The standard LDR (Light Dependent Resistor) symbol consists of a resistor base—either a rectangle or a zigzag line—enclosed in a circle, with two inward-pointing arrows indicating light sensitivity. Because global schematic standards differ, the exact shape of the resistor base changes depending on whether your design software defaults to IEC or IEEE/ANSI conventions. Below is the complete reference for identifying, interpreting, and testing LDRs in the wild.
LDR Symbol Reference Table: IEC vs IEEE Standards
When reading a schematic or selecting a footprint in CAD software like KiCad or Altium, you will encounter one of two primary symbol standards. The Electronics Club reference guide and standard IEC documentation outline these differences clearly. Use this table to identify which standard applies to your region and software.
| Standard Designation | Visual Base Shape | Arrow Direction | Primary Region / Usage | CAD Library Default |
|---|---|---|---|---|
| IEC 60617 | Empty Rectangle | Inward (toward center) | EU, UK, AU, International | KiCad (official lib), Altium |
| IEEE 315 / ANSI Y32.2 | Zigzag Line | Inward (toward center) | USA, Canada, Legacy MIL-SPEC | Legacy Eagle, older OrCAD libs |
| Unenclosed Variant | Rectangle or Zigzag | Inward (toward center) | Informal / Hobbyist sketches | Custom / User libraries |
Rows People Get Wrong: LDR vs. Photodiode vs. Phototransistor
The most common mistake when reading optoelectronic schematics is confusing the LDR symbol with other light-sensitive components. The arrows always point inward for light-sensing devices and outward for light-emitting devices (like LEDs), but the base shape dictates the actual component behavior.
- The Photodiode Confusion: A photodiode symbol also features inward-pointing arrows and a circle, but the base is a diode triangle and bar, not a resistor. If you see a triangle, it is a photodiode. Photodiodes operate in reverse bias and generate microamps of current proportional to light; they do not act as variable resistors.
- The Phototransistor Confusion: A phototransistor symbol has inward arrows pointing at a transistor structure (usually an NPN bipolar junction with the base terminal omitted or left floating). Phototransistors (like the TEPT5600) offer built-in gain and switch much faster than LDRs, making them ideal for digital encoder wheels, whereas LDRs are too slow (typically 20ms to 50ms response time) for high-speed data.
- The Missing Circle Trap: In informal schematics or hasty whiteboard sketches, engineers often omit the enclosing circle. If you see a standard resistor with two inward arrows pointing at it, treat it as an LDR. The circle is technically required by strict IEC 60617 rules to denote the "optical coupling" or environmental sensitivity, but its absence in practice does not change the component's identity.
Identifying and Testing Unmarked or Faded Photoresistors
When scavenging parts from old streetlights, alarm systems, or night-light PCBs, you will frequently find LDRs with faded silkscreen markings or no schematic documentation. According to All About Circuits component references, physical identification and multimeter verification are your best tools.
Physical Identification Traits
Standard through-hole CdS LDRs almost universally come in 5mm (e.g., GL5516) or 7mm (e.g., GL5537) epoxy-coated discs. If you look closely at the face of the component under a magnifying glass, you will see a distinct serpentine or zigzag trace of semiconductor material bridging two interlocking comb-like electrodes. This physical layout is unique to photoresistors; photodiodes and phototransistors look like standard black or clear epoxy LEDs with a distinct flat edge on the rim to denote polarity.
The Multimeter Verification Test
To confirm an unmarked component is indeed an LDR and to roughly bin its light/dark resistance profile, follow this bench procedure:
- Set your DMM: Turn your multimeter dial to the resistance (Ω) setting, specifically the 2MΩ range.
- Measure Dark Resistance: Cover the face of the component completely with your finger or a piece of black electrical tape. Wait 10 seconds for the carrier recombination to stabilize. A standard GL5528 will read between 500kΩ and 1.2MΩ. If it reads open (OL) or short (0Ω), the component is dead.
- Measure Light Resistance: Shine a standard 60-lumen white LED flashlight directly at the face from 2 inches away. The resistance should drop rapidly. A typical GL5528 will fall to 10kΩ – 20kΩ. High-sensitivity variants (like the VT90N) may drop to 2kΩ.
- Check for Memory Effect: LDRs suffer from "light memory" or hysteresis. If the component was stored in a bright environment, its dark resistance may read artificially low until it sits in total darkness for several hours. Do not discard a component just because its initial dark resistance reads 200kΩ instead of 1MΩ; let it rest in a dark drawer overnight and retest.
Frequently Asked Questions
What is the difference between the LDR symbol and the photodiode symbol?
The core difference lies in the base shape inside the circle. The LDR symbol uses a resistor base (a rectangle in IEC standards or a zigzag in IEEE standards) because it functions as a passive, light-variable resistor. The photodiode symbol uses a diode base (a triangle pointing toward a vertical bar) because it functions as a semiconductor junction that generates a small current when photons strike the depletion region. Both use inward-pointing arrows to denote light reception, but their circuit behaviors are entirely different.
Which LDR symbol standard does KiCad or Altium use by default?
Modern CAD ecosystems have largely standardized on IEC 60617 for international compatibility. In KiCad's official symbol library, the LDR (designated as R_Photo or PHOTO) uses the IEC rectangle base enclosed in a circle with inward arrows. Altium's default unified library also favors the IEC rectangle. If you are working with legacy US-based MIL-SPEC schematics or older versions of Autodesk EAGLE, you will frequently encounter the IEEE/ANSI zigzag base. You can swap between these in KiCad by toggling the symbol style in the schematic editor preferences, though the IEC rectangle is recommended for new 2026 designs.
How do I safely interpret an LDR symbol if the schematic circle is missing?
If you see a resistor symbol (rectangle or zigzag) with two arrows pointing inward toward it, but the enclosing circle is missing, you should safely interpret it as an LDR. The circle is a formal IEC requirement to denote that the component's properties are altered by external environmental factors (in this case, photons). However, in fast-paced engineering environments, hand-drawn schematics, or simplified educational diagrams, the circle is routinely omitted to save space. The inward arrows are the critical identifier. Just ensure the arrows aren't pointing away from the resistor, which would technically imply a light-emitting resistor (a component that doesn't practically exist) or be a drafting error for an LED.
Can I replace an LDR with a phototransistor in an existing circuit?
Not without redesigning the biasing network. An LDR is a passive, bidirectional resistor that simply changes its ohmic value based on lux levels; it requires no polarity and operates on basic voltage divider principles. A phototransistor is an active, polarized semiconductor device that requires a specific collector-emitter bias voltage and acts as a current source rather than a variable resistor. Dropping a phototransistor into a footprint designed for an LDR will result in a non-functional circuit, as the microcontroller's ADC pin will either float or be pulled permanently high/low depending on the orientation.
