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PIR and WiFi CSI both detect people without cameras. They measure completely different physical quantities, and each excels in different scenarios.

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Sensing mechanism

PIR detects changes in infrared radiation within a ~120 degree cone. A motionless person emits constant IR: no change, no trigger. After a timeout, the sensor reports empty. This is the “lights off while I am sitting still” failure mode, a fundamental property of differential IR sensing, not a firmware bug. WiFi CSI detects changes in radio channel state across the whole room. A still person modulates phase via breathing (micro-Doppler at 0.1 to 0.5 Hz). Macro-motion variance may be near zero while periodic phase modulation confirms presence.

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Coverage geometry

One PIR per zone, aimed carefully. Two CSI nodes cover a room regardless of furniture layout.

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Event semantics

PIR emits motion pulses: edge triggers when IR changes. Automations must infer presence from motion timeouts, which fail for still occupants. CSI supports presence holds: a sustained occupied state while a person breathes in the space. See smart-home automation for event design guidance.

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Cost and power

PIR costs 2to2 to 5, draws microwatts in standby, and achieves decades of battery life. It is the cheapest motion trigger available. CSI costs 5to5 to 10 per ESP32 node, draws milliwatts (needs power), but provides presence holds that PIR cannot.

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When to choose which

PIR: cheapest motion trigger, outdoor lighting, zones where still-detection does not matter. WiFi CSI: occupancy-aware HVAC, presence-based automation, elder-care awareness, anywhere “is someone here” matters more than “did something move.”