Introduction
Energy harvesting – powering devices from ambient energy (light, vibration, heat) – is becoming practical. Ultra-low power PIR sensors are key enablers for maintenance-free wireless motion sensors.
The Power Challenge
A typical wireless sensor must operate for years on a small battery or harvested energy. Every microamp counts. Standard PIR modules (HC-SR501) draw 50-65 µA – too high for many energy-harvesting designs.
Ultra-Low Power PIR Sensors
Modern PIR sensors can operate at astonishingly low currents:
| Sensor | Current | Voltage |
|---|---|---|
| Panasonic EKMB (1µA type) | 1 µA | 2.3-4.0V |
| Excelitas PYD 2597 | 2 µA | 1.4-3.6V |
| AM312 | 35 µA | 2.7-12V |
Energy Harvesting Sources
Solar (Indoor Light)
Indoor solar cells can generate 10-100 µA under typical office lighting. Combined with a 1 µA sensor, this is sufficient.
Thermoelectric (Body Heat)
A thermoelectric generator on a warm pipe or human body can generate microwatts to milliwatts.
Vibration (Piezoelectric)
Vibration energy harvesters can power intermittent operation.
RF Harvesting
Ambient RF energy (WiFi, cellular) can provide nanowatts to microwatts – enough for wake-up but not continuous operation.
System Architecture
An energy-harvesting PIR sensor typically includes:
- Energy harvester (e.g., solar panel).
- Power management IC with maximum power point tracking.
- Energy storage (capacitor or thin-film battery).
- Ultra-low power PIR sensor (1-2 µA).
- Microcontroller with deep sleep modes.
- Wireless transmitter (e.g., BLE, LoRa, Sub-GHz).
Duty Cycling and Wake-up
The sensor runs continuously at 1 µA. The microcontroller sleeps (µA or nA) and is woken by the sensor’s interrupt output only when motion occurs. This keeps average power extremely low.
Example: Solar-Powered Motion Sensor
A solar-powered sensor using:
- Panasonic EKMB (1 µA)
- nRF52 microcontroller (2 µA sleep)
- Solar cell (50 µA average in office light)
- Supercapacitor (1F) for storage
Can operate indefinitely without battery replacement, transmitting motion events via BLE.
Design Considerations
- Cold start: System must start from zero stored energy.
- Storage sizing: Must survive periods of low energy (night).
- Efficiency: Every conversion stage loses power.
- Transmit power: Most energy is used for wireless transmission – minimize frequency and duration.
Commercial Examples
- EnOcean: Wireless sensors using energy harvesting, some with PIR.
- Everspring: Solar-powered motion sensors.
- Panasonic: Energy-harvesting PIR sensors (EKMB series).
Conclusion
With 1-2 µA PIR sensors, energy harvesting becomes practical for motion detection. These sensors enable truly maintenance-free IoT devices.