LED · Exterior view
Epoxy lens · Ø 5 mmCathode frame (anvil)Anode frame (post)Cathode leg (−) · short legAnode pin (+) · long leg
Clear colored epoxy dome, flat cathode side on the lower edge, long anode and short cathode leg
5 mm diameter, body height ≈ 8.6 mm (3 mm type also common)
Diodes

LED

In simple terms
What is this?

A small diode that gives off light; it glows as current flows and works in one direction only.

What is it for?

Lights up as a power and status indicator; it shows the circuit is working.

Where do you see it?

The small lights on a modem, a charger and a TV.

How do you spot it?

A domed coloured plastic body and two legs; the long leg is positive, the flat edge of the dome is on the negative side.

Caution:

Do not connect it without a series resistor, or it burns out; long leg to plus.

Technical definition

A light-emitting diode: as current passes from anode to cathode, the semiconductor chip gives off light. Its color depends on the chip’s material, and the forward voltage changes with color. It is always used with a series resistor.

Forward voltage≈2.0 V (red)
Current20 mA (max.)
Reverse voltagemax. 5 V
Schematic symbol Same in IEC and ANSI

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 2
1Anode (A)Long leg; goes to plusA
2Cathode (K)Short leg, flat edge of the bodyK
Features typical
Forward voltage (red)
1.8–2.2 V
Forward voltage (blue/white)
3.0–3.3 V
Continuous current
20 mA (max.)
Recommended current
5–15 mA
Reverse voltage
max. 5 V
Resistor calculation
R = (Vsupply − Vf) / I
Example
5 V, red, 15 mA → 200 Ω (pick 220 Ω)
Example part
5 mm red LED
Common mistakes
Connecting without a resistorAn LED does not limit its own current. An LED connected directly to 5 V burns out right away from overcurrent. Calculate and fit a series resistor.
Fitting it backwardsThe long leg goes to plus. An LED fitted backwards won't light; reverse voltage above 5 V can damage it.
A single resistor for parallel LEDsEach LED’s forward voltage is slightly different; if you connect them in parallel with a single resistor, the current does not split equally. Give each LED its own resistor.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement
On real boards
USB-C Li-ion charger board
Red charge indicator · Blue complete indicator
Where it is used
Power and status indicatorNight lightIR emitter and optocouplerLight decoration
Related lessons
Your first LED circuit
Module 1 · From zero to your first circuitOhm’s law
Module 1 · From zero to your first circuitDiode test
Module 2 · Measuring with a multimetersoon
Try it in the Lab