LED (light-emitting diode)

Diode / Optoelectronic

A diode that emits light when forward current flows. Polarised - only works one way. Needs a current-limiting resistor.

Schematic symbol

LED

How led (light-emitting diode) appears in a circuit diagram.

Standard indicator (3mm/5mm)SMDHigh-powerRGBInfrared (IR)UVAddressable (WS2812/NeoPixel)

Voltage drop when lit (~1.8-2.2V red, ~3-3.4V blue/white).

Operating current, typically 10-20mA for indicators.

Determines the colour of light.

degrees

How wide the light spreads.

Shockley diode equation
An LED is a diode: current rises exponentially with voltage, so you can never set brightness with volts — a few tenths too many and it dies.
Ohm's lawV = I × R
The series resistor does the real work: R = (Vsupply − Vf) / I is the single most-used calculation on this whole site.
Watt's lawP = V × I = I²R = V²/R
P = Vf × I is what the package must shed; it is why a 1 W LED needs a metal-core board and a 20 mA indicator does not.
Inverse square law of illuminationE = I / d²
Illuminance from the LED falls as 1/d², which is why a bright indicator is blinding at 10 cm and invisible across a lit room.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

Longer leg = anode (+), flat side / shorter leg = cathode (-).

Resistor for 5V supply, red LED, 10mA: R = (5 - 2) / 0.01 = 300Ω (use 330Ω).

Pick colour/brightness, then size the resistor with the LED-resistor calculator: R = (Vsupply - Vf) / If.

5 mm through-hole

Check first: Longer leg is the anode (+). The flat on the rim and the shorter leg mark the cathode (−).

2 leads (anode +, cathode -). Polarised.

Burns out if run without a resistor (overcurrent); dims with age; wrong polarity = no light.

Multimeter diode mode across leads (red probe to anode) - should glow faintly. Reverse = nothing.

Any LED of same colour/package; recheck the series resistor for the new Vf.