PIR Sensor Wiring & Positioning: Detection Zones, Override Switches & False Triggers

Quick Answer: A PIR (Passive Infrared) sensor detects movement by sensing infrared radiation changes across its detection zone. For a 2-wire PIR replacing a switch, the sensor acts as an automatic switch in the live. For a 3-wire sensor with a constant live, a neutral is needed. Avoid aiming sensors at heat sources, windows, or vents to prevent false triggers.

Summary

PIR sensors are used for security lighting, automatic room lighting, and intruder alarm systems. For domestic and commercial lighting, the sensor replaces or supplements the manual switch — it switches the live to the light on detection and holds it on for an adjustable time period before switching off. They are cost-effective, energy-saving, and increasingly expected in commercial properties.

Wiring PIR sensors is straightforward but there are common mistakes: using the wrong type for a given wiring configuration, mounting in positions that guarantee false triggers (opposite a radiator, beside a window, in a drafty corridor), and not providing an override facility for applications where constant illumination is sometimes needed.

There are two main wiring types: the 2-wire PIR (effectively a smart switch that needs only L and switched-L, no permanent neutral) and the 3-wire PIR (needs L, N, and switched-L for the load). Understanding which you have before you start avoids wiring errors and trips to the supplier.

Key Facts

• PIR principle — Detects change in infrared (heat) radiation across two halves of the sensor lens; movement creates a temperature differential
• 2-wire PIR — Connects in series with the live; no neutral required; suitable for replacing standard switches in existing 2-core wiring
• 3-wire PIR — Requires permanent live, neutral, and switched live; common for wall/ceiling sensors with more features
• Detection range — Typically 5-12m for domestic PIRs; commercial sensors up to 20m
• Detection angle — Typically 100-180° horizontal; 90-110° vertical depending on sensor type
• Ceiling mount detection radius — Varies by height; a sensor at 2.4m ceiling with 120° angle covers approximately 6m diameter
• Lux level adjustment — Most PIRs have a daylight sensor (LDR) and a sensitivity threshold to avoid triggering in daylight
• Time delay adjustment — Adjustable hold-on time, typically 5 seconds to 10 minutes
• Sensitivity adjustment — Some sensors have a range/sensitivity control
• Override function — Double-click or constant activation allows manual override to hold lights on continuously
• Switch override — A conventional switch in parallel with the PIR output allows manual override
• Maximum load — Check manufacturer specs; most domestic PIRs are rated for incandescent/LED loads (100-1000W resistive)
• LED compatibility — Minimum load of some PIRs can cause LED flicker; check compatibility
• Temperature range — Standard PIRs operate from -20°C to +50°C; check for outdoor installations
• Outdoor IP rating — Minimum IP44 for external use; IP65 for exposed locations

Quick Reference Table

Detailed Guidance

Wiring a 2-Wire PIR Sensor

A 2-wire PIR simply replaces a standard light switch in the switch position. The existing wiring from the consumer unit to the switch and then to the light fitting remains unchanged.

Existing circuit to switch:

• Brown (permanent live) comes from consumer unit
• Blue/grey (switched live) returns to light fitting

Connection at PIR back box:

• Brown (permanent live) → Live terminal on PIR
• Blue/grey (switched live) → Load terminal on PIR

The PIR completes the circuit internally when it detects movement — just like a switch closing.

Important: Check the PIR's minimum load. Many 2-wire PIRs need a minimum load (often 5-25W) to power their internal electronics. With very efficient LED fittings, this minimum load may not be met, causing the LED to flicker dimly when the PIR is in standby. Solutions: (a) use a 3-wire PIR with a permanent neutral, (b) add a parallel resistor (bleeder resistor), or (c) check the PIR manufacturer's LED compatibility list.

Wiring a 3-Wire PIR Sensor

A 3-wire PIR requires a permanent live and neutral at the sensor, plus the switched live output:

This requires either 3-core cable run from the light fitting (where both live and neutral are available) or a new run from the consumer unit. It avoids the minimum load problem since the sensor is independently powered.

Wiring with a Manual Override Switch

Many applications need the ability to hold the light on regardless of PIR activity (meeting rooms, storage areas where someone may be stationary). Two methods:

Method 1 — Parallel switch: Wire a conventional switch in parallel with the PIR's switched live output. When the switch is closed, the light is permanently on. When open, the PIR controls the light. Simple and reliable; the switch is clearly labelled "hold on."

Method 2 — Double-click override (integral): Many modern PIRs have an integral override: quickly switching the supply off and on twice activates a hold-on mode, switching off and on again to cancel. This works without any additional wiring. Check the sensor manual for the specific activation sequence.

Method 3 — Bypass switch: A 2-way switch circuit where one position bypasses the PIR entirely (direct connection) and the other routes through the PIR. More complex wiring but gives cleaner control.

Detection Zone Planning

Getting the detection zone right is essential:

Room coverage:

• A single sensor in the centre of the ceiling using a 360° lens covers a circular area equal to approximately (sensor height × 2) in diameter — so a sensor at 2.4m covers roughly 5m diameter
• For rectangular rooms, two sensors at opposite ends pointing toward the centre may give better coverage
• Aim to have direct line of sight to all areas where occupants typically stand or sit

What triggers a PIR:

• A warm body moving across the detection zone (temperature differential between body and background)
• The sensor is most sensitive to movement across its beam, less sensitive to movement directly toward or away from it

False triggers — common causes:

Commercial Occupancy Sensors

In commercial buildings, PIR sensors for lighting control must comply with Building Regulations Part L (Conservation of Fuel and Power), which requires presence detection and automatic switch-off in offices, storage areas, and similar spaces.

Commercial sensors often:

• Include ultrasonic or microwave technology alongside PIR for better detection of stationary occupants (dual-technology)
• Link to DALI or KNX building management systems
• Feature adjustable hold-on times of up to 30 minutes
• Provide daylight dimming as well as on/off control

Outdoor PIR for Security Lighting

For external security lighting:

• Use a PIR rated IP44 minimum; IP65 for fully exposed locations
• Angle the sensor to cover the approach paths, not public roads or neighbouring gardens
• Set the lux threshold so the light only activates at night (most outdoor PIRs have this)
• Set hold-on time to 1-3 minutes (shorter for pathways, longer for driveways)
• Consider dual-technology (PIR + microwave) sensors to reduce false triggers from windblown vegetation
• All outdoor lighting circuits must have 30mA RCD protection (see outdoor electrics)

Frequently Asked Questions

My PIR light stays on permanently — what's wrong?

Three likely causes: (1) The sensitivity is set too high, picking up heat from a radiator or sunlight — reduce sensitivity or reposition; (2) The time delay is set to maximum — reduce the hold-on time; (3) The sensor is stuck in override mode — check the manual for the reset procedure (usually cycling power). If it's a new installation, check the wiring — a short between Live and Load terminals would hold the light on permanently.

Can I use a standard PIR with LED lights?

Usually yes, but check the minimum load specification. Many PIRs require a minimum 5-25W load to operate correctly. Modern high-efficiency LED fittings might not meet this threshold. Symptoms include LED flickering when PIR is in standby mode. Solution: use a PIR specifically rated for LED loads (many modern units are), or use a 3-wire sensor that doesn't rely on the load circuit for its own power.

Where should I not put a PIR?

Avoid: directly facing a window (sunlight and external heat changes cause false triggers), above or near a radiator or convector heater, in the path of air conditioning vents, directly facing a corridor where it will trigger from distant movement unnecessarily, and in positions where it will detect movement in an adjacent room through thin walls (microwave sensors are particularly susceptible to this).

What's the difference between a PIR and a microwave sensor?

A PIR detects infrared (heat) movement — it requires line of sight and detects best when movement crosses its field of view. It doesn't penetrate walls. A microwave sensor emits microwaves and detects the Doppler shift from moving objects — it doesn't need line of sight, works well for slow movement, and can detect through thin partitions (which can cause false triggers from adjacent spaces). Dual-technology sensors combine both, requiring both to trigger simultaneously — this significantly reduces false activations.

Regulations & Standards

• BS 7671:2018+A2:2022 — Wiring Regulations; all fixed wiring must comply

• Building Regulations Part L — Conservation of fuel and power; requires automatic lighting controls in commercial buildings

• Part P of the Building Regulations — Any fixed wiring in a dwelling is notifiable work; competent person schemes apply

• IET Wiring Regulations 18th Edition — Governing standard for fixed electrical installation

• Electrical Safety First Technical Guidance — Consumer and trade guidance

• CIBSE Guide LG9 — Lighting guide for offices; includes occupancy sensor guidance

• outdoor electrics — RCD protection and IP ratings for outdoor sensor installations

• smart home wiring — Integrating sensors with smart home systems

• cable sizing — Cable sizing for lighting circuits