Light Emitting Diodes (LEDs)

1. Definition

1.1 LED

A Light Emitting Diode (LED) is a semiconductor device that emits light when an electric current passes through it.
Unlike incandescent bulbs, LEDs produce light by electroluminescence, where electrons recombine with holes in the semiconductor material, releasing energy in the form of photons.

LED examples

LED Explained

LED Explained

LED symbol

LED symbol

1.2 LED Colors and Materials

The color of an LED depends on the semiconductor material and the energy gap:

  • Red, Yellow, Green: typically made from gallium arsenide phosphide (GaAsP) or gallium phosphide (GaP).
  • Blue, White: often made from gallium nitride (GaN).
  • Infrared: gallium arsenide (GaAs).

Infrared LED LED Infrared

2. Features

  • Forward Voltage (Vf): usually 1.8–3.3 V depending on color.
  • Forward Current (If): typically 10–20 mA for standard LEDs.
  • Light Intensity: measured in millicandela (mcd) or lumens.
  • Viewing Angle: determines the spread of emitted light.
  • Polarity Sensitive: will not light if connected backwards.
  • Colors & Types: single-color, RGB, infrared, UV, high-power, and indicator LEDs.

4. How to Use

Safety Note

LEDs must be used with a current-limiting resistor or constant current driver to prevent damage.

4.1. Identify LED polarity

  • Cathode (negative): typically the shorter lead, flat spot on the rim, or larger internal electrode.
  • Anode (positive): longer lead and smaller internal electrode.

LED polarity

4.2. Choose the right LED

  • Select color/wavelength based on application.
  • Match forward voltage and current to the driver circuit.
  • Choose intensity and viewing angle for visibility needs.

4.3. Connect it correctly

  • Connect anode to positive, cathode to ground through a resistor.
  • Calculate resistor value:

R = Vsupply - Vf / If

Example: For 5 V supply, red LED (Vf = 2 V), If = 20 mA:
R = 5-2 / 0.02 = 150 Ω

4.4. Test and verify

  • Use a multimeter’s diode test mode to check polarity and function.
  • LED should light dimly under diode test (forward bias).

4.5. Install in circuit

  • On breadboard: insert leads in*

5. Video Explanation

video coming soon