Everything about the humble LED

Why LEDs come in different colours

The colour of an LED has nothing to do with the plastic of its case — a clear LED can glow red, a coloured one can glow white. The colour is set by the semiconductor material of the tiny crystal inside: different materials release photons of different energy, and photon energy is colour. That’s why you can’t just “paint” an LED a new colour.

Note white: there’s no “white” semiconductor — a white LED is a blue die with a yellow phosphor coating that mixes up to white.

What’s inside a 5 mm LED

An LED isn’t just coloured plastic — it’s a little optical machine. The light is born in a semiconductor die less than a millimetre across, held in a reflector cup that aims the light forward, connected by a hair-thin gold bond wire, and focused by the epoxy lens. The two legs are not interchangeable: the long leg is the anode (+), the short leg — next to the flat side of the rim — is the cathode (−).

Why each colour has its own voltage

When current flows, electrons drop to a lower energy level and emit a photon. A bluer, higher-energy photon needs a bigger energy drop — which means a higher forward voltage (Vf). So red sits around 2 V while blue, white and UV need 3–3.8 V. This matters directly on a 3.3 V ESP32: a blue or white LED is already close to the rail, so it needs only a small resistor (and a red one needs a bit more).

Sizing the series resistor

Never wire an LED straight to a pin. An LED barely limits its own current, so without a resistor it pulls too much and burns out (and can stress the pin). The resistor sets the current with Ohm’s law — subtract the LED’s forward voltage from the supply, divide by the current you want:

// Series resistor for an LED:  R = (Vsupply − Vf) / I
//
// Red LED (Vf ≈ 2.0 V) at 10 mA on a 3.3 V ESP32 pin:
//   R = (3.3 − 2.0) / 0.010 = 130 Ω  →  use 150 Ω (nearest common)
// Same red LED on a 5 V Arduino pin:
//   R = (5.0 − 2.0) / 0.010 = 300 Ω  →  use 330 Ω

void setup() {
  ledcAttach(2, 5000, 8);      // GPIO2, 5 kHz, 8-bit — hardware PWM dimming
}

void loop() {
  for (int d = 0; d <= 255; d++) { ledcWrite(2, d); delay(4); }   // fade up
  for (int d = 255; d >= 0; d--) { ledcWrite(2, d); delay(4); }   // fade down
}

Aim for 5–15 mA for a normal indicator (an ESP32 pin should stay under ~20 mA anyway; see 3.3 V vs 5 V). Rather than recompute every time, these are the everyday values (blue/white LED at 10 mA):

The quick rule of thumb: on a 3.3 V ESP32 pin use 33–100 Ω; on a 5 V Arduino pin use 220–330 Ω. Want smooth dimming instead of on/off? Drive the pin with hardware PWM — the resistor stays exactly the same.