Your first LED circuit

An LED needs a resistor in series. Without it, too much current flows and the LED burns out in seconds.

Find the resistor value

R = (Vsupply − Vled) ÷ I

For a red LED (about 2 V) on 5 V at 15 mA: (5 − 2) ÷ 0.015 = 200 Ω. Pick the next bigger standard value, 220 Ω.

Build it

1. Long leg of the LED (+) goes to the resistor.
2. The resistor goes to Arduino pin D13.
3. Short leg of the LED (−) goes to GND.
4. Upload the Blink example. The LED flashes once a second.

Try the LED resistor calculator in Tools to check other colours.