Introduction
When you need to detect motion or presence, you have several technology choices. Each has strengths and weaknesses. This article compares PIR (Passive Infrared), microwave Doppler, and mmWave radar to help you choose.
PIR (Passive Infrared)
How it works: Detects changes in infrared radiation emitted by warm objects (like humans). Passive – no energy emitted.
Pros:
- Low cost ($2-10)
- Low power (µA to mA)
- Simple interface (digital output)
- No licensing required
Cons:
- Only detects moving warm objects (not stationary people)
- Can be fooled by heat sources, sunlight, pets
- Limited range (typically 5-10m)
- Doesn’t work through glass or walls
- Blind spots due to lens pattern
Best for: Indoor occupancy detection, lighting control, security alarms, battery-powered devices.
Microwave Doppler (e.g., RCWL-0516, HB100)
How it works: Emits microwave signals (typically 5.8GHz or 10GHz) and detects frequency shift (Doppler) from moving objects.
Pros:
- Detects motion through non-metal materials (plastic, drywall, glass)
- Broader detection area
- Not affected by heat or light
- Can detect very small movements
Cons:
- Higher power consumption (tens of mA)
- May cause interference with other devices
- Can penetrate walls – may detect motion in adjacent rooms (false triggers)
- Regulatory restrictions on frequency bands
- Only detects motion (not presence)
Best for: Through-wall detection, outdoor lighting, automatic doors.
mmWave Radar (e.g., HLK-LD2410, LD2450)
How it works: Uses frequency-modulated continuous wave (FMCW) radar at 24GHz or 60GHz to detect both motion and stationary presence by measuring distance and micro-movements (breathing).
Pros:
- Detects stationary people (presence sensing) via breathing
- Can measure distance, direction, and speed
- Works through thin materials (plastic, fabric)
- Unaffected by heat, light, air currents
- High sensitivity and resolution
Cons:
- More expensive ($10-30)
- Higher power consumption (tens to hundreds of mA)
- Complex configuration (UART commands)
- Potential interference in dense deployments
- Requires careful antenna design
Best for: Smart home presence detection (lights that stay on while you’re sitting), people counting, gesture recognition.
Comparison Table
| Feature | PIR | Microwave | mmWave |
|---|---|---|---|
| Cost | Low | Low | Medium |
| Power consumption | Very low | Medium | Medium-High |
| Through-wall | No | Yes | Limited |
| Stationary detection | No | No | Yes |
| Immunity to heat/light | Poor | Good | Excellent |
| Detection range | 5-10m | 10-15m | 5-10m |
| Complexity | Simple | Simple | Complex |
Choosing the Right Technology
- Simple motion detection (lights, alarms) indoors: PIR is cheapest and easiest.
- Through-wall or outdoor detection: Microwave.
- Presence sensing (stay on when seated): mmWave.
- Battery-powered IoT sensors: PIR (low power).
- Advanced applications (people counting, gesture): mmWave.
Dual-Technology Sensors
Many commercial motion detectors combine PIR and microwave (dual-tech) to reduce false alarms: both must trigger to activate. This offers the best of both worlds but increases cost and complexity.
Future Trends
mmWave is rapidly dropping in price and becoming the go-to for smart home presence. However, PIR remains dominant for simple, low-power applications. Expect more integration and hybrid sensors in the coming years.