The standard schematic symbol for a photocell (specifically a photoresistor or Light Dependent Resistor) is a zig-zag resistor line enclosed in a circle, with two inward-pointing arrows indicating light sensitivity, as defined by the IEEE 315 standard. In mains electrical wiring diagrams, a dusk-to-dawn photocell switch is drawn as a standard single-pole switch with a dashed line connecting to a light-sensor node, wired using black (line), white (neutral), and red (switched load) conductors in North America.
The Complete Photocell Symbol & Wiring Reference Table
Use this table to identify the correct schematic symbol and wiring color code based on whether you are building a low-voltage logic circuit or wiring a 120V/240V outdoor lighting control. Read the left column to identify your application, then follow the row to find the exact symbol and wire colors.
| Component Type | IEEE 315 Schematic Symbol | IEC 60617 Schematic Symbol | Mains Wiring Colors (US/NEC) | Mains Wiring Colors (IEC/EU) |
|---|---|---|---|---|
| Photoresistor (LDR) 5V-12V Logic |
Resistor zig-zag in a circle, 2 inward arrows | Rectangle with diagonal line, 2 inward arrows | N/A (Use Red/Black for VCC/GND) | N/A (Use Brown/Blue for VCC/GND) |
| Photodiode High-speed sensing |
Diode triangle/bar in a circle, 2 outward arrows | Diode symbol in a box, 2 outward arrows | N/A | N/A |
| Mains Photocell Switch 120V/240V Outdoor Lighting |
Single-pole switch with dashed line to light node | Switch with dashed line to sensor node (S-) | Black (Line), White (Neutral), Red (Load) | Brown (Line), Blue (Neutral), Black (Load) |
Regional Variants: NEC vs. IEC vs. Old UK Standards
While the schematic symbols for low-voltage components remain relatively consistent globally, the wiring colors for mains-voltage photocell switches vary drastically by region. Misinterpreting these colors will result in a dead short or a shocked installer.
North America (NEC / NFPA 70)
In the US and Canada, a standard 3-wire photocell switch uses Black for the ungrounded line (hot), White for the grounded neutral, and Red for the switched load feeding the luminaire. The neutral wire is required to power the internal solid-state relay or electromechanical contactor inside the photocell housing.
Europe & International (IEC 60446)
Under IEC harmonized standards, the color code shifts. The line conductor is Brown, the neutral is Blue, and the switched load is Black (or sometimes Grey). If you are retrofitting a US-manufactured Intermatic sensor into an EU junction box, you must map the US Black to EU Brown, US White to EU Blue, and US Red to EU Black.
Old UK (Pre-2006)
If you are troubleshooting an older installation in the UK, you will encounter the legacy color code: Red (Line), Black (Neutral), and Yellow/Green (Earth). Do not confuse the old UK Black (neutral) with the modern IEC Black (switched load). Always verify with a meter before cutting wires in pre-2006 UK junction boxes.
Rows People Get Wrong: Photocell vs. Photodiode Symbols
When reading schematics or designing PCB footprints, engineers and hobbyists frequently confuse the symbol for a photocell (LDR) with a photodiode or phototransistor. Here is how to tell them apart and why it matters in practice.
- The Arrow Direction Trap: A photoresistor (LDR) absorbs light to change its bulk resistance, so the arrows point inward toward the component. A photodiode emits light (if used as an LED) or is modeled with current flowing outward in photovoltaic mode, so its arrows point outward. If the arrows point out, you are looking at a photodiode, not a standard CdS photocell.
- The Circle vs. Box Debate: In IEEE 315 (US standard), discrete components are almost always drawn inside a circle. In IEC 60617 (European standard), the circle is largely abandoned in favor of rectangular boxes or bare symbols. If your schematic uses rectangles for resistors and diodes, it is an IEC-based drawing.
- The Missing Third Pin: A photocell (LDR) is a passive, two-terminal device. It has no polarity. If the symbol shows three pins (Emitter, Base, Collector) with light arrows pointing at the base junction, you are looking at a phototransistor (like the TEPT5600), which requires a bias voltage and acts as an amplifier, not a simple variable resistor.
Decision Path: Selecting the Right Photocell Component
Stop guessing which part to buy. Follow this decision tree to terminate on the exact component and part number for your specific application.
| Application Scenario | Operating Voltage | Required Response Time | Concrete Part Pick |
|---|---|---|---|
| Arduino/ESP32 Light Meter Reading ambient room lux via ADC |
3.3V or 5V DC | Slow (20-50ms) | GL5528 CdS Photoresistor (10-20kΩ at 10 lux, pair with a 10kΩ pull-down resistor) |
| High-Speed Optical Encoder Detecting slot interrupts on a motor |
5V DC | Fast (Microseconds) | BPW34 PIN Photodiode (Reverse-biased with a 100kΩ transimpedance amplifier) |
| 120V Porch Light Automation Dusk-to-dawn outdoor lighting control |
120V AC, 60Hz | Slow (Minutes, with delay) | Intermatic K4221 Photocell (Rated 120V, 15A Tungsten, includes 3-wire pigtail) |
Safe Interpretation When Markings Are Faded or Missing
On the jobsite or when salvaging components from old PCBs, the silkscreen symbol for a photocell may be faded, or the component itself may be unmarked. Never guess based on physical appearance alone—a dark epoxy blob could be an LDR, a photodiode, or a thermistor. Use this multimeter verification protocol to safely identify the component.
- Isolate the Component: If testing in-circuit, ensure the board is completely de-energized and capacitors are discharged. Ideally, desolder one leg of the component to prevent parallel circuit paths from skewing your resistance reading.
- Set Multimeter to Resistance (Ω): Connect your probes across the two leads. Polarity does not matter for a true photocell (LDR), as it is a passive resistor.
- The Dark Test: Cover the component completely with your finger or a piece of electrical tape. Wait 5 seconds. A standard CdS photocell will show a massive resistance spike, typically reading between 500kΩ and 2MΩ in total darkness.
- The Light Test: Shine a bright LED flashlight or phone screen directly onto the sensor face. A healthy photocell will drop in resistance immediately, typically settling between 1kΩ and 10kΩ depending on the light intensity.
- The Polarity Check (Identifying Photodiodes): If the resistance remains extremely high (OL) in both light and dark when tested as a standard resistor, switch your meter to Diode Test mode. If it reads a voltage drop (e.g., 0.4V) in one direction and OL in the other, and that forward voltage changes slightly under a bright light, you have a photodiode, not a photocell.
By relying on the exact resistance delta between dark and light states, you can confidently identify an unmarked LDR without needing the schematic symbol. For a deeper dive into how these components behave in AC circuits, refer to the All About Circuits component reference guide for standardized symbol libraries.
