Introduction

Lighting up an LED is often the first step in electronics. It sounds simple, but getting the resistor value right requires Ohm’s Law — one of the most fundamental concepts in electronics.

In this article, we’ll calculate the correct resistor value and build a working LED circuit on a breadboard.

What is Ohm’s Law?

Ohm’s Law states the relationship between voltage (V), current (I), and resistance (R):

V = I × R

To find the resistance needed, we rearrange it:

R = V / I

Calculating the Resistor Value

A typical red LED has these specifications:

  • Forward voltage (Vf): 2.0V
  • Recommended current (If): 10mA (0.01A)

With a 5V Arduino supply:

R = (5V - 2V) / 0.01A = 300Ω

The closest standard value is 330Ω, which is what we’ll use.

Circuit Diagram

Connect the components as follows:

  • Arduino 5V pin → 330Ω resistor → LED anode (+) → LED cathode (−) → GND

Building on a Breadboard

  1. Insert the 330Ω resistor into the breadboard
  2. Connect the LED anode to one end of the resistor
  3. Connect the LED cathode to the GND rail
  4. Connect the resistor’s other end to the Arduino 5V pin

Your LED should light up immediately — no code required for this basic circuit!

Summary

  • Always use a current-limiting resistor with LEDs
  • Calculate resistor value with: R = (Supply voltage − LED forward voltage) / desired current
  • 330Ω is a safe choice for 5V Arduino + standard LED