What Is a Photocell Relay? Types and Selection Criteria

A photocell relay is a twilight switch that measures ambient light with a photocell eye and switches lighting on when it gets dark and off when it gets light.

A photocell relay is a twilight switch that measures the ambient light level with a light sensor (photocell eye), switches lighting on when it gets darker than the set level and switches it off when it gets light. It is used in street, garden, residential complex, parking, sign and facade lighting so that lamps switch on and off automatically with daylight. It is not the same product as a motion-detecting sensor or the safety photocell on automatic doors.

Types

• Adjustable threshold photocell relay: The switch-on level is set in lux with an adjustment knob. Narrow-range models (Entes FG-4, 1–3 lux) switch on in full darkness; wide-range ones (Entes FG-4A 1–10 lux, Schneider Electric IC2000 2–2000 lux) switch on earlier when needed.
• Fuse protected: The output circuit is protected by a fuse on the relay (Entes FG-4R).
• DIN rail housing: Mounts on the rail inside a panel; a separate photocell eye placed outdoors senses the light (Tense FTV-04, V-Otomat DIN housing; Schneider Electric IC2000 modular type).
• Replacement and separate photocell eye: An eye that has failed or must be placed further away is sold separately (Entes FG-GÖZ, Tense FT-G).

Selection criteria

1. Lux setting range: If street and area lighting is to switch on in full darkness, a narrow, low range is enough. For shop windows, signs or covered car park entrances that should light up earlier, choose a wide-range model.
2. Contact current and load type: The current values in the product names (Tense FTV-04 5 A, Tense FT-GR 7 A, Schneider Electric CCT15285 16 A) are the output contact ratings. Luminaires with LED drivers and ballasts draw a high inrush current at switch-on; switch large luminaire groups with a contactor driven by the relay.
3. Eye and mounting location: If the housing goes inside a panel, you need a type with an eye mounted separately outdoors. Check the eye's cable length and outdoor protection class in the product datasheet.
4. Supply voltage: The Entes FG-4 series runs on 190–255 V AC; the 250 V AC in the Tense model names is the rating of the relay output contact, so check the supply voltage on the product page. The supply must match the control circuit voltage.
5. Delay and control: Prefer a model with delayed operation so that the relay does not switch on brief light changes such as car headlights or lightning; check the product page for a manual control option.

Common mistakes

• Placing the eye where it sees the lamp: When the lamp lights up, the eye senses brightness and the relay switches the lamp off; the lighting keeps flashing on and off.
• Pointing the eye into direct sun or shade: An eye in direct sun switches on late, one in constant shade switches on early. Place the eye where it sees daylight but no artificial light, usually facing north.
• Setting the threshold in daytime: Set the threshold at the dusk level at which the lighting should come on, turning the knob slowly.

Brands in the catalog

The Photocell Relays category has 6 active products: Entes 3 (FG-4, FG-4A, FG-4R), Tense 2 (FT-GR, FTV-04), Schneider Electric 1 (CCT15285 IC2000). For Schneider Electric CCT15285 and Entes FG-4A, price and lead time are given on quotation; the other models can be ordered directly. Replacement photocell eyes are in the Photocell Relay Accessories category.

Frequently asked questions

What is the difference between a photocell relay and an astronomical timer relay?

A photocell relay measures ambient light with an eye; on an overcast day it switches on early. An astronomical timer relay uses no sensor; it calculates sunrise and sunset from location and date and is not affected by a dirty eye or shade.

How is a photocell relay wired?

The relay is supplied with phase and neutral; the output contact switches the phase of the lighting circuit or, for large loads, a contactor coil. On models with a separate eye, the eye is connected to its own terminals. Take the terminal numbers from the wiring diagram on the product.

How is the lux setting of a photocell relay adjusted?

At the dusk level at which the lighting should come on, turn the knob slowly from the lowest value and leave it at the point where the relay switches. Make the setting with the eye in its permanent position.

Why does the photocell relay switch the lamp on in daytime?

Usually the eye is dirty or in shade, or the threshold is set too high. If the lighting flashes on and off at night, the eye is seeing the lamp's light; move it to a place without artificial light.