Photocell / Dusk Sensor Symbol
Definition: The Photocell / Dusk Sensor symbol represents an automatic light-control device, designated P or LS in schematics, that uses a light-dependent switching mechanism to turn connected lighting circuits ON at dusk (low ambient light) and OFF at dawn (high ambient light), per IEC 60617 and NEMA standards.
Also known as: dusk-to-dawn sensor, dusk sensor, photoelectric switch, light-activated switch, LDR switch, twilight switch, photo switch.
What the Photocell / Dusk Sensor symbol means
The Photocell / Dusk Sensor symbol denotes an automatic switching device that measures ambient light levels and controls a connected load — almost always an outdoor lighting circuit. It replaces manual switching for security lights, street lights, and area lighting by responding to natural light transitions at dawn and dusk.
In electrical wiring diagrams this symbol appears in lighting control circuits, connected in series with or controlling the hot (line) conductor feeding luminaires. The symbol signals to the reader that the lighting circuit operates automatically based on light level rather than a manual switch.
How to identify the Photocell / Dusk Sensor symbol
The Photocell symbol is typically drawn as a circle or small square representing the sensor body, with an arrow or light-wave symbol (diagonal arrow or the letters 'hν') pointing toward it to indicate incoming light. Two connection terminals labelled 'In' (line/supply) and 'Out' (to load) represent the switched contacts. Some standards show it as a circle with a photosensitive element symbol (a curved line or eye) inside. On wiring diagrams it often appears simply as a circle with the label 'PC' or 'PHCL'.
Function in a circuit
A Photocell contains a photoelectric sensing element (typically a cadmium sulfide LDR or silicon photodiode) connected to a threshold-switching circuit and an internal relay or triac. When ambient light falls below the set threshold (dusk), the output switches to pass current to the connected luminaires; when light rises above the threshold (dawn), the output opens to switch the lights off. Most units include a built-in time delay (typically 30–120 s) to prevent false triggering from transient shadows or headlight glare.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60669-2-1 covers general-purpose automatic switches including photoelectric controls for household and similar use. IEC 60617 represents automatic switches with a contact symbol plus the photoelectric control indicator. IEC 60598 governs the luminaires that photocells control. |
|---|---|
| ANSI/IEEE 315 | ANSI/NEMA WD 6 defines residential wiring device configurations. NEMA photocontrol standards specify twist-lock mounting (NEMA 1, NEMA 3 twist-lock receptacles for outdoor photocells). UL 773 covers unattended free-standing outdoor luminaire controllers (photocells). |
| Key difference | IEC diagrams typically show the photocell as a switch symbol with a light-sensitive element notation. ANSI/NEMA wiring diagrams often represent photocells as a circle with a twist-lock symbol or simply the abbreviation 'PC'. Functional meaning is identical in both conventions. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Supply voltage: 120 V AC (North America) or 230–240 V AC (Europe/international); load capacity: 1000–2000 W (incandescent), 500–1000 W (LED/CFL); turn-on threshold: typically 1–5 foot-candles (10–50 lux); turn-off threshold: typically 3–10 fc (30–100 lux); operating temperature: −40 °C to +60 °C.
Where the Photocell / Dusk Sensor symbol is used
- Outdoor area and security lighting: automatically turning on floodlights, wall packs, and pole lights at dusk without manual intervention
- Street lighting control: utility and municipality street light systems use photocells to switch entire circuits of HID or LED street lights
- Car park and parking structure lighting: automating perimeter and entrance lighting based on natural light levels
- Commercial signage and display lighting: controlling illuminated signs to be on only during low-light hours
- Residential exterior lighting: porch lights, driveway lights, and garden lighting controlled automatically
- Greenhouse lighting supplementation: triggering grow-lights when natural daylight drops below a set level
Example
In a residential outdoor lighting circuit, the Photocell is wired in series with the hot (line) conductor supplying a 150 W LED wall pack. The In terminal connects to the 120 V line; the Out terminal feeds the luminaire. At dusk, when ambient light drops below approximately 2 foot-candles, the photocell's internal relay closes and the wall pack illuminates; at dawn it opens and the light extinguishes.
Key facts
- The Photocell / Dusk Sensor symbol represents an automatic light-level-activated switching device that turns lighting loads ON at dusk and OFF at dawn.
- Its two circuit terminals are labelled In (supply/line) and Out (to load); the photosensitive element is not a separate pin but an integral part of the enclosure.
- UL 773 is the North American safety standard for unattended free-standing outdoor photocells (luminaire controllers), and NEMA specifies the twist-lock mounting base.
- Photocells use a cadmium sulfide (CdS) LDR, silicon photodiode, or phototransistor as the sensing element; CdS types are being phased out in some regions due to RoHS restrictions.
- Most photocells include a built-in time delay of 30–120 seconds to prevent false switching from vehicle headlights, lightning, or passing shadows.
- Standard NEMA twist-lock photocell sockets are used on North American street light poles and area light fixtures, allowing the photocell to be replaced without rewiring.
- Typical turn-on sensitivity is 1–5 foot-candles (10–50 lux), adjustable on commercial units; turn-off threshold is set higher (3–10 fc) to create hysteresis and prevent oscillation at twilight.
- The photocell symbol on a wiring diagram indicates that the lighting circuit is automatically controlled and does not require a manual wall switch for normal operation.
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Frequently asked questions
What does the photocell symbol mean in a wiring diagram?
The Photocell symbol in a wiring diagram means the lighting circuit is automatically switched based on ambient light level. The photocell turns the connected lights ON at dusk when light falls below its threshold and OFF at dawn when light rises above the threshold, with no manual switching required.
What does the photocell symbol look like?
The Photocell symbol is typically drawn as a circle or small rectangle with a light-wave or diagonal arrow symbol (indicating incoming light) pointing toward it, and two connection terminals labelled In and Out. In wiring diagrams it may appear simply as a circle with the label 'PC' or 'PHCL' to indicate its function.
What are the terminals (pins) on a photocell?
A photocell has two circuit terminals: In (connected to the AC line supply) and Out (connected to the luminaire load). The photosensitive element is internal to the device and is not a separate wiring terminal. Three-wire photocells add a neutral wire for the internal electronics supply.
What is the difference between a photocell and a photoelectric sensor in wiring diagrams?
A photocell (dusk sensor) is a self-contained AC power-switching device that controls lighting circuits based on ambient light level. A photoelectric sensor is a proximity/detection device (with separate VCC, GND, and signal output) used in automation to detect the presence or absence of objects using a light beam. The two serve entirely different circuit functions.
What standard governs photocell design and installation?
In North America, UL 773 covers unattended outdoor luminaire photocell controllers and NEMA standards define the twist-lock socket interface. In Europe, IEC 60669-2-1 applies to automatic switches including photoelectric controls. NEC Article 411 covers lighting control in residential and commercial installations.
Why does a photocell have a time delay built in?
The built-in time delay (typically 30–120 seconds) prevents the photocell from false-triggering when a passing cloud briefly reduces light, or when vehicle headlights momentarily illuminate the sensor at night. The delay requires the light level to remain above (or below) the threshold for the full delay period before the switch changes state.
What voltage and load ratings are typical for photocells?
Residential and commercial photocells are typically rated for 120 V or 277 V AC (North America) or 230 V AC (Europe), with load ratings of 1000–2000 W for resistive/incandescent loads and 500–1000 W for LED/CFL loads. High-capacity commercial units for HID street lights may be rated up to 1000 W at 480 V.
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