The standard circuit symbol for an LED (Light Emitting Diode) is a semiconductor diode—a triangle pointing toward a vertical line—with two small arrows radiating outward to indicate light emission. The anode connects to the triangle base, and the cathode connects to the vertical line. Below is the complete reference for standard and variant LED symbols, followed by regional differences and a direct decision path for selecting physical components.
The Complete LED Circuit Symbol Reference Table
How to read this table: The 'Core Shape' defines the base geometry. The 'Modifiers' are the specific additions that change a standard rectifier diode into a specific optoelectronic component. Always verify the arrow direction to distinguish emitters from sensors.
| Component | Core Shape | Modifiers / Arrows | Pinout Standard |
|---|---|---|---|
| Standard LED | Triangle + Vertical Bar | Two outward arrows (angled away) | Anode (Triangle), Cathode (Bar) |
| RGB LED (Common Cathode) | Three Triangles + 1 Bar | Three sets of outward arrows (Red, Green, Blue) | 3 Anodes, 1 Shared Cathode |
| RGB LED (Common Anode) | 1 Triangle + 3 Bars | Three sets of outward arrows | 1 Shared Anode, 3 Cathodes |
| Infrared (IR) LED | Triangle + Vertical Bar | Outward arrows, often with 'IR' or '940nm' text | Anode, Cathode (Standard) |
| Ultraviolet (UV) LED | Triangle + Vertical Bar | Outward arrows, often with 'UV' text | Anode, Cathode (Standard) |
| OLED (Organic LED) | Triangle + Vertical Bar | Outward arrows, enclosed in a distinct box or labeled 'OLED' | Anode, Cathode (Standard) |
| Photodiode (Contrast) | Triangle + Vertical Bar | Two INWARD arrows (pointing at the junction) | Anode, Cathode (Reverse bias operation) |
Regional Standards: IEC 60617 vs. IEEE 315
While the triangle-and-bar geometry is universal, the enclosure rules differ depending on whether your schematic follows international or North American standards. Understanding this prevents misreading optoisolators as discrete LEDs.
| Feature | IEC 60617 (International / EU) | IEEE 315 / ANSI Y32.2 (North America) |
|---|---|---|
| Component Enclosure | No circle. The symbol stands alone unless part of a multi-pin IC or optocoupler block. | Often enclosed in a circle to explicitly denote a discrete semiconductor device. |
| Arrow Styling | Arrows are strictly parallel or slightly divergent, typically drawn at a 45-degree angle from the cathode. | Arrows may diverge more widely; older drafts sometimes place them near the anode, though modern practice aligns with IEC. |
| Optoisolator Context | LED and Phototransistor share a single large dashed box indicating the dielectric barrier. | LED and sensor are drawn in separate circles, linked by a dashed line or enclosed in a larger solid box. |
For a deep dive into standard semiconductor symbols, the All About Circuits semiconductor reference provides excellent baseline schematics, while the IEC official symbols portal maintains the definitive global registry.
Rows People Get Wrong: Symbol Confusions
Misidentifying an LED symbol on a schematic can lead to ordering the wrong component or wiring a circuit backward. Here are the most common bench-level mistakes:
The most frequent error is confusing an LED with a photodiode. If the arrows point inward toward the triangle/bar junction, it is a light sensor (photodiode), not an emitter. Wiring a photodiode as a light source will result in a dark circuit and potential reverse-bias breakdown if the voltage exceeds its low PIV rating.
- Laser Diodes: A laser diode symbol looks like an LED but includes a circle around the entire symbol with a specific inner notation (or the text 'LASER'). Do not substitute a standard high-brightness LED for a laser diode; the driver circuit expects the sharp forward voltage knee and coherence properties of a laser junction.
- Zener Diodes: A Zener diode has no arrows. Instead, the vertical cathode bar has bent ends (resembling a 'Z' or a lightning bolt). It is used for voltage clamping, not illumination.
- Varactor Diodes: Often confused with dual-LED setups, varactors show two diodes facing each other (anode-to-anode or cathode-to-cathode) and are used for voltage-variable capacitance in RF tuning, not lighting.
Faded Silkscreen & Missing Schematics: Physical Identification
When repairing legacy gear, you will often encounter PCBs where the silkscreen 'A' (Anode) and 'K' (Cathode) markings are rubbed off, or the schematic is entirely missing. You must safely interpret the physical LED to determine polarity and health.
Set your multimeter to the diode test mode (usually indicated by a diode symbol). Place the red probe on one lead and the black probe on the other. If the LED lights up dimly and the meter reads between 1.8V (Red) and 3.3V (Blue/White), your red probe is on the Anode. If it reads 'OL' or '1', swap the probes. If it reads 0.00V or beeps continuously in both directions, the LED is shorted and dead.
If you cannot use a meter, rely on these physical geometry rules for through-hole and SMD LEDs:
- The Flat Spot (Through-Hole): The rim of the LED bulb has a flat edge. The lead adjacent to this flat edge is always the Cathode.
- Lead Length (Through-Hole): On unused components, the factory cuts the Cathode lead shorter than the Anode lead.
- The Internal Anvil (Through-Hole): Look inside the plastic dome. The larger metal paddle inside the bulb is the Cathode; the smaller post is the Anode. (Note: Some specialty high-power LEDs invert this rule, so always verify with a meter if unsure).
- SMD Markings: For 0603 and 0805 SMD LEDs, look for a tiny green line, a dot, or a chamfered corner on the component body. This mark indicates the Cathode. On the PCB pad, the silkscreen 'K' or a notch in the copper pour also points to the Cathode.
Decision Path: Matching Schematic Symbols to Physical BOM Parts
Use this decision tree to translate the schematic symbol on your drawing into a concrete, purchasable part number for your Bill of Materials (BOM). Stop at the first row that matches your schematic and application.
| If Your Schematic Shows... | And Your Application Requires... | Buy This Exact Part Number |
|---|---|---|
| Standard LED (2 pins, outward arrows) | Through-hole indicator, 5mm, Red, 20mA | Kingbright WP710A10LSRD (Standard 5mm Red, 2.0Vf) |
| Standard LED (2 pins, outward arrows) | SMD space-constrained indicator, 0603, Red | Lite-On LTST-C190KRKT (0603 Red, 2.0Vf, 130° viewing angle) |
| RGB LED (4 pins, 3 Anodes / 1 Cathode) | Full-color status indicator, Common Cathode, 5mm | Wurth Elektronik 150141M173100 (RGB CC, 2.0Vf R, 3.2Vf B/G) |
| IR LED (Outward arrows + 'IR' text) | Remote control transmission or IR break-beam sensor (940nm) | Vishay TSAL6200 (940nm IR, 100mA peak, 60° angle) |
| UV LED (Outward arrows + 'UV' text) | Fluorescence excitation or resin curing (395nm) | Lumileds L1CU-3950000000000 (395nm UV, 3.1Vf, 1A max) |
For comprehensive component datasheets and physical pinout diagrams, Components101's LED reference is an excellent bench companion. Always verify the forward voltage (Vf) of your selected part against your current-limiting resistor calculations using Ohm's Law: R = (V_source - Vf) / I_desired. Never wire an LED directly to a voltage source without a driver or resistor, regardless of the symbol variant.
