Presence vs Motion: What PIR Sensors Miss at Night
Key numbers
- 24 GHz
- mmWave frequency the LD2410 radar in RoomGuardian V2 operates at
- 38 °C
- ambient temperature above which PIR sensitivity drops in Kolkata summers
- 2
- sensor layers per room — PIR AM312 for fast entry detection, LD2410 for sustained presence
- 0
- internet connections required for occupancy-based automations to work
One of the more common Home Assistant frustrations in Indian flats: the motion sensor keeps the AC running when the room is empty, or turns off the light when you are reading in bed. Both failures have the same cause — a PIR sensor reads motion, not occupancy. This guide explains the difference, why it matters in practice, and how RoomGuardian V2 (forthcoming from GrihaTEK) uses both sensor types to address it.
What a PIR sensor actually measures
PIR stands for passive infrared. The sensor contains a pyroelectric element that responds to changes in infrared radiation — specifically, heat patterns moving across its detection zone. A warm body moving through the zone changes the infrared gradient and fires the sensor. That word — moving — is the constraint. The PIR AM312, which RoomGuardian V2 uses, triggers on movement. If you are sitting still at your desk, or lying in bed at 1am, the infrared pattern across the sensor is static and the sensor reads zero. In a consumer motion sensor this is usually intentional. The sensor consumes a fraction of a milliwatt and handles entry/exit use cases correctly. For a hallway light or a security trigger, PIR is the right tool. For room-level occupancy — whether a person is present, regardless of activity — it is the wrong tool.
What mmWave radar measures instead
The LD2410, the other sensor in RoomGuardian V2, uses millimetre-wave radar. It emits a low-power 24 GHz radio signal and analyses the reflected return. Because it is active (emitting), it does not depend on temperature differential or ambient infrared. It reads the Doppler shift of the reflected signal — the tiny frequency change caused by any movement in its field, including breathing. A person asleep or reading in bed causes a consistent micro-Doppler return from the rise and fall of their chest. The LD2410 sees this as occupancy. The PIR AM312 sees nothing. The two sensors are complementary rather than redundant: both trigger when someone walks through a room, but only the LD2410 holds the occupied state when a person is seated still or asleep.
Why Indian conditions make this more relevant
Three factors specific to Indian homes amplify the practical cost of using PIR alone for occupancy.
Split AC control
Many Home Assistant setups use IR blasters to control split units. The automation is typically: turn AC on when motion detected, off after N minutes of no motion. With a PIR sensor, the timer restarts every time the person moves, but not when they are asleep. The AC turns off in the first quiet hour.
Power cuts
When grid power fails, state-recovery on restart matters. An AC that was on before the cut should come back on after it. PIR-based occupancy checks fail here because there is no motion at the moment of restart. Reliable presence data — held across the cut — produces better restart decisions.
Humidity and temperature swings
PIR sensors show reduced sensitivity in very high ambient temperatures: the body's thermal signature drops relative to a hot background. Kolkata summers, with ambient temperatures above 38 °C, push some PIR elements toward their detection limits. mmWave is immune to ambient temperature.
How RoomGuardian V2 uses both
RoomGuardian V2 includes the PIR AM312 and the LD2410 as distinct sensing layers. The PIR handles event detection — someone entered, something moved — with low latency and near-zero power draw. The LD2410 handles sustained presence detection for occupancy state. The firmware design, in progress as of August 2026, combines the two into a single room-state signal: occupied, unoccupied, or uncertain. The uncertain state deliberately delays any occupancy-based automation rather than making the wrong call. RoomGuardian V2 is forthcoming. GrihaTEK is in the hardware fabrication phase for the rev B PCB as of August 2026. The unit is designed for builders and facility managers, not individual consumers.
Frequently asked questions
- What is the difference between a PIR sensor and mmWave presence radar?
- PIR detects movement by sensing changes in infrared heat across its field. mmWave radar detects presence by measuring Doppler shifts in reflected radio signals. PIR misses stationary occupants; mmWave does not.
- Can a motion sensor tell if a person is asleep in a room?
- A PIR-only motion sensor cannot reliably detect a sleeping person. mmWave presence sensors, which measure micro-movements including breathing, can. RoomGuardian V2 includes both for this reason.
- Why do smart home motion sensors turn off lights when someone is sitting still?
- Most consumer motion sensors use PIR, which requires physical movement to trigger. If you are stationary for longer than the sensor's detection window, it reports no occupancy. The solution is to combine PIR with a presence sensor that holds the occupied state without requiring motion.
- Do PIR sensors work reliably in hot Indian summers?
- PIR sensitivity decreases as the ambient temperature approaches body temperature, because the thermal contrast the sensor depends on is reduced. In rooms above 38 °C, PIR false-negatives become more common. mmWave does not have this limitation.
- Is RoomGuardian V2 available to buy?
- No. RoomGuardian V2 is forthcoming — GrihaTEK is currently in the hardware fabrication phase. The unit is being designed for builders, housing developers, and facility managers who want pre-installed multi-sensor room monitoring.