, draws more power, and needs placement thought.

The radar options in this index

The LD2410 gives presence and distance on a budget; the LD2450 adds multi-target X/Y coordinates for zone automation; the SEN0395 is the long-range single-zone workhorse; the Aqara FP2 is the packaged, no-soldering option with in-app zones.

The PIR options

Choosing, room by room

• Hallways, stairs, pantries — PIR. Motion is the event you care about, latency matters, stillness never happens.
• Offices, reading corners, bedrooms, bathrooms — mmWave. Long stretches of stillness are exactly the case PIR cannot hold.
• Outdoors — PIR, reluctantly: radar sees swaying branches and rain as motion. Pick a PIR with adjustable sensitivity.
• Shared walls — remember radar penetrates thin partitions; aim it away from the neighbour's corridor or fence it with zones.

All of these and their siblings live in the Presence & Motion Sensors category, which compares every documented option in one table.