The light dependent resistor symbol universally features a standard resistor body enclosed in a circle, with two inward-pointing arrows representing incident photons. Under the US-based IEEE 315 / ANSI Y32.2 standard, the resistor body is drawn as a zigzag line. Under the international IEC 60617 standard, it is drawn as a solid rectangle. If you are reading a schematic today, look for the inward arrows—that is the definitive differentiator from light-emitting components like LEDs.
Complete LDR Symbol and Schematic Reference
Before wiring up a breadboard or routing a PCB, you need to know exactly what your EDA tool is placing. Here is the complete reference table for photoresistor schematic symbols, mapped to their governing standards and common CAD library names.
| Symbol Variant | Governing Standard | Visual Description | Common CAD Library Name | Primary Region |
|---|---|---|---|---|
| Zigzag + Circle + Inward Arrows | IEEE 315 / ANSI Y32.2 | Zigzag resistor inside a circle, two arrows pointing at the center | Photoresistor_US, LDR_IEEE | North America |
| Rectangle + Circle + Inward Arrows | IEC 60617 | Rectangular box inside a circle, two arrows pointing at the center | Photoresistor_EU, LDR_IEC | Europe / International |
| Zigzag + Inward Arrows (No Circle) | IEEE 315 (Simplified) | Zigzag resistor with arrows, enclosing circle omitted for space | LDR_Small, Photoresistor_N | Global (Modern CAD) |
| Rectangle + Inward Arrows (No Circle) | IEC 60617 (Simplified) | Rectangular box with arrows, enclosing circle omitted | LDR_IEC_Small | Global (Modern CAD) |
IEEE vs. IEC: Regional Standards and CAD Variants
The primary split in schematic documentation is the resistor body itself. North American engineering firms and legacy US military schematics strictly use the IEEE 315 zigzag. European automotive and industrial schematics default to the IEC 60617 rectangle.
In modern EDA tools like Altium Designer, KiCad, and Autodesk EAGLE, the enclosing circle is frequently treated as optional. While older strict interpretations of the standards mandate the circle to denote the 'enclosure' of the photosensitive material, modern dense PCB layouts often drop it to save routing space. If you see inward-pointing arrows hitting a bare zigzag or rectangle, it is still an LDR.
Rows People Get Wrong (and Physical Verification)
The most common schematic misinterpretations happen when engineers confuse the light dependent resistor symbol with other optoelectronic components. Here is how to tell them apart, and how to verify the physical part if the schematic is missing or the component is unmarked.
The Confusion Matrix
- Photodiode: The arrows point outward (away from the junction), indicating light emission or the generation of current from light. The body is a diode triangle, not a resistor.
- Phototransistor: The arrows point inward, but the body is a Bipolar Junction Transistor (BJT) symbol with a collector and emitter, lacking a base connection.
- LED (Light Emitting Diode): Arrows point outward, body is a diode triangle.
Safe Interpretation of Unmarked Physical Parts
If you have a bin of unmarked orange/red components and need to confirm they are Cadmium Sulfide (CdS) LDRs and not thermistors or varistors, use a multimeter.
- Set your DMM to the highest resistance range (usually 2MΩ or 20MΩ).
- Probe the two leads. Under standard room lighting (~100 lux), a typical hobbyist LDR like the GL5528 will read between 10kΩ and 20kΩ.
- Cover the sensor face completely with your finger or black electrical tape. The resistance should spike to >1MΩ (dark resistance).
- Shine a high-lumen flashlight directly at the face. The resistance should drop to roughly 1kΩ to 3kΩ.
If the resistance drops when heated by a soldering iron but ignores light, you are holding an NTC thermistor, not an LDR.
Sensor Selection Decision Path
Do not default to an LDR just because it is cheap. Use this decision tree to terminate your design phase with a concrete part number based on your circuit's actual physical requirements.
| Application Requirement | Required Response Time | Spectral Target | Concrete Part Pick |
|---|---|---|---|
| Streetlight activation, night-light toggling, slow ambient tracking | Slow (20ms - 50ms rise/fall) | Visible Light (Peak ~540nm) | GL5528 (CdS) or PDV-P8104 (RoHS Alt) |
| Optical encoders, IR data transmission, high-speed counting | Fast (Microseconds) | Visible or Near-IR | PT333-3C (Phototransistor) or BPW34 (Photodiode) |
| Flame detection, specific UV index monitoring | Moderate to Fast | UV Spectrum (200nm - 400nm) | GUVA-S12SD (UV Photodiode) |
| IR Remote control decoding (38kHz carrier) | Demodulated Digital Output | Infrared (940nm) | TSOP38238 (IR Receiver Module) |
Worked Example: Sizing the Voltage Divider
An LDR cannot be read directly by an Arduino or ESP32 ADC pin because it varies resistance, not voltage. You must build a voltage divider. Let's calculate the exact series resistor value for a 5V system using a GL5528 LDR, referencing standard voltage divider principles.
Knowns:
- V_in = 5.0V
- LDR Dark Resistance (R_dark) = 1,000,000Ω (1MΩ)
- LDR Light Resistance (R_light) = 2,000Ω (2kΩ)
- Target: Maximum voltage swing at the ADC pin (V_out) between dark and light conditions.
The Math:
To maximize the swing, the fixed series resistor (R1) should be roughly the geometric mean of the LDR's minimum and maximum resistance, or chosen based on the specific threshold you want to trigger at. For general ambient light detection, a 10kΩ fixed resistor is the industry standard baseline.
Using the formula: V_out = V_in * (R_LDR / (R1 + R_LDR)) (assuming LDR is tied to GND and R1 is tied to 5V).
- In Bright Light (R_LDR = 2kΩ):
V_out = 5.0 * (2000 / (10000 + 2000)) = 5.0 * (2000 / 12000) = 0.83V (ADC reads ~170 on a 10-bit scale) - In Total Darkness (R_LDR = 1MΩ):
V_out = 5.0 * (1000000 / (10000 + 1000000)) = 5.0 * (1000000 / 1010000) = 4.95V (ADC reads ~1013 on a 10-bit scale)
This provides a massive, easily readable swing from 0.83V to 4.95V. If you place the LDR on the top (VCC side) and the fixed resistor on the bottom (GND side), the logic inverts: bright light yields ~4.16V and darkness yields ~0.05V. For a deeper look at component behavior and standard schematic representations, consult the LDR component profiles on Components101 or the All About Circuits photoresistor experiments.
Final Rule: Always place a 0.1µF ceramic capacitor in parallel with the bottom resistor of your voltage divider when feeding an ESP32 or Arduino ADC. LDRs are highly susceptible to 50Hz/60Hz AC mains flicker from room lighting, which will cause your ADC readings to jitter wildly without hardware low-pass filtering.
