LEDs — How to Connect One Without Burning It (Complete Guide)

From a single indicator LED to RGB strips — everything you need to know about light-emitting diodes

Introduction

The LED is the first component most people connect to a microcontroller — and the first component most people burn out. Connecting an LED directly to 5V without a resistor destroys it in seconds. Done correctly, LEDs are incredibly versatile: indicators, displays, backlights, decorative lighting, and communication signals. This post covers physics, current calculations, LED types, RGB LEDs, PWM dimming, and addressable strips like WS2812B.

Part 1: How LEDs Work

As covered in Post 7, an LED is a diode that emits light when forward biased. When electrons from the N-side recombine with holes on the P-side, they release energy as photons. The colour depends on the semiconductor material and the photon energy level.

Colour	Semiconductor Material	Forward Voltage	Wavelength
Infrared (IR)	Gallium Arsenide (GaAs)	1.2-1.5V	700-1000nm
Red	Gallium Arsenide Phosphide	1.8-2.2V	620-750nm
Orange	GaAsP / AlGaAs	2.0-2.2V	590-620nm
Yellow	Gallium Phosphide (GaP)	2.0-2.2V	570-590nm
Green	Indium Gallium Nitride (InGaN)	1.9-2.4V	495-570nm
Blue	Indium Gallium Nitride	2.8-3.3V	450-495nm
White	Blue LED + Yellow phosphor	3.0-3.4V	380-750nm (broad)
UV	Aluminium Gallium Nitride	3.0-4.5V	200-400nm

Figure 1 — Typical LED forward voltage by colour; higher-energy photons (blue/white/UV) need higher forward voltage.

Part 2: The Resistor Calculation — The Most Important Formula

An LED is a diode — once conducting, its resistance drops very low. Without a current-limiting resistor, current is limited only by wire resistance (nearly zero), and the LED pulls as much current as the source can provide. This destroys the LED in under a second.

🔑  KEY CONCEPT

R = (Vsupply − Vled) ÷ Iled

Where Vsupply = your power supply voltage, Vled = LED forward voltage, Iled = desired current (typically 10-20mA = 0.01-0.02A).

Scenario	Vsupply	Vled	Desired Current	R calculation	Use
Red LED on Arduino 5V	5V	2.0V	15mA (0.015A)	(5-2)/0.015 = 200Ω	220Ω
Blue LED on Arduino 5V	5V	3.2V	15mA	(5-3.2)/0.015 = 120Ω	120Ω or 150Ω
Red LED on 3.3V MCU	3.3V	2.0V	10mA (0.01A)	(3.3-2)/0.01 = 130Ω	150Ω
Red LED on 9V supply	9V	2.0V	20mA (0.02A)	(9-2)/0.02 = 350Ω	330Ω or 390Ω
White LED on 12V	12V	3.2V	20mA	(12-3.2)/0.02 = 440Ω	470Ω

Part 3: Identifying LED Polarity

Connect an LED backwards and it won’t light (and won’t be damaged in most cases — LEDs are simply reverse-biased). How do you know which leg is which?

• Longer leg = ANODE (+) — connects toward positive voltage
• Shorter leg = CATHODE (−) — connects toward ground
• Flat edge on the LED body rim = CATHODE side
• Inside the LED dome: the larger electrode = CATHODE (the cup); the smaller wire = ANODE
• Multimeter diode test: LED glows when red probe on anode (long leg), black on cathode

Part 4: Multiple LEDs

Multiple LEDs in Parallel (Same Pin)

Each LED gets its own resistor and connects from the same pin to GND — each has full voltage and draws its own current. Total current = sum of all LED currents.

⚠️  WARNING

Arduino digital output pins can source/sink a maximum of 40mA, and the absolute maximum for ALL pins combined is 200mA.

With three LEDs at 15mA each = 45mA from one pin — this exceeds the single-pin limit.

For multiple LEDs, use a transistor or dedicated driver.

Multiple LEDs in Series

Connect LEDs end-to-end with one shared resistor — all LEDs get the same current, and voltage drops add up: R = (Vsupply − Vled1 − Vled2 − … − VledN) ÷ Iled. Example: 3 red LEDs in series on 12V: R = (12 − 2 − 2 − 2) ÷ 0.02 = 300Ω, use 330Ω. Advantage: more efficient than parallel (one resistor, lower power loss). Downside: if one LED fails open, all go dark.

Figure 2 — LEDs in series (left, one shared resistor) vs parallel (right, each LED needs its own).

Part 5: PWM Dimming

PWM (Pulse Width Modulation) dims an LED by switching it on and off very fast (typically 490-1000 Hz). The human eye sees the average brightness — 50% on-time = 50% brightness, 10% on-time = 10% brightness. On Arduino: analogWrite(pin, value), where value is 0 (off) to 255 (full on). Works on PWM-capable pins (3, 5, 6, 9, 10, 11 on Uno).

Part 6: RGB LEDs and Addressable Strips

RGB LED (4-pin)

An RGB LED contains three separate LEDs (Red, Green, Blue) in one package with four pins: one common anode (or cathode) plus one for each colour. Mixing different intensities of R, G, B using PWM creates millions of colours.

• Common Cathode: common pin to GND, individual pins to PWM outputs via resistors
• Common Anode: common pin to VCC, individual pins LOW through resistors to GND

Addressable LED Strips (WS2812B / NeoPixel)

Each WS2812B LED contains its own control IC. A single Arduino data wire controls hundreds of individually addressable full-colour LEDs. The protocol sends 24-bit colour data (8 bits each for R, G, B) to each LED in sequence at 800kHz.

• Power: each LED draws up to 60mA at full white. 60 LEDs = 3.6A! Use a dedicated 5V supply.
• Library: Adafruit NeoPixel or FastLED — both are excellent.
• Connection: 100-500Ω resistor in series with the data line prevents ringing. 1000µF capacitor across power.

Common Mistakes Beginners Make

• Connecting LEDs directly to 5V without a resistor
• Using one resistor for parallel LEDs
• Reversing LED polarity
• Exceeding Arduino GPIO current limits
• Powering long WS2812B strips directly from the Arduino
• Forgetting a common ground with external power supplies

Frequently Asked Questions

Q: Can I just use a lower voltage supply to skip the resistor?

A: Not reliably. Even a supply voltage close to the LED’s forward voltage can allow current to shoot up unpredictably because LED resistance drops sharply once conducting. A current-limiting resistor sized correctly is always the safer, more predictable approach.

Q: Why do blue and white LEDs need a smaller resistor than red on the same supply?

A: Blue and white LEDs have a higher forward voltage (~3.2V vs ~2V for red). Since R = (Vsupply − Vled) ÷ I, a higher Vled leaves less voltage to drop across the resistor, so the required resistance is smaller for the same target current.

Q: What happens if I wire an RGB LED with the wrong common pin type?

A: If you wire a common-anode LED as if it were common-cathode (or vice versa), the individual colour channels won’t light correctly, or will stay on/off inverted from what you expect. Always confirm which type you have — usually printed on the datasheet or determined by testing with a multimeter’s diode mode.

Q: Do WS2812B strips need a microcontroller with special hardware?

A: No special hardware is required, but the timing is strict (precise microsecond-level pulses), so libraries like Adafruit NeoPixel or FastLED handle the low-level bit-banging so you don’t have to write the protocol yourself.

Q: Is PWM dimming bad for LED lifespan?

A: No — PWM at typical frequencies (490Hz+) is a standard, safe way to dim LEDs and doesn’t shorten their life, since each pulse still keeps the LED within its rated forward current when on.

Q: How many LEDs can I drive directly from one Arduino pin?

A: None should be driven at more than the per-pin limit (40mA on most Arduino boards), and the combined draw across all pins shouldn’t exceed the chip’s total limit (200mA on an Uno). For more than a couple of LEDs at once, use a transistor, MOSFET, or dedicated LED driver IC instead of pulling all the current through GPIO pins.