LEDs & Photodiodes

What is an LED?

A Light-Emitting Diode (LED) is a special diode that emits light when forward-biased. When current flows through the PN junction, electrons recombine with holes and release energy as photons. The colour of the light depends on the semiconductor material and the energy gap between bands.

LED Forward Voltages by Colour

• •Red — V_f ≈ 1.8–2.0 V
• •Orange / Yellow — V_f ≈ 2.0–2.2 V
• •Green — V_f ≈ 2.0–3.0 V (varies widely between standard and high-brightness)
• •Blue — V_f ≈ 3.0–3.5 V
• •White — V_f ≈ 3.0–3.5 V (white LEDs are blue LEDs with a phosphor coating)
• •Infrared (IR) — V_f ≈ 1.1–1.5 V

The Current-Limiting Resistor

LEDs are specified for a certain operating current — typically 10–20 mA for standard indicator LEDs. Without a resistor to limit the current, the LED would draw excessive current and burn out almost instantly. The resistor value is calculated from the supply voltage, the LED forward voltage, and the desired current.

Brightness & Efficiency

• •Luminous intensity is measured in millicandela (mcd). A standard indicator LED is about 20–50 mcd; high-brightness LEDs can exceed 10,000 mcd.
• •Viewing angle — how wide the light spreads. Narrow angle (15°) = focused beam. Wide angle (120°) = broad illumination.
• •Efficiency — LEDs convert 20–50% of electrical energy to light (far better than incandescent bulbs at ~5%).
• •More current = more brightness, but only up to the rated maximum. Beyond that, heat destroys the LED.

Multi-colour & RGB LEDs

An RGB LED contains three separate LED chips (Red, Green, Blue) in a single package with either a common cathode or common anode. By varying the current through each chip, you can mix any colour. Addressable LEDs like WS2812B (NeoPixel) integrate an RGB LED with a tiny controller IC — each LED in a strip has its own address and can be individually controlled over a single data wire.

Photodiodes

A photodiode works in reverse: instead of converting current to light, it converts light to current. When photons hit the PN junction, they generate electron-hole pairs, producing a small current proportional to the light intensity. Photodiodes are operated in reverse bias (photoconductive mode) for speed, or zero-bias (photovoltaic mode — like a tiny solar cell) for precision.

• •Phototransistor — a photodiode + transistor in one package. More sensitive but slower.
• •Solar cell — essentially a large-area photodiode optimised to generate power from sunlight.
• •IR receiver — photodiode sensitive to infrared, used in TV remotes and proximity sensors.