Vol. IV — Electronics · Tool
An LED wants current, not voltage. The series resistor drops what the LED does not use. Set the supply, the LED’s forward voltage, and how many milliamps you want. The ohms update as you type.
The battery voltage and the LED’s forward voltage differ by a leftover amount. That leftover sits across the resistor: R = (Vs − Vf) / I. Without the resistor the LED tries to take as much current as the supply will give, and it cooks.
Typical indicator LEDs run around 5–20 mA. The nearest E24 value is the common through-hole part you can buy; a slightly higher ohm value means a little less current, which is usually safer.
After you pick ohms, decode the bands on Resistor codes, or practice the idea in the Circuit explorer LED lesson.