Photocell Wiring Diagram: LDR Dusk-to-Dawn Automatic Switching
This is a free printable photocell diagram: download the diagram as SVG or open it and print to paper or PDF.
A photocell diagram shows how a light-dependent resistor senses ambient light levels to automatically switch exterior lighting on at dusk and off at dawn.
A photocell (also called a photoelectric switch or dusk-to-dawn switch) uses the properties of a light-dependent resistor (LDR) to control a switching device — typically a relay or a triac — that switches a lighting load. The operating principle is straightforward: the LDR's resistance decreases dramatically as light intensity increases. In bright daylight, LDR resistance can be below 1 kΩ; in darkness it rises to several megaohms.
In a basic discrete circuit, the LDR forms a voltage divider with a fixed resistor connected to a DC supply derived from the mains by a small transformer or capacitive dropper. The midpoint of the divider connects to the base (or gate) of a switching transistor or thyristor. In daylight, the LDR's low resistance pulls the midpoint voltage down, keeping the transistor off. As ambient light falls at dusk, LDR resistance rises, the midpoint voltage increases, and the transistor switches on, energising a relay coil. The relay's normally open contacts close to supply the lighting load.
Commercial integrated photocell units — the type used in exterior lighting columns and security floodlights — encapsulate the entire circuit and present three external connections: line (L), neutral (N), and a switched line output. The LDR is mounted under a semi-transparent dome facing skyward. These units typically include hysteresis in the switching circuit to prevent rapid on-off cycling at twilight. The switching threshold is usually set at approximately 35–50 lux, with a hysteresis of 10–15 lux.
An adjustable trip-point version incorporates a potentiometer in the voltage divider to allow the installer to set the light level at which switching occurs.
Time-delay functions are sometimes combined with photocell switching: the unit switches on at dusk but switches off after a preset time rather than waiting until dawn.
All external wiring must comply with applicable wiring regulations. Units for outdoor use must be rated for outdoor installation (IP rating appropriate to exposure).
How to wire photocell diagram
- Select a suitable photocell unit Choose a photocell rated for the supply voltage (typically 230 V AC or 120 V AC) and with a switched current rating exceeding the total connected load. Verify the IP rating suits the installation environment.
- Isolate the supply circuit Switch off and lock the circuit breaker feeding the lighting circuit. Verify absence of voltage at the installation point using a calibrated voltage tester.
- Mount the photocell Position the photocell where its dome faces open sky without obstruction. It must not face the luminaires it controls (to avoid self-cycling) and should be shielded from artificial light sources such as street lamps that could prevent it from switching on.
- Wire the photocell inline Connect the incoming mains line conductor to the photocell's L terminal. Connect neutral to N. Connect the photocell's switched output to the line input of the luminaire. Neutral feeds directly to the luminaire without passing through the photocell. The earth conductor connects to the luminaire and the enclosure earth terminal.
- Seal cable entries In outdoor installations, use weatherproof cable glands at all cable entry points to maintain the IP rating of the enclosure. Apply appropriate sealant where specified by the installation instructions.
- Restore power and function test Re-energise the circuit. Test dusk operation by covering the photocell dome fully — the luminaire should switch on within the response time stated by the manufacturer (typically 5–30 seconds). Uncover the dome; the luminaire should extinguish.
Specifications
| Typical switching threshold | 35–50 lux (dusk-to-dawn operation) |
|---|---|
| Hysteresis (typical) | 10–15 lux |
| Response time (typical) | 5–30 seconds |
| LDR dark resistance | 1 MΩ–10 MΩ |
| LDR light resistance (1000 lux) | 100 Ω–500 Ω |
| Outdoor IP rating (minimum) | IP44; IP65 for exposed locations |
| Operating temperature range | -20°C to +50°C |
Safety warnings
- All wiring of mains-voltage photocell units must be carried out by a qualified electrician in compliance with applicable regulations (NEC/NFPA 70, BS 7671, AS/NZS 3000, IEC 60364).
- Verify the circuit is dead using a calibrated voltage tester before working on any wiring. Do not assume the circuit is de-energised because the luminaire is not illuminated — the photocell may be preventing the lamp from switching on in daylight.
- For outdoor installations, ensure all enclosures, photocell units, and cable glands carry an IP rating appropriate to the installation environment.
- Do not exceed the photocell's rated switching current. Overloading a photocell causes contact or triac failure and may cause a fire.
- Cadmium sulphide LDRs contain hazardous material. Dispose of in accordance with local WEEE or hazardous materials regulations.
Tools needed
- Calibrated non-contact voltage tester
- Insulated screwdrivers
- Wire strippers and crimping tool
- Multimeter (resistance and AC voltage functions)
- IP-rated cable glands (appropriate size)
- Lux meter (for threshold adjustment)
- Weatherproof sealant where required by installation
Common mistakes
- Positioning the photocell where the luminaire it controls illuminates the dome — causes self-cycling as the lamp switching on causes the sensor to switch the lamp off again.
- Undersizing the photocell switching current rating for the connected load, causing premature failure of the relay contacts or triac.
- Failing to seal cable entries on outdoor enclosures, allowing moisture ingress that corrodes terminals and causes erratic switching.
- Wiring the neutral through the photocell instead of the switched line — the switching action occurs in the line conductor.
- Not allowing for the photocell's response time when testing: covering the dome and immediately expecting the lamp to respond may give the impression of a fault.
Troubleshooting
- Luminaire does not switch on in darkness
- Cause: Photocell incorrectly wired, switching threshold set too low, or photocell receiving stray light Fix: Verify L, N, and output wiring. Cover the dome and test. If lamp now illuminates, reposition the photocell away from stray light sources. If still dead, verify supply voltage at photocell input terminals.
- Luminaire does not switch off in daylight
- Cause: Photocell dome obstructed or LDR failed Fix: Clean the dome and ensure nothing is blocking it. In a discrete circuit, measure LDR resistance in daylight — it should be well below 10 kΩ. If resistance remains high, the LDR has failed and should be replaced.
- Rapid on-off cycling at twilight
- Cause: Insufficient hysteresis, or self-cycling from the lamp reflecting onto the photocell Fix: Reposition the sensor away from the lamp's illuminated area. If an adjustable unit, increase the hysteresis or raise the trip-point threshold slightly.
Frequently asked questions
What is an LDR and how does it detect light?
An LDR (light-dependent resistor) is a passive semiconductor component whose resistance varies inversely with incident light intensity. It uses cadmium sulphide or similar photosensitive material. In darkness, resistance is very high (megaohms); in bright sunlight, it drops to hundreds of ohms. This resistance change drives the switching circuit.
Why does my photocell switch the light on and off rapidly at dusk?
Rapid cycling at twilight indicates insufficient hysteresis in the switching circuit. The ambient light level is hovering near the trip point. A quality photocell unit includes built-in hysteresis. If the problem persists, the unit may be positioned where it receives reflected light from the lamp it controls — this is called 'self-cycling' and requires relocating the sensor.
What IP rating is required for an outdoor photocell?
For exposed outdoor installations, IP44 (splash-proof) is the minimum. Installations subject to direct rain or water jets require IP65 (dust-tight and jet-proof). Check that the photocell's IP rating is appropriate for its installation location and complies with local wiring regulations.
Can a photocell switch LED luminaires?
Yes, but the photocell must be rated for the minimum and maximum load current of the LED luminaires. Some older photocell relay types require a minimum load current that LED luminaires do not draw, causing the relay contacts to fail to break cleanly. Always verify the load compatibility of the photocell against the luminaire specifications.
How do I adjust the switching threshold on a photocell?
On adjustable units, a small trimmer potentiometer is accessible through a slot in the housing. Rotating it changes the voltage divider ratio and therefore the light level at which the circuit trips. Adjustment is best performed on-site at the target ambient light level, or in a controlled environment with a lux meter as reference.
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