04·Resistors and LEDs: why a diode needs a resistor

Introduction

An LED is a nonlinear component — above its forward voltage its current rises steeply, so without current limiting it is easy to burn out. This lesson shows how to size a series resistor from the formula R = (V_supply − V_LED) / I_LED, how to compute the power dissipated in the resistor, how to read the color code, and what currents safely load the Arduino pins. All on real values: 5 V, 3.3 V, 220–1000 Ω resistors, currents in mA.