Blink an LED with Arduino — your first circuit, explained

#What you will build

A red LED that switches on for one second, off for one second, forever. It sounds tiny — but it is the same thing every Arduino project does: a program decides when a pin is on or off, and a circuit turns that into something you can see. Once you understand this one, motors, buzzers and robots are the same idea with different parts.

#How it works, in one picture

An Arduino pin is like a tiny switch that the program flips between 5 V (HIGH) and 0 V (LOW).

• When pin 13 is HIGH, current flows out of the pin, through the LED, through the resistor, and back into GND. The LED lights.
• When pin 13 is LOW, both ends of the circuit are at 0 V. No current flows, so the LED goes dark.

Current always needs a complete loop: out of the pin, through the parts, and back to GND. Break the loop anywhere and nothing happens.

#Why the LED needs a resistor

An LED has almost no resistance once it starts to glow. Wired straight to 5 V it would try to pull far more current than it can take — and burn out (in our 3D lab it really does, with a puff of smoke 💨).

The 220 Ω resistor limits the current. With a red LED (about 1.9 V across it), the resistor gets the rest:

14 mA is bright, safe for the LED (max about 20 mA) and safe for the pin (max 40 mA).

#The wiring

1. Push the LED into the breadboard. The long leg is the anode (+), the short leg the cathode (−).
2. Put the 220 Ω resistor so that one of its legs is in the same row of five holes as the LED's short leg.
3. A jumper wire from pin 13 to the LED's long leg.
4. A jumper wire from the resistor's free leg to GND.

Here is the same circuit as a schematic, the way electronics engineers draw it. Pins with the same label are connected:

#The code, line by line

void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(13, HIGH);
  Serial.println("LED on");
  delay(1000);
  digitalWrite(13, LOW);
  Serial.println("LED off");
  delay(1000);
}

• setup() runs once, when the board powers up or resets.
• pinMode(13, OUTPUT) makes pin 13 an output: the program drives it, instead of reading it.
• Serial.begin(9600) opens the serial connection so the board can print messages to your computer (the Serial Monitor) at 9600 bits per second.
• loop() runs again and again, forever.
• digitalWrite(13, HIGH) puts 5 V on the pin — the LED turns on.
• delay(1000) waits 1000 milliseconds = 1 second. The board does nothing else meanwhile.
• digitalWrite(13, LOW) puts 0 V on the pin — the LED turns off.

#Common mistakes

• The LED is in backwards. It only conducts one way. Flip it: long leg toward the pin.
• No resistor. The LED is bright for a moment, then dead. Always add one.
• Both LED legs in the same row. The five holes of a row are joined inside the breadboard, so the LED is short-circuited and never lights. See how a breadboard works.
• Wire to the wrong pin. The code says 13 — the wire must go to 13.

#Try these challenges

1. Make it blink twice as fast (change both delays to 500).
2. Make a heartbeat: on 100 ms, off 100 ms, on 100 ms, off 700 ms.
3. Move the LED to pin 12 — change the wire and the code.
4. In the 3D lab, delete the resistor and put a jumper wire in its place (from the LED's short-leg row to GND). Press Run and watch what happens — then press Replace part.

#Check yourself

#Questions people ask

#Can I use any pin, not just 13?

Yes — pins 2 to 13 all work as outputs. Change the number in pinMode and digitalWrite to match.

#Why 220 Ω? Can I use 330 Ω or 1 kΩ?

Anything from about 150 Ω to 1 kΩ works. Bigger is dimmer and gentler; 220 Ω or 330 Ω is the usual choice.

#Do I need an Arduino to try this?

No. Every project here runs in our free 3D lab in your browser — the code really compiles and runs on a simulated ATmega328P, the same chip as on the UNO.

#What's next

You now know how to control something. Next, learn how to read something: a push button — your first input.

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Last updated 2026-10-29