The photo resistor symbol (commonly referred to as an LDR or Light Dependent Resistor) varies significantly depending on whether your schematic follows North American IEEE/ANSI standards or international IEC standards. Unlike standard fixed resistors, LDRs do not use EIA color codes; identification relies on schematic symbols, printed part numbers, and module pinouts. Below is the complete reference for interpreting these components on paper and on the bench.

The Photo Resistor Symbol: IEC vs IEEE/ANSI Standards

When reading a schematic, the first step is identifying which drafting standard the engineer used. The core difference lies in how the base resistor shape is rendered. According to the IEC standard symbols database, international schematics use a rectangular box for all resistors, while North American standards retain the legacy zigzag line.

Table 1: Photo Resistor Symbol & Designator Reference
Standard Symbol Description Reference Designator Primary Usage Region
IEEE 315 / ANSI Y32.2 Circle enclosing a zigzag line, with two inward-pointing arrows. RL or R (e.g., RL1) North America
IEC 60617 Standard resistor rectangle with two inward-pointing arrows. R or RL (e.g., R1) Europe, UK, Global
Legacy / Old British Circle with a zigzag line (no enclosing box), arrows sometimes dashed. RL Legacy UK/AU schematics
Optocoupler Variant Resistor symbol enclosed in a box with a photodiode, arrows internal. U or IC Global (Isolator circuits)

In practice, the inward-pointing arrows are the universal indicator of light reception. If the arrows point away from the component, you are looking at a light-emitting device (like an LED or infrared emitter), not a photo resistor. The reference designator RL (Resistor, Light) is preferred to distinguish it from standard fixed resistors (R), though many legacy schematics simply use R and rely on the symbol arrows for context.

Physical Markings and Spec Data (The "Color Code" Reality)

Callout: Why There Are No LDR Color Codes
Standard 1/4W through-hole resistors use the 4-band or 5-band EIA color code. Photo resistors do not use color codes. Because their resistance changes dynamically with illuminance, a static color band would be meaningless. Instead, bare CdS (Cadmium Sulfide) photocells are identified by physical diameter, epoxy coating color, and printed alphanumeric part numbers on the face or packaging.

To select or replace an LDR, you must match its dark resistance, light resistance, and spectral peak. The Adafruit photocell guide notes that different chemical dopings shift the peak wavelength sensitivity, which is critical if your application involves infrared or specific LED colors rather than ambient white light.

Table 2: Common Bare LDR Part Numbers & Specifications
Part Number Dark Resistance (MΩ) Light Resistance (kΩ @ 10 lux) Peak Wavelength (nm) Typical Application
GL5528 1.0 MΩ 10 - 20 kΩ 540 nm (Green/Yellow) Streetlights, nightlights
GL5516 0.5 MΩ 5 - 10 kΩ 540 nm (Green/Yellow) Camera light meters
VT90N 20.0 MΩ 2 - 5 kΩ 620 nm (Red) High-sensitivity alarms
PDV-P8001 0.05 MΩ (50kΩ) 0.1 kΩ (100Ω) 700 nm (Near-IR) IR beam break sensors

Notice the logarithmic relationship in the data above. An LDR does not drop resistance linearly as light increases; it follows a steep curve. A GL5528 might read 1 MΩ in total darkness, drop to 15 kΩ at 10 lux (dim twilight), and fall to 1.5 kΩ at 100 lux (overcast day). This non-linearity is why LDRs are rarely used for precision scientific light measurement, but excel at simple threshold triggering.

LDR Module Pinouts and Wiring References

While a bare photo resistor has only two non-polarized leads, most hobbyists and prototypers use LDRs mounted on breakout boards. These modules integrate a voltage divider and often an LM393 comparator to provide clean digital signals to microcontrollers like the Arduino or ESP32.

Below is the standard pinout reference for the ubiquitous 3-pin and 4-pin LDR modules (often sold as KY-018 or generic analog light sensor boards):

Table 3: Standard LDR Breakout Module Pinouts
Pin Label Function Wiring Target (ESP32/Arduino) Internal Circuit Path
VCC Power Input 3.3V or 5V Feeds the top of the LDR voltage divider and LM393 VCC.
GND Ground GND Common ground for the divider and comparator.
AO (or OUT) Analog Output ADC Pin (e.g., ESP32 GPIO 34) Direct tap from the midpoint of the LDR and fixed resistor divider.
DO Digital Output GPIO Pin (e.g., Arduino D2) Output of the LM393 comparator; threshold set by onboard trimpot.

Wiring Caveat: The analog output (AO) on these cheap modules is highly dependent on the fixed resistor paired with the LDR on the board. Most modules use a 10kΩ fixed resistor. If you are using a GL5528 (which drops to ~15kΩ at 10 lux), your voltage divider will output roughly half of VCC at twilight. If you swap in a PDV-P8001 without changing the module's fixed resistor, your analog readings will be heavily skewed toward the rails.

Symbol & Table Rows People Get Wrong (Plus Faded Marking Recovery)

When interpreting schematics or sorting through a bin of salvaged components, misidentification is common. Here are the specific rows and symbols that trip up even experienced bench technicians:

  • Confusing LDR with Photodiode (Symbol Error): In both IEC and IEEE standards, a photodiode is drawn as a standard diode symbol with arrows pointing away from the junction (indicating light emission in standard diodes, but modified for reception context) OR arrows pointing toward the junction but lacking the resistor zigzag/rectangle. If there is no resistor body in the symbol, it is a photodiode or phototransistor, meaning it requires reverse-bias or base-bias wiring, unlike the passive LDR.
  • Assuming Polarity (Physical Error): Because electrolytic capacitors and diodes have strict anode/cathode markings, beginners often look for a stripe or flat edge on an LDR. Bare CdS cells are completely non-polarized. You can solder them in either direction.
  • Misreading the IEC Rectangle (Schematic Error): In older IEC schematics, the rectangle used for the LDR is identical to a standard fixed resistor or a capacitor (if drawn without the internal gap). The only differentiator is the two inward arrows. If a schematic is poorly scanned and the arrows are lost in the noise, an LDR can easily be mistaken for a standard 10kΩ pull-down resistor, leading to a non-functional light-sensing circuit.

Safe Interpretation When Markings Are Faded or Missing

Salvaged LDRs often suffer from degraded epoxy coatings, UV yellowing, or rubbed-off part numbers. If you have an unmarked photo resistor and need to determine its specifications for a replacement, do not guess based on physical size alone. Follow this 2-point bench test:

  1. Measure Dark Resistance: Place the LDR in a completely light-tight enclosure (a closed film canister or a heavy black cloth wrapped tightly). Connect your multimeter across the leads. Wait 30 seconds for the dark resistance to stabilize (CdS cells have a "memory effect" and take time to reach maximum resistance after light exposure). Record the value in MΩ.
  2. Measure Calibrated Light Resistance: Expose the LDR to a known light source. If you lack a calibrated lux meter, use a standard 60W-equivalent LED bulb (approx. 800 lumens) placed exactly 1 meter away in a dark room. This approximates 10 to 20 lux. Record the resistance in kΩ.
  3. Match to Table 2: Compare your two data points to the reference table above. If your dark resistance reads ~1 MΩ and your 10-lux resistance reads ~15 kΩ, you are holding a GL5528. If the dark resistance is exceptionally high (>10 MΩ), you likely have a VT90N or similar high-sensitivity cell.

By relying on measured electrical characteristics rather than faded physical markings or ambiguous schematic scans, you ensure your light-sensing circuit will trigger at the exact thresholds your firmware expects.