How do motion sensors detect motion?

#sensors #motionsensors #ultrasonic #hcsr501

Here’s the quick, practical rundown—by sensor type—of how motion sensors detect motion and what’s really going on inside.

1) PIR (Passive Infrared) — most common in rooms & lights

Principle: A pyroelectric element generates a tiny voltage when the infrared (heat) pattern changes across it. A Fresnel lens splits the view into zones; when a warm body moves between zones, the element sees a rising then falling IR signal.
Processing: AC-coupled, band-pass (~0.1–10 Hz) to ignore slow temperature drift and catch human motion; a comparator asserts “motion.”
Pros/Cons: Low power, cheap, doesn’t see through glass; line-of-sight only; false triggers from drafts/sunlight if aimed poorly.

2) Microwave radar (Doppler / FMCW, e.g., 5.8 GHz or 60 GHz)

Principle: Transmit CW or chirps; measure the frequency shift on the echo (Doppler: Δ𝑓≈2𝑣𝑓0/𝑐). Any moving target changes phase/frequency → motion.
Processing: Mix TX and RX to baseband (“I/Q”), filter for human-motion bands; FMCW/mmWave can estimate range and even micro-motions (breathing).
Pros/Cons: Works in the dark and through some materials (thin walls/plastic); more sensitive, but can overreach (detect motion outside a room).

3) Ultrasonic (active sonar)

Principle: Emit 40 kHz pings; watch echo time or Doppler. If distance or phase changes over time → motion.
Processing: Time-of-flight/phase comparison; threshold on change rate.
Pros/Cons: Good coverage, not light-dependent; reflections vary with fabrics/angles; drafts/fans can cause noise.

4) Vision (camera, thermal imager)

Principle: Frame differencing/optical flow—if enough pixels change between frames, flag motion. Thermal cameras do the same in long-wave IR.
Processing: Background modeling, filters, sometimes ML for people/pets.
Pros/Cons: Rich info (who/where), but power, privacy, and lighting matter (for visible).

5) Time-of-Flight (ToF) lidar / structured light

Principle: Measure distance per pixel; motion = distance map changing over time.
Processing: Per-pixel delta + clustering.
Pros/Cons: Works in the dark; short-range, costlier than PIR.

6) Magnetic/reed or Hall switches (doors/windows)

Principle: Detect magnet presence; “motion” = state change (open/close).
Pros/Cons: Binary, robust; only detects that one event.

7) Pressure mats / vibration (piezo, MEMS accelerometers)

Principle: Force or vibration change indicates footsteps or object movement.
Pros/Cons: Simple, local; prone to environmental vibrations.

Why many sensors combine tech

“Dual-tech” (e.g., PIR + microwave) requires both to trigger → far fewer false alarms (drafts trigger PIR, swaying plants trigger radar, but rarely both).

Output & integration (what you get electrically)

Digital: open-collector/relay or logic “motion” pin (PIR modules like HC-SR501; radar RCWL-0516).
Analog: some give I/Q or amplitude for custom processing (mmWave, ultrasonic).
MCU readout: poll a GPIO or use interrupts; add debounce/hysteresis (e.g., require signal > threshold for 100–300 ms).

Placement tips to reduce false triggers