The photoresistor symbol represents a Light-Dependent Resistor (LDR), a passive component whose resistance drops as incident light intensity increases. Whether you are reading a legacy US military schematic or a modern European datasheet, correctly identifying this symbol—and the physical component it represents—is the first step in designing reliable light-sensing circuits. Below is the definitive reference for schematic symbols, physical epoxy color codes, and a decision framework for selecting the exact part number for your workbench.
Photoresistor Schematic Symbols & Physical Markings Reference
Use this table to cross-reference the symbol on your schematic with the physical component in your bin. The epoxy coating color on standard through-hole LDRs is not just decorative; it indicates the semiconductor material and peak spectral sensitivity.
| Symbol / Marking | Standard / Type | Meaning in Practice | Typical Peak Wavelength |
|---|---|---|---|
| Rectangle + Circle + 2 Inward Arrows | IEEE/ANSI Std 315 (Modern US) | Standard LDR. The circle denotes an optically active component; inward arrows mean light controls the device. | N/A (Schematic only) |
| Rectangle + 2 Inward Arrows (No Circle) | IEC 60617 (International) | Standard LDR. IEC omits the enclosing circle for most passive opto-components to reduce schematic clutter. | N/A (Schematic only) |
| Zig-Zag Line + Circle + 2 Inward Arrows | IEEE/ANSI (Legacy US) | Legacy LDR. Uses the old US resistor zig-zag base. Common in pre-1990s aerospace and automotive diagrams. | N/A (Schematic only) |
| Orange / Red Epoxy Coating | Physical (Cadmium Sulfide - CdS) | Visible light sensor. Matches the human eye's photopic response curve. Most common in hobby and consumer electronics. | 520nm - 540nm (Green/Yellow) |
| Dark Red / Maroon Epoxy | Physical (Cadmium Selenide - CdSe) | Extended red response. Slower reaction time than CdS, but sensitive further into the near-infrared spectrum. | 680nm - 720nm (Deep Red) |
| Black / Opaque Epoxy with IR Window | Physical (Lead Sulfide - PbS) | Infrared sensor. Used in flame detection and gas analysis. Requires specialized bias circuits; not for standard 5V logic. | 1000nm - 3000nm (IR) |
Regional Standard Variants: IEEE vs. IEC vs. Legacy
When tracing a circuit, the region where the schematic was drafted dictates which symbol variant you will encounter. Understanding these differences prevents misidentifying an LDR as an LED or a photodiode.
North America (IEEE/ANSI Std 315)
In the US and Canada, the modern standard uses a rectangular box to represent the resistive element, enclosed in a circle to denote optical interaction. However, if you are servicing older equipment (pre-1980s), you will frequently see the legacy zig-zag resistor symbol inside the circle. Both use two inward-pointing arrows at a 45-degree angle.
Europe and International (IEC 60617)
The IEC standard strips away the enclosing circle. The symbol is simply a rectangle with two inward arrows. Because the IEC uses a rectangle for all resistors, the LDR symbol looks exactly like a standard resistor with light arrows added. If you are reading a schematic from a European manufacturer like Siemens or Philips, expect this cleaner, circle-free variant.
Which Standard Applies to You?
If you are designing a new board in Altium or KiCad in 2026, default to the IEC 60617 rectangle-with-arrows symbol. It is the globally accepted modern standard and renders cleaner on dense PCB silkscreens and schematic PDFs. Use the IEEE circled variant only if you are explicitly required to match legacy US military or aerospace drawing standards (e.g., MIL-STD-806).
The 'Rows People Get Wrong' Guide to LDR Markings
Misreading a schematic symbol or misidentifying a physical component on the bench leads to fried microcontrollers and non-functional circuits. Here are the most common pitfalls.
- Confusing the LDR with a Photodiode: A photodiode symbol uses a triangle and a line (the standard diode symbol) with inward arrows. An LDR uses a rectangle (resistor). Physically, a photodiode is polarity-sensitive (has an anode and cathode) and responds in nanoseconds. An LDR is non-polarized (works in either direction) and responds in tens of milliseconds.
- Confusing the LDR with a Thermistor: On a schematic, an NTC/PTC thermistor symbol looks like a resistor with a diagonal line through it and a flat base (representing temperature). On the bench, both can look like small epoxy-coated discs. The bench test: Cover the component with your hand. If the resistance changes based on your body heat, it is a thermistor. If it only changes when you shine a flashlight on it, it is an LDR.
- Assuming All LDRs React to IR Remote Controls: Standard orange CdS photoresistors (like the ubiquitous GL5528) are effectively blind to the 940nm infrared light emitted by TV remotes and IR obstacle sensors. If your circuit needs to see IR light, an orange LDR will not work, regardless of how bright the IR LED is.
Decision Path: Selecting the Exact Photoresistor Part Number
Stop guessing which LDR to order. Use this decision tree to terminate your search with a specific, purchasable part number based on your circuit's actual requirements.
| Application Requirement | If Yes... | Concrete Part Pick |
|---|---|---|
| Do you need to detect ambient visible light for slow-switching applications (e.g., streetlights, nightlights, solar trackers)? | Yes → You need standard CdS with high dark resistance and slow decay. | GL5528 (Dark: ~1MΩ, Light: ~10kΩ) |
| Do you need visible light detection, but require a lower baseline resistance for higher current circuits without an op-amp? | Yes → You need a lower-resistance CdS cell. | GL5516 (Dark: ~0.5MΩ, Light: ~5kΩ) |
| Do you need fast response times (<5ms) for audio compressors, optical encoders, or strobe synchronization? | Yes → Standard hobby LDRs are too slow (20-50ms decay). You need a professional-grade fast cell. | Excelitas VT935G (Fast decay, linear response) |
| Do you need to detect Infrared (IR) light, flame, or heat signatures? | Yes → STOP. Do not use a standard photoresistor. Pivot to a photodiode or specialized PbS cell. | BPW34 Photodiode (For near-IR/visible) or PbS Cell (For mid-IR) |
Safe Interpretation When Epoxy Markings Are Faded or Missing
When scavenging parts from old equipment or dealing with a bin of unlabeled LDRs, the epoxy color might be faded, or the component might be bare. Do not trust a visual guess; verify the component electrically using a digital multimeter (DMM) before soldering it into a 3.3V or 5V microcontroller circuit.
The Bench Verification Protocol
- Set your DMM to the 2MΩ (Mega-ohm) range. Connect the probes to the LDR leads (polarity does not matter; LDRs are non-polarized).
- Test Dark Resistance: Cup your hand completely over the LDR to block all ambient light, or do this in a dark room. A healthy standard CdS LDR (like the GL5528) will read between 200kΩ and 2MΩ. If it reads near 0Ω or 'OL' (open loop/infinity), the component is shorted or has a broken internal lead.
- Set your DMM to the 20kΩ range.
- Test Light Resistance: Shine a standard smartphone flashlight (approx. 50-100 lumens) directly onto the face of the component from 2 inches away. The resistance should drop rapidly to between 1kΩ and 10kΩ.
- Check for Memory Effect: Turn off the flashlight. The resistance should slowly climb back up over 10 to 50 milliseconds. If it instantly snaps back to 1MΩ, you likely have a photodiode or phototransistor misidentified as an LDR, not a true photoresistor.
By combining the correct schematic symbol interpretation with this bench-test protocol, you eliminate the guesswork from light-sensing circuit design. For 90% of DIY and educational projects involving visible light detection, the GL5528 remains the undisputed, cost-effective default choice. Reference the Electronics Tutorials LDR guide for deeper math on calculating your voltage divider bias resistors based on these exact dark/light thresholds.
