LED
An LED emits light when current flows through its junction in the forward direction. The colour is set by the semiconductor recipe; the brightness is set by the current you push through it.
Electrons crossing the junction drop into lower-energy holes, releasing the difference as photons. Different semiconductor materials drop different energy gaps, which is why LEDs come in red, green, blue, and beyond.
In plain terms
A water mill that, instead of grinding grain, glows when water flows through it. Reverse the flow and the mill stops.
Also called: light-emitting diode.
Why designers use it
- Show power, status, or activity to the user.
- Backlight an LCD, keypad, or button.
- Illuminate scenes — from a flashlight to a stadium.
- Carry data over fibre or short-range IR remotes.
Best for
- Indicators
- Backlights
- General lighting
- Optical signalling
Key specifications
- Forward voltage: 1.8 V (red) – 3.4 V (blue/white)
- Forward current: 1 mA – 1 A
- Luminous flux: 0.01 lm – 5,000 lm
- Wavelength: 200 nm (UV) – 2 µm (IR)
- Lifetime (L70): 20,000 – 100,000 h
When not to use it
- Without a current-limiting resistor or driver — they self-destruct quickly.
- Where a diffuse glow is needed and the LED's narrow beam would create harsh hotspots.
Common mistakes
- Wiring an LED across a battery without a series resistor.
- Reversing the polarity — most LEDs simply don't light, but high-voltage drives can damage them.
- Using a generic resistor value across all colours; forward voltages differ widely.
Where you will find it
- Every TV remote talks to your TV by blinking an infrared LED in coded pulses — invisible to your eye, but if you point the remote at a phone camera and press a button you'll actually see it flashing.
- Modern car headlights are arrays of high-power LEDs whose beam pattern is steered by a tiny computer that dims individual LEDs to avoid blinding oncoming drivers.
- Pulse-oximeters clip to your finger and shine a red and an infrared LED through your blood; comparing how much of each colour passes through tells the device how oxygenated you are — the same trick used in COVID wards.
A short history
Semiconductor light emission was reported by H. J. Round in 1907, long before practical LED indicators. Rubin Braunstein reported infrared emission from gallium arsenide in 1955, and Nick Holonyak Jr. developed a practical visible-spectrum LED in 1962. These milestones illustrate why 'the first LED' needs context: observing light, building an infrared emitter and making a useful visible indicator are different achievements.
Good to know
- Blue LEDs were so hard to make that the breakthrough waited until 1993 — the trio responsible (Akasaki, Amano, Nakamura) won the 2014 Nobel Prize in Physics.
- A modern white LED is actually a blue LED shining onto a yellow phosphor coating — the eye averages the two and sees white.
- The world's tiniest LEDs (microLED) measure about 5 µm across — small enough to fit 200 of them in the diameter of a human hair.