Master the Basics: What is an LED?

What Is an LED? Light Emitting Diodes, or LEDs, are special types of diodes that emit light when current flows through them. Like all diodes, LEDs allow current to pass in only one direction, but they are designed to release photons—particles of light—when operating.

The LED symbol looks like a regular diode with two arrows pointing outward, symbolizing the emitted light.

Physical Types of LEDs LEDs come in many shapes and sizes:

• Surface-mount LEDs are tiny rectangles used in compact electronic devices.

• Through-hole LEDs are more common in breadboarding and hobby projects. These typically have a plastic bulb and two long metal leads.

To determine polarity:

• The longer lead is the positive (anode).

• The shorter lead and the flat side of the bulb mark the negative (cathode).

If your LED doesn't light up, try flipping it around—it may be in backwards.

Forward Voltage and Color Each LED color has a different forward voltage:

• Red LEDs: 2.0–2.4V

• Blue LEDs: 3.0–3.4V

Blue LEDs usually handle more current and are brighter than red LEDs. Brightness is measured in millicandela (mcd):

• Low-intensity indicator: 50–100 mcd

• Bright LED flashlight: 10,000+ mcd

Controlling LED Brightness with Resistors LEDs don’t naturally limit current. If you connect one directly to a power source without protection, it can burn out quickly. To avoid this, use a current-limiting resistor in series with the LED.

Resistor Rule of Thumb For general use with 3.3V or 5V sources, a 330Ω resistor is typically safe for most standard LEDs.

Want Maximum Brightness? Use Ohm’s Law Let’s say:

• You’re using a 5V supply

• Your red LED has a 2V forward drop

• You want to allow 20mA max current

Then:

Resistance = (5V - 2V) / 20mA = 150Ω

Since 150Ω isn’t a common resistor, round up to the nearest safe value—220Ω. This protects the LED by reducing the current to about 13.5mA.

Never round down—you risk sending too much current through the LED.

Adjusting Brightness Use a higher resistor value to dim the LED:

• 220Ω: Brightest (safely)

• 10kΩ: Medium brightness

• 100kΩ: Very dim (~44μA current)

Conclusion LEDs are versatile, efficient, and fun to use in all kinds of projects. Just remember:

• They're polarized, so install them in the right direction

• Use a resistor to limit current and prevent burnout

• Calculate resistor values with Ohm’s Law to get the brightness you want

Whether you're building an indicator light or an LED matrix, understanding the basics of how LEDs work will help you design safer and more reliable circuits.