Skill: PIR Sensors — What They Are & How They Work
Skill level: 3
Wiring
Learning Objective:
Understand the function of a PIR (Passive Infrared) sensor — how it detects movement, where it’s used in gate systems and security, and how to install and test it correctly.
1. What Is a PIR Sensor?
A PIR (Passive Infrared) sensor detects movement by sensing changes in infrared (IR) radiation, which is emitted as heat by people, animals, and vehicles.
Unlike active sensors, it does not emit a signal — it passively observes the environment.
When a warm object moves across its detection zones, the sensor recognises the change in IR energy and produces an output (often a relay contact).
PIRs are widely used because they are low power, reliable, and simple to integrate with gates, lighting, and alarms.
2. Where PIR Sensors Are Used
Driveway approach detector — Open gates automatically when someone approaches
Lighting control — Switch on lights when movement is detected
Intruder alarms — Trigger alarms when motion occurs in protected zones
Intercom systems — Wake up a screen when a visitor approaches the gate post
📌 PIRs are often the first layer of detection, giving early warning or convenience functions.
3. How a PIR Sensor Works
Inside the PIR are two halves of an infrared-sensitive element.
A Fresnel lens focuses IR radiation into these halves, splitting the detection area into multiple zones.
When something warm moves across the zones, the difference in detected IR between the two halves is measured.
The circuit interprets this as “motion” and switches the output, usually via a relay contact (NO/NC).
PIRs respond better to side-to-side movement across their field of view rather than straight towards them.
4. Adjustable Settings
Most PIRs allow fine-tuning for different environments:
Time delay — How long the relay stays on after detection (5 seconds up to several minutes).
Sensitivity — Adjusts how easily it triggers, useful in windy or high-traffic areas.
Detection range — Some allow altering the effective distance, e.g. 5–15 metres.
Lux control (lighting PIRs) — Ensures the PIR only triggers in low-light conditions.
Correct adjustment avoids nuisance activations and ensures reliable performance.
5. Wiring and Power
Typical PIR wiring is simple but varies slightly by manufacturer:
Red/Brown = +V power (usually 12–24V DC, sometimes 230V AC for lighting types).
Blue/Black = 0V / neutral.
Yellow/White = Output (relay contact to gate input, lighting trigger, or alarm).
Outputs are typically volt-free relay contacts (NO/NC), making them versatile for different systems.
📌 Always check the label or datasheet before wiring — misconnection can damage the PIR or control board.
6. Installation and Testing
Height: Mount between 1.8–2.4m for optimal coverage.
Positioning: Avoid aiming at trees, roads, or reflective surfaces.
Sealing: Use glands or silicone on cable entries to prevent water ingress.
Labelling: Mark cables for future maintenance.
Testing procedure:
Power the unit.
Walk across the detection zone — listen for the relay click or watch the indicator LED.
Verify the output with a multimeter or directly on the control system.
Adjust sensitivity or delay to fine-tune operation.
7. Safety and Good Practice
PIRs often run at low voltage, but some lighting types operate at 230V AC — confirm supply before working.
Always isolate and prove dead before wiring (see SSOW for electrical isolation procedures).
Seal all enclosures to prevent water ingress — moisture is a common cause of false triggering.
Avoid routing PIR cables near high-voltage lines to reduce electrical noise pickup.
Test thoroughly after installation to ensure reliable detection.
For security applications, always use PIRs as part of a wider system (e.g. beams, cameras) — never rely on a single device.