Day 2: Blinking an External LED & Understanding the Breadboard

Welcome back, Anish. Yesterday we made the tiny built-in LED on the Arduino blink. Cool, but small. What if we want to blink a bigger LED of our own? That’s where we go today. We’re going to wire up an external LED on a breadboard and blink it with the same kind of code from Day 1.

What you need today

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• Arduino Uno + USB cable
• Breadboard
• 1 LED (any color — red is classic)
• 1 220Ω resistor (red-red-brown-gold stripes)
• 2 jumper wires (male-to-male)

What is a breadboard?

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A breadboard is a flat plastic board full of tiny holes. It is basically a LEGO board for electricity. Under the plastic, some holes are already connected to each other with little metal strips. That means if you plug a wire into hole A and an LED leg into a connected hole B, they are now touching — without any soldering.

graph TB
    TOP["+ rail — all connected left-to-right (use for power)"]
    subgraph Middle["Middle area"]
        COL["Each vertical column of 5 holes
is connected top-to-bottom
(with a gap in the very middle)"]
    end
    BOT["− rail — all connected left-to-right (use for GND)"]

Two rules to remember:

1. The long rails on the top and bottom (marked + and −) are connected along the whole length. Use them for power and ground.
2. In the middle, each column of 5 holes is connected. The gap in the very middle splits the board into a top half and bottom half.

Once you see it a few times, it becomes obvious.

Why do we need a resistor?

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An LED is like a thirsty plant. If you give it too much water (too much electricity), it dies — sometimes with a little pop. The Uno sends out 5 volts from each pin, which is way more than an LED can handle on its own.

The resistor is like a narrow straw. It slows the electricity down so the LED gets just the right amount. A 220Ω resistor is a safe, gentle value for most LEDs with a 5V Arduino.

Rule: every LED needs a resistor in series with it. No resistor = burnt-out LED. Do not skip this.

The LED has a + side and a − side

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An LED only works one way around. Look at your LED carefully:

• The longer leg is the positive side (called the anode, +)
• The shorter leg is the negative side (called the cathode, −)

There is also a flat spot on the plastic rim near the shorter leg, in case the legs get bent.

If you plug the LED in backward, it won’t blow up — it just stays dark. Flip it and try again.

The circuit

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We’re going to use pin 8 this time, not pin 13. Why? Because pin 13 is already wired to the built-in LED on the board. If we use pin 13, both LEDs would blink — the built-in one and our new one. Our external LED would not feel special. On any pin from 2 to 12, our LED gets the spotlight to itself.

graph LR
    PIN8["Arduino
Pin 8"] -->|jumper wire| R["220Ω
Resistor"]
    R --> ANODE["LED long leg
( + anode )"]
    ANODE --> CATHODE["LED short leg
( − cathode )"]
    CATHODE -->|jumper wire| GND["Arduino
GND"]

Step-by-step on the breadboard:

1. Plug the LED into the breadboard so the two legs are in different columns (e.g. long leg in column 10, short leg in column 11). If both legs were in the same column, they’d be shorted — nothing would light.
2. Plug one end of the 220Ω resistor into the same column as the long leg of the LED. Plug the other end of the resistor into an empty column (e.g. column 8).
3. Use a jumper wire from Arduino pin 8 to column 8 on the breadboard (where the free end of the resistor is).
4. Use a second jumper wire from the GND pin on the Arduino to the column with the LED’s short leg.

That’s the whole circuit. Electricity flows: Pin 8 → resistor → LED long leg → LED short leg → GND. A loop, all the way back to the Uno.

The code

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Open a new sketch in the Arduino IDE and type this in:

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void setup() {
  pinMode(8, OUTPUT);     // Pin 8 is an output now (not pin 13)
}

void loop() {
  digitalWrite(8, HIGH);  // LED ON
  delay(500);             // wait half a second
  digitalWrite(8, LOW);   // LED OFF
  delay(500);             // wait half a second
}

Click Upload. Your external LED should blink on and off every half second.

If nothing happens:

• Is the LED plugged in the right way around? (long leg toward the resistor)
• Is the resistor in the same column as the long leg?
• Is the GND wire going to a GND pin on the Uno (not 5V)?
• Is the jumper going from pin 8 (not pin 13)?

What is new in today’s code?

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Notice how all four instructions from Day 1 are still here: pinMode, digitalWrite, delay. The only thing that changed is the pin number:

• Day 1: pinMode(13, OUTPUT) → controls the built-in LED
• Day 2: pinMode(8, OUTPUT) → controls our LED wired to pin 8

That’s it. You already knew how to blink; we just moved the blink to a different pin. This is the big idea: Arduino code for any output pin looks almost identical. Once you can do one, you can do them all.

Also notice we used delay(500) instead of delay(1000). That’s half a second instead of a full second — twice as fast. Same instruction, different number.

Try this

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1. Move the LED to pin 9. Change pinMode(8, OUTPUT) to pinMode(9, OUTPUT) and both digitalWrite(8, ...) lines to digitalWrite(9, ...). Unplug the jumper wire that went to pin 8, move it to pin 9. Upload. Still blinks, right? You just changed pins with a tiny code edit.
2. Reverse the LED (take it out, flip it, plug it back in). What happens? Nothing — it stays dark. That’s polarity. Put it back the right way.
3. Remove the resistor — but only for 1 second. Touch the LED with your finger as you upload. Do you feel it getting hot? Put the resistor back right away. LEDs without resistors die fast.

Bonus: Two LEDs blinking alternately

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If you have a second LED and a second 220Ω resistor, try this. Add a second LED on pin 12 exactly the same way you wired the first one.

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void setup() {
  pinMode(8, OUTPUT);
  pinMode(12, OUTPUT);
}

void loop() {
  digitalWrite(8, HIGH);   // LED 1 ON
  digitalWrite(12, LOW);   // LED 2 OFF
  delay(500);
  digitalWrite(8, LOW);    // LED 1 OFF
  digitalWrite(12, HIGH);  // LED 2 ON
  delay(500);
}

Two LEDs taking turns — like a police car light. See how we just added more instructions inside the same loop()? Arduino runs them top to bottom, then starts the loop over. That’s how you build anything: a little more, a little more, a little more.

What you learned today

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• How a breadboard is wired inside
• Why every LED needs a resistor (so you don’t burn it out)
• LEDs have a + side (long leg) and a − side (short leg)
• Any digital pin on the Arduino can control an external LED
• You can control multiple pins by calling pinMode and digitalWrite for each one
• Code that worked for pin 13 works for pin 8 — just change the number

What is next

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Day 3 — we flip it around. Instead of the Arduino telling something what to do, the Arduino listens to a button and reacts. You will learn your first input and your first if statement.

Nice work, Anish.