Key Differences Between PIR and Active Infrared Sensors

When selecting sensors, many engineers and developers often treat "infrared sensor" as a general term, ignoring the fundamental differences between PIR (passive) and active infrared sensors (barrier or reflective) in principle, application, and performance. This confusion can lead to wrong product selection, high false alarms, or functionality issues. Nexisense, specializing in infrared detection technology, provides complete solutions for both types. This article systematically compares physical principles, detection targets, advantages, disadvantages, and applications.

PIR Sensor (Passive)

The PIR sensor, or passive infrared sensor, does not emit any energy and relies entirely on natural infrared radiation emitted by objects.

Operating Principle

It is based on the pyroelectric effect. Inside, the sensor uses pyroelectric crystals (e.g., modified lithium niobate or PZT ceramics) that generate electrical signals when exposed to changing far-infrared radiation (8–14 μm, mainly emitted by ~37°C human body). The signal is converted and amplified through an internal JFET.

The key characteristic is that it is sensitive only to the rate of temperature change: a stationary heat source does not produce output; only moving targets generate detectable alternating signals.

Typical Structure and Enhancements

• Dual or quad complementary design: crystals with opposite polarity in series cancel slow environmental temperature changes and enhance dynamic movement signals.

• Fresnel lens: divides the detection area into alternating light and dark zones; a moving person generates continuous pulses, easily detected by the circuit.

Main Features

• Completely passive, very low power consumption (static current in microamps).

• Detects only moving warm objects (humans, animals).

• Output typically digital (high/low); some digital models support I²C or custom protocols.

• Does not distinguish object material, only detects thermal changes.

Typical Applications

Automatic corridor or staircase lighting; human intrusion detection in security systems; automatic AC or TV activation in smart homes; people monitoring in warehouses or parking lots.

Common Infrared Sensors (Active)

Refers mainly to barrier and reflective infrared sensors that actively emit infrared light.

Operating Principle

Composed of an IR LED (emitter, ~940nm) and phototransistor or PIN diode (receiver).

• Barrier type: emitter and receiver separated; blockage of the beam triggers output change.

• Reflective type: emitter and receiver integrated; reflected light from objects triggers the output.

They do not rely on temperature, only on obstruction or reflection changes.

Main Features

• Active emission, higher power consumption (LED continuous or pulsed).

• Detects any opaque object (humans, vehicles, boxes, metals).

• Output can be digital or analog (reflection intensity can estimate distance).

• Poor detection of transparent or highly absorbent black objects.

• Very fast response (microseconds), suitable for high-speed counting or precise positioning.

Typical Applications

Counting objects on production lines; detecting material position; obstacle avoidance for cleaning robots or automatic doors; paper detection in printers; elevator door safety; remote control signaling.

Core Differences Comparison

Nexisense Applications

Nexisense PIR series (GDA223-F digital, HDA-243A analog) optimize anti-interference algorithms and lens matching to reduce false alarms. Active IR sensors offer reliable barrier/reflective modules with industrial protection and long-range capabilities. Both support standard protocols such as Modbus-RTU.

FAQ

1. Why doesn't a PIR sensor detect stationary people? It responds only to the rate of temperature change; stationary heat sources produce no signal.

2. Can an active IR sensor detect transparent glass? Usually not; glass transmits near IR, so the beam is not effectively blocked.

3. Can pets trigger a PIR sensor? Normal lenses may trigger it; pet-immune lenses (~1 m blind zone) are recommended.

4. Power consumption difference? PIR is microamps, suitable for battery; active IR continuously emits light, consuming tens to hundreds of times more.

5. Does sunlight affect PIR sensors? Slow changes are canceled by dual-element design; fast shadows may cause false alarms, algorithmic filtering helps.

6. Active IR detection range? Barrier: tens of meters; reflective: centimeters to meters depending on power and environment.

7. Can PIR measure temperature? No, it only signals motion; temperature measurement requires a thermopile.

8. Can both sensor types be combined? Yes, PIR for human presence, active IR for precise location or counting.

9. Do Nexisense PIR sensors support digital output? Yes, GDA series with I²C or Modbus interface.

10. Which is better for smart homes? PIR for occupancy and security (low power, discreet); active IR for door/window state detection.

Conclusion

PIR and active infrared sensors, although both infrared, differ fundamentally in principle, target, and application: PIR passively detects moving heat sources and excels at living being detection, while active IR emits light for object detection, offering broader versatility. Nexisense provides both types for flexible selection, enabling accurate and reliable perception systems in IoT and smart control environments.