Diodes
LED
A diode that gives off light when current flows forwards through it. The colour depends on the semiconductor material, and so does the forward voltage: about 1.8 V for red and about 3 V for blue or white.
Think of it as: A one-way valve that glows when water rushes through it.
Pins
- A (anode, +)
- : The longer leg. Goes towards the positive supply.
- K (cathode, −)
- : The shorter leg, next to the flat edge of the rim. On SMD LEDs there is a mark (often green) on this end.
Through-hole LEDs are 3 mm or 5 mm domes; SMD LEDs are tiny chips (0603, 0805) with a mark on the cathode end.
What to look for inside
- A red LED. The voltage across it slowly rises to about 1.95 V and falls again.
- Below about 1.6 V the barrier in the middle (dark band) holds the carriers apart: no current, no light.
- Past the knee, electrons and holes rush into the middle and meet. Each meeting gives off one small packet of light (a photon): the red wiggles.
- More current means more meetings per second, so a brighter LED.
The numbers that matter
| Spec | Typical |
|---|---|
Forward voltage V_F Use it to work out the series resistor. | 1.8 V red · 2.1 V green · 3 V blue/white |
Forward current I_F Modern LEDs are bright at 2 to 5 mA. 20 mA is usually the maximum. | 2 to 20 mA |
Reverse voltage LEDs are poor at blocking. More than about 5 V backwards can kill them. | ≈ 5 V |
Luminous intensity How bright, measured straight ahead. | mcd |
Where you’ll use it
- Indicator lights: power on, status, errors.
- Driven straight from a GPIO pin through a resistor, or through a transistor for more current.
- Lighting: white LED strips, torches, displays.
Common mistakes
- No series resistor. The current is limited only by luck, and the LED burns out.
- Fitting it backwards. It simply stays dark (and can be damaged by more than 5 V).
- Running LEDs in parallel off one resistor. They never share evenly; give each its own resistor.
Lessons that use it
- Lesson 0.1: Your First Circuit: Voltage, Current & Resistance
- Lesson 3.1: Diodes & LEDs: Forward Voltage and the I-V Curve
Go deeper
Series resistor: e.g. (5 V − 1.8 V) ÷ 10 mA = 320 Ω, use 330 Ω.
Shorter wavelength (blue) means more energy per photon, which is why blue LEDs need a higher voltage.