Lighting Controls: PIR Sensors, Time Switches, and Dimmer Modules — An Electrician's Installation Guide

Lighting controls reduce energy waste, improve occupant comfort, and increasingly form part of Building Regulations compliance obligations. For UK electricians, understanding PIR occupancy sensors, time switches, photocell controls, and dimmer modules is essential work — from domestic loft conversions and stairwells to commercial corridor and office fit-outs. This guide covers selection, wiring, BS 7671 requirements, and common installation pitfalls across all mainstream control types.

Why Lighting Controls Matter for Tradespeople

Part L of the Building Regulations (Conservation of Fuel and Power) requires that new dwellings and certain extensions include lighting controls that automatically limit energy use. In non-domestic settings, CIBSE SLL Code for Lighting and Part L2A/L2B impose similar obligations — including automatic occupancy control in areas where manual switching is impractical. Beyond compliance, correctly selected controls reduce the risk of a client ringing back six months later complaining about a £400 electricity bill because the stairwell light has been burning 24 hours a day.

From a commercial standpoint, fitting lighting controls also represents upsell revenue per job. A PIR sensor and corridor LED batten combination is a straightforward add-on for most first-fix and second-fix electricians.

PIR Occupancy and Presence Detectors

How PIR Detection Works

Passive infrared (PIR) sensors detect movement by sensing changes in infrared radiation emitted by warm bodies (primarily humans) moving through their field of view. A fresnel lens focuses heat signatures onto a pyroelectric sensor, which generates a voltage when the infrared pattern changes. "Passive" means the sensor does not emit any radiation itself — it only detects incoming IR.

A key distinction exists between occupancy detectors and presence detectors:

• Occupancy detector (PIR): Detects gross movement — walking, arm sweeping. Suitable for corridors, stairwells, toilets. Will switch off if occupant sits still at a desk for >5 minutes.
• Presence detector (microwave or dual-technology): Detects minor movement — typing, writing, breathing at rest. Required for offices, meeting rooms, classrooms. Uses Doppler microwave or combined microwave+PIR to prevent false switch-off.

For domestic work, PIR is almost always sufficient. For commercial offices, presence detection is the correct specification.

Coverage Patterns and Mounting Heights

PIR sensors are rated by detection angle and mounting height range:

• Ceiling-mount 360° sensor: Typical coverage 6m × 6m at 2.5m height. Used in square rooms, open-plan areas.
• Wall-mount 180° sensor: Coverage 12m × 12m at 2.5m height. Used in corridors, near doors.
• Corridor sensor (narrow angle, 15° × 120°): Detection up to 12m down a 1.2m-wide corridor from a ceiling-mount position.
• High-bay sensor: Rated to 8–10m mounting height for warehouse and industrial applications.

A common mistake is mounting a ceiling PIR directly above the entry point of a room — by the time an occupant walks under it, the sensor has not yet detected them and the light is off. Mount sensors so the detection zone covers the likely entry path from 2–3m away.

Detection is strongest for movement across the sensor's field of view (transverse movement) and weakest for movement towards the sensor (radial movement). Position accordingly — in a corridor, mount on the end wall facing down the corridor, not on the side wall.

Hold-On Timer and Lux Level

Most commercial PIR sensors include two adjustments:

• Time delay (hold-on timer): How long the light stays on after the last detected movement. Domestic: typically 1–10 minutes. Commercial corridors: 3–5 minutes. Toilets: 5–10 minutes.
• Lux level (ambient light threshold): The sensor will not switch the light on if ambient light is already above this level. Set correctly, it prevents lights activating in daylight. Set too high, lights never come on; too low, lights activate unnecessarily at dusk.

Wiring PIR Sensors — Switched Live Method

Most standalone PIR sensors interrupt a switched live, not the neutral. The wiring method depends on whether the sensor has a permanent live terminal:

Two-wire PIR (series wired): Connected in-line with the switched live only. Internal power drawn from supply current. Cannot be used with LED loads below ~25W as insufficient current flows to power the sensor — will cause flickering or failure to switch off. Always check the minimum load specification.

Three-wire PIR (separate line/neutral/load): Has independent permanent live and neutral connections for internal power, plus a switched load output. Suitable for any load size including low-power LEDs. Preferred for commercial and any LED installation. Terminal labelling: L (permanent live), N (neutral), COM (switched output) — though manufacturers vary.

Under BS 7671 (18th Edition, Regulation 559.6.1.5.4), lighting points wired using the current-carrying conductor method (loop-in ceiling roses) must provide a switched live and neutral at the fitting position. Most modern luminaire wiring already provides this.

Load Types and Compatibility

PIR sensors are rated by load type and maximum wattage:

Load Type	Typical Max Load	Notes
Resistive (incandescent, halogen)	1000–2000W	Easiest load — not commonly encountered now
LED (non-dimmable)	200–400W	Check minimum load; electronic PSU inrush can be high
LED (dimmable)	Requires compatible dimmer output PIR	Specialist product; most PIRs are on/off only
Fluorescent (capacitive)	Check rating — often derated	High inrush; verify sensor switching capacity
Motor	Not suitable — use dedicated controls	High inrush damages PIR contacts

Always verify the manufacturer's load rating matches the luminaire type. A sensor rated "400W LED" is not interchangeable with one rated "400W total" — fluorescent capacitive loads may derate capacity significantly.

Time Switches

Mechanical vs Electronic Time Switches

Time switches interrupt a circuit at pre-programmed on/off times. Two main types:

Mechanical (pin-type) time switches: 24-hour rotating drum with push-in or clip-in pins at 15- or 30-minute intervals. Simple and reliable, with battery backup on better models. Accuracy ±15 minutes per day (acceptable for security lighting). No programming interface — physical pin manipulation only. Common brands: Hager, Legrand, BG. Suitable for simple flood lighting, overnight security circuits.

Electronic (digital) time switches: LCD display with pushbutton or touchscreen programming. 7-day (individual day), 5/2-day (weekday/weekend), or 8-bank programmable. Can store 8–16 on/off events per day. Accuracy ±1 second per day (quartz crystal). Battery-backed memory retains programme through power cuts. More suitable for commercial applications, access control circuit timing, and HVAC plant sequencing.

Astronomical (astro) time switches: Programmed with geographic coordinates (latitude/longitude) to calculate local sunrise and sunset. Switch relative to these times (e.g., "on at sunset, off at sunrise" or "on 30 minutes after sunset"). Eliminates the need for seasonal manual adjustment. Preferred for external lighting and street furniture circuits. Most major DIN-rail mount manufacturers offer astro variants at modest price premium.

Wiring Time Switches

DIN-rail mounted time switches (the most common type for commercial panels) are wired with permanent live to a supply terminal and switched live output to the load. They require a neutral for their own electronics. Terminal designations vary by manufacturer:

• Hager: 1 = supply L, 2 = supply N, 3 = load switched output
• Legrand: A1 = supply L, A2 = supply N, 1 (or 11) = NO output, 2 (or 12) = NO+NC common

Always consult the terminal diagram inside the product lid or on the data sheet. Many time switches have separate NO and NC outputs — use the normally-open (NO) output for lighting unless the application requires fail-safe energised operation (uncommon in lighting).

Standalone socket-outlet time switches for plug-in loads work differently — the socket face is always live; the time switch only controls which on/off programme is active. These do not require electrical installation work and are outside BS 7671 scope.

Combining Time Switches with PIR

A common commercial arrangement combines a time switch with PIR sensors:

• Time switch provides an "enable window" (e.g., 17:00–08:00 — outside occupied hours)
• PIR sensors switch individual luminaires within that window
• During occupied hours (08:00–17:00), lighting is controlled by manual switch or BMS

This requires the time switch to feed the supply to the PIR sensors, not directly to the luminaires. In panel design, the time switch output feeds a dedicated MCB, which feeds the PIR sensor supply terminals. The PIR sensors then control the individual lighting circuits.

Photocell (Dusk-to-Dawn) Controls

Photocells (also called photoelectric cells, LDRs, or twilight switches) measure ambient light and switch circuits at a threshold lux level. Typically:

• Switch-on threshold: 50–70 lux (dusk, equivalent to heavy overcast or early evening)
• Switch-off threshold (hysteresis): 100–150 lux (higher than switch-on to prevent rapid on-off cycling at threshold)

Photocells are rated for the same load types and maximum currents as PIR sensors. Most are IP65-rated for external mounting on building facades or column tops.

Positioning is critical:

• Mount facing north (or away from direct sun) to prevent false morning switch-off due to direct sunrise illumination
• Avoid mounting near other luminaires the photocell is controlling — light from the controlled fitting will illuminate the photocell, causing it to switch off immediately (hunting)
• Mount clear of shadows from roof overhangs that would delay switch-on until long after natural dusk

For road and car park lighting columns, the photocell typically mounts in a NEMA socket at the top of the column — a 3-pin twist-lock socket (NEMA 5-20 or ANSI C136.10 standard) integral to the control gear compartment. The photocell plugs directly in, with no separate wiring required beyond the column's internal supply.

Dimmer Switches and Dimmer Modules

Dimming Technology Types

Not all dimmers work with all LED luminaires. The dimming technology must match the luminaire's driver:

Leading-edge (LE) dimmer (TRIAC): Traditional standard for resistive and inductive loads. Chops the front of the AC sine wave. Works well with incandescent and some halogen transformers. Often incompatible with LED drivers — causes buzzing, flickering, reduced dimming range, or minimum load issues.

Trailing-edge (TE) dimmer (MOSFET/IGBT): Chops the rear of the sine wave. Smoother, quieter operation. Compatible with most LED drivers marked "TE dimmable" or "MOSFET dimmable". Generates less RF noise. Preferred for LED loads. Higher cost than LE.

0–10V (analogue) dimming: The dimmer supplies a 0–10V DC signal to the luminaire driver via a separate two-core control cable. 0V = off (or minimum), 10V = full output. Widely used for commercial LED panels, LED battens, high-bays. Simple and reliable. Requires a separate control wiring run in addition to mains. Maximum control wire run typically 50m.

DALI (Digital Addressable Lighting Interface, IEC 62386): Digital two-wire bus connecting a controller to individual DALI-addressed drivers. Each driver can be individually addressed, grouped, and dimmed independently. Scene-setting, logging, and fault reporting possible. Used in offices, retail, and prestige commercial projects. Requires a DALI controller (or BMS integration). DALI control wires typically run alongside mains in the same conduit — low voltage (max 16V) and safe to do so.

DSI (Digital Serial Interface): Older Tridonic protocol, largely superseded by DALI. Still encountered on legacy systems.

RF wireless dimming: Proprietary wireless control signals to compatible LED drivers (Casambi, EnOcean, OSRAM TRALED). No control wiring required. Suitable for retrofits where additional cable runs are impractical.

Selecting a Dimmer for LED Loads

The key parameters when specifying a dimmer for LED:

1. Dimming technology: Confirm the LED driver is TE-compatible or specify the correct technology (DALI, 0–10V, etc.)
2. Minimum load: Most TRIAC dimmers require a minimum load (often 10–40W) to function correctly. A single 5W LED will not reach this — the dimmer will flicker or fail to switch off. Either specify a dimmer with no minimum load, or fit a dummy load resistor (wasteful — avoid where possible).
3. Maximum load: Total wattage of all connected LED luminaires. Derate by 25% for LED from the resistive rating (e.g., a 400W dimmer derated to 300W LED due to electronic inrush).
4. Number of gangs and intermediate switching: Confirm whether the installation requires 2-way or intermediate switching. Many push-button dimmers require specific master/slave configurations for multi-way dimming control.
5. Compatibility list: Major dimmer manufacturers (Lutron, Varilight, Retrotouch, MK) publish approved LED compatibility lists. Cross-check the specific LED driver/luminaire against the list before purchase.

Wiring Dimmers

Replacing a conventional switch with a dimmer requires:

• Confirming a neutral is present at the switch position (required by most modern electronic dimmers)
• A back-box depth of at least 35mm (most dimmers require 35–47mm; standard 25mm back-boxes are too shallow)
• No overloading of the dimmer — total wattage of connected luminaires within derating

Under BS 7671 Regulation 559.4.1, dimmer switches must be rated for the connected load, including inrush current. Electronic dimmers with integral overload protection satisfy this requirement for LED loads.

For 2-way dimming (two switch positions on a staircase, for example), the master dimmer unit connects at the incoming supply position. The remote (slave) unit contains no dimming electronics — it provides a switching input to the master. Slave units are not interchangeable with master units; always order the correct companion slave.

Emergency Lighting Interaction

Where automatic lighting controls (PIR, time switch) are fitted in areas that also require emergency lighting under BS 5266, the emergency circuit must be wired independently — never through the control device. Emergency luminaires must illuminate on mains failure regardless of the position of the automatic control. Wiring emergency fittings through a PIR or time switch, which may be in its "off" state, would result in a BS 5266 non-compliance and potential liability.

Maintained emergency fittings (which are on whenever mains is present) require permanent live supply from an emergency circuit, not via any controls. Non-maintained fittings (which illuminate only on mains failure) are powered by the battery pack independent of controls.

Part L Compliance and Documentation

For new domestic dwellings (Part L1A) and extensions, Approved Document L requires that fixed lighting outlets are either fitted with energy-efficient lamps or provided with automatic energy-saving controls. One approach satisfying the requirement is that at least 75% of fixed lighting outlets use lamps with a luminous efficacy greater than 45 lamp-lumens per circuit-watt.

For commercial new builds and major refurbishments (Part L2A/L2B), the lighting energy performance target is typically expressed in terms of average installed power density (W/m²) and must include automatic controls that switch off or reduce lighting when the area is unoccupied. A building log book entry or O&M manual documenting the control system is required for handover.

NICEIC, NAPIT, and other approved contractor schemes require that lighting control systems installed under notifiable Part P work are tested and certified on a Minor Works or Installation Certificate as appropriate. A standard lighting circuit with an additional PIR in lieu of a manual switch is a minor alteration to an existing circuit — Minor Works Certificate. New circuit installation incorporating controls requires a full Electrical Installation Certificate.

Common Faults and Troubleshooting

PIR Will Not Switch Off

• Hold-on timer set too long — reduce setting
• Lux threshold too high — the sensor is seeing daylight levels but activating anyway (check sensitivity adjustment)
• Sensor detecting reflected heat from a radiator, boiler flue, or HVAC diffuser within its field of view — relocate or mask the sensor
• Faulty sensor — confirm by disconnecting and manually operating the circuit

PIR Will Not Switch On

• Lux threshold too high — the control sees room as adequately lit (lower lux sensitivity)
• Sensor aimed at wall or floor outside detection zone — check mounting position and tilt
• LED load too low for two-wire sensor minimum — upgrade to three-wire sensor
• No movement within detection zone — test with transverse movement across the sensor face

LED Flickering with Dimmer

• Leading-edge dimmer with TE-only LED driver — replace dimmer with trailing-edge
• Load below dimmer minimum — add luminaires or use a low-minimum-load dimmer
• Dimmer minimum setting too low — adjust the trim pot (usually accessed via a screwdriver hole) to set minimum level above flicker threshold
• Phase cut interference from another dimmer on the same circuit — separate the circuits

Time Switch Losing Programme

• Battery backup flat or absent — replace CR2032 or AA backup battery
• Supply voltage interruptions causing memory reset — investigate supply quality
• Summer/winter clock change not manually adjusted (mechanical units only) — adjust or upgrade to astro switch

Key Products and Brands

The UK market for lighting controls is well-served across price points:

• PIR sensors: Hager EE801 (200W LED ceiling mount), Timeguard NS001, BG Electrical BPIR, Schneider Merten (premium ceiling presence)
• Time switches: Hager EG103 (7-day electronic), Legrand 412611 (DIN astro), BG Electrical BGTM7 (7-day digital)
• Photocells: Timeguard NightEye (IP65, 10A), Hager EE101 (NEMA socket)
• Dimmer switches: Varilight V-Pro (TE, extensive LED compatibility), Lutron Caséta (wireless), Retrotouch Crystal CT series (glass), BG Electrical Evolve
• Commercial dimmer modules: Helvar, Lutron EcoSystem, Tridonic DALI drivers, Osram DALI

Summary

Lighting controls — whether a simple PIR in a domestic corridor, a time-switch-controlled external flood circuit, or a DALI-addressed office system — follow the same principles: match the control technology to the load, confirm minimum/maximum load ratings, position sensors for the coverage pattern required, and document the installation correctly. Getting these basics right eliminates callbacks and satisfies Part L compliance obligations without over-engineering the installation.

For further reading on related topics, see our guides to LED GU10 lamps, LED panel lights, fire-rated LED downlights, wiring accessories, emergency lighting, electric shower installation, and cable sizing.