Your First External LED — Arduino L1

Learning Goals 5 min

By the end of this lesson you will be able to:

Wire an external LED to digital pin 13 on the Arduino through a 220 Ω current-limiting resistor, with the LED in the correct polarity.
Tell the anode (long leg) from the cathode (short leg) of any LED by sight, and explain why the resistor must always be in series — never in parallel — with the LED.
Re-upload the Blink sketch from L01-02 and watch your external LED blink in step with the onboard L LED on pin 13.

Warm-Up 10 min

Last lesson we decoded resistor colour bands. Today we put that 220 Ω red-red-brown-gold resistor to work alongside an LED — the most-used pair of parts in all of electronics.

Quick-fire puzzle

Aisyah holds up a brand-new red LED and a 9 V battery. She is about to clip the LED's two legs straight onto the battery terminals — no resistor, no Arduino — just LED + battery.

What do you predict will happen in the first 1–2 seconds?
Using Ohm's Law and assuming the LED itself has almost no resistance, roughly how much current would try to flow?
What single component, costing about RM 0.20, would have saved the LED?

Reveal the answer

The LED will flash brightly for a fraction of a second, then burn out. You may see a tiny puff of smoke.
I = V ÷ R. With almost no resistance in the path, R is tiny — so I is huge. Hundreds of milliamps. A red LED is rated for about 20 mA. It cannot survive a flood that large.
A resistor. Putting one in series with the LED limits the current to a safe value. That is the entire job of the 220 Ω resistor in today's circuit.

Every external LED you ever wire — in this course, on a hobby project, inside a real product — sits next to a resistor for this reason.

New Concept 20 min

The big idea — an LED is a one-way valve

An LED is like a tiny water valve that only opens in one direction. When current is pushed in through the long leg (the anode) it flows out the short leg (the cathode) and the bulb glows. Push current the wrong way — in through the short leg — and the valve stays shut. No light.

Anatomy of an LED

Feature	What it tells you
Long leg	This is the anode (+). Current enters here.
Short leg	This is the cathode (−). Current leaves here.
Flat side on the round body	The flat side is on the cathode. Useful when the legs have been trimmed equal.
Inside the bulb	The smaller metal piece is the anode wire; the bigger cup-shaped piece is the cathode.

Wiring this lesson's circuit

One LED. One resistor. Two jumper wires. The Arduino's pin 13 supplies the signal; the Arduino's GND pin completes the circuit.

From	To	Notes
Arduino pin `D13`	Breadboard `A6`	Yellow jumper. This is the signal wire — when pin 13 goes HIGH, it pushes 5 V into the circuit.
Resistor leg 1	Breadboard `D6`	Same column as the yellow wire (col 6). The column-tie strip joins them.
Resistor leg 2	Breadboard `D11`	The resistor lies flat across row D, bridging col 6 and col 11.
LED anode (long leg)	Breadboard `B11`	Same column as the resistor's right leg (col 11). Current arrives from the resistor.
LED cathode (short leg)	Breadboard `B10`	Adjacent column. The LED bridges col 10 ↔ col 11.
Breadboard `C10`	Breadboard − rail	Black jumper. Brings the current out of the LED back toward GND.
Breadboard − rail	Arduino `GND`	Second black jumper. Returns the current to the Arduino.

The sketch — same Blink as L01-02

We do not need a new sketch today. The original Blink sketch you uploaded in L01-02 toggles pin 13 on and off once a second. As long as your external LED is wired to pin 13, that same code will make it blink.

// The same Blink sketch from ARD-L01-02 — no changes needed.
void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

LED_BUILTIN is just a built-in name for pin 13. We will dissect every line of this sketch in L01-08. Today, our job is the wiring.

Why it matters

An LED-plus-resistor is the single most common indicator circuit on the planet. Inside your TV remote, your Wi-Fi router, your microwave, your phone charger — every "I am on" light is the circuit you are about to build.

Worked Example 20 min

Build the circuit step by step. Don't power the Arduino on until step 6.

Plug the resistor in. Push one leg of the 220 Ω resistor into D6 and the other leg into D11. It lies flat across row D.
Plug the LED in. Put the LED's long leg (anode) into B11 — the same column as the resistor's right leg. Put the short leg (cathode) into B10.
Add the yellow signal wire. One end into Arduino pin D13, the other end into A6 on the breadboard.
Add the first black wire. One end into C10 on the breadboard, the other end into any hole on the − rail (top of the breadboard).
Add the second black wire. One end into the − rail, the other end into the Arduino's GND pin.
Power up. Plug the Arduino into your laptop with the USB cable. The Blink sketch from L01-02 should already be on the board.

The full wiring diagram

Trace the current: when pin 13 goes HIGH, 5 V leaves Arduino D13, runs up the yellow wire to A6, crosses the 220 Ω resistor in row D to D11, travels up column 11's tie strip to the LED's anode at B11, lights the bulb, exits the cathode at B10, drops out at C10 onto the − rail, and returns through the second black wire to the Arduino's GND pin. Maths: I = (5 − 2) ÷ 220 ≈ 13.6 mA — bright but safe.

Expected behaviour

The onboard L LED on the Arduino blinks once per second (the same as in L01-02).
Your external red LED blinks in perfect step with it.
If only the onboard LED blinks but yours does not, your wiring has a problem. Try flipping the LED — that fixes most first-time builds.

Try It Yourself 20 min

Three small experiments on the circuit you just built. No new sketch — just hardware swaps and a maths check.

🟢 Easy

Goal: Swap the 220 Ω resistor for a 1 kΩ resistor (brown-black-red-gold). The LED still blinks — but does it look brighter, dimmer or the same? Use Ohm's Law to prove what your eyes see.

Given: V_pin = 5 V, LED drops ≈ 2 V, so the resistor sees 3 V. R = 1 000 Ω.
Find: I in milliamps
Formula: I = V ÷ R
Working: I = ____ ÷ ____ = ____ A = ____ mA
Compared to the 220 Ω value (≈ 13.6 mA), the LED is: brighter / dimmer / same? ____

🟡 Medium

Goal: Swap the red LED for a green or yellow LED (same wiring, same resistor). Watch how bright it looks. Different colour LEDs "use up" different amounts of voltage. Recalculate the current using each LED's forward voltage from the table.

LED colour	Forward voltage (V_F)	V across the 220 Ω resistor	Current
Red	~ 2.0 V	5 − 2.0 = 3.0 V	≈ 13.6 mA
Yellow	~ 2.1 V	____ V	____ mA
Green	~ 3.0 V	____ V	____ mA

Formula: I = (V_pin − V_F) ÷ R
Question: Which colour LED draws the least current with this resistor, and why does that match what your eyes see?

🔴 Stretch

Goal: Re-wire the LED so it stays on all the time, instead of blinking — without changing the sketch. (Hint: pin 13 turns on and off because the sketch tells it to. Which Arduino pin is always 5 V?)

Question: Where would you move the yellow wire to make this happen? ____
Question: Name one real product that uses an always-on LED like this. ____

Reveal the wiring change

Move the yellow wire from Arduino pin D13 to the Arduino's 5V pin. The 5V pin is always at 5 V the moment the board is powered, so current flows continuously: 5V → resistor → LED → GND. The LED stays on as long as the USB cable is plugged in. Real-world examples: the power LED on a phone charger, the standby light on a TV, the always-on dot next to your router's WPS button.

Mini-Challenge 15 min

The two-rail power indicator

Real makers don't bring 5 V and GND straight from the Arduino pins to every component. They use the breadboard's + rail and − rail as a power bus, then pull side wires off as needed. Today's challenge: rebuild the always-on version of your LED circuit using both rails properly.

Your task:

Run a red jumper from Arduino 5V to any hole on the breadboard's top + rail.
Run a black jumper from Arduino GND to any hole on the breadboard's top − rail.
Plug your 220 Ω resistor between a hole on the + rail and one of the rows in the top half (e.g. A20).
Plug your red LED so its anode shares the column with the resistor's right leg, and its cathode goes to a fresh column.
Run a short black jumper from the LED's cathode column to the − rail.

It works if:

The LED lights up the instant the USB cable is plugged in.
Nothing else on the Arduino changes — pin 13's onboard L LED keeps blinking (because the Blink sketch is still running), but your external LED is steady-on, not blinking.
The wires touching the Arduino are only at 5V and GND — pin 13 is no longer involved.

Reveal one valid wiring answer

5 V from the Arduino runs up to the + rail (red wire). A short red jumper pulls 5 V down into column 16 of the breadboard. The 220 Ω resistor sits across row D, from D16 to D21. Column 21's tie strip lifts the current to the LED anode at B21; the bulb glows. The cathode at B20 drops out at C20 onto the − rail, which loops back to GND. Maths: I = (5 − 2) ÷ 220 ≈ 13.6 mA — the same brightness as the worked example, but now powered directly from the rails.

Recap 5 min

An external LED needs three things to work: a current source (a 5 V pin or any digital pin set HIGH), a current-limiting resistor in series, and the correct polarity (long leg toward the +, short leg toward the −). Wire all three the right way and the bulb lights up. Skip the resistor and the LED dies; flip the polarity and the LED just stays dark.

LED: Light-Emitting Diode. A tiny one-way valve that glows when current flows from anode to cathode.
Anode: The long leg of an LED. The + side. Current enters here.
Cathode: The short leg of an LED, flagged by the flat side of the body. The − side. Current leaves here.
Forward voltage (V_F): The voltage an LED "uses up" while lit. Red ≈ 2.0 V, yellow ≈ 2.1 V, green ≈ 3.0 V, blue/white ≈ 3.2 V.
Current-limiting resistor: A resistor placed in series with an LED to keep the current at a safe level — typically 220 Ω for 5 V circuits.
Series: One-after-the-other. Current has only one path through the components. The opposite is parallel.

Homework 5 min

Rebuild and prove. At home (or in study group), rebuild the lesson's circuit using a different colour LED — green, yellow, blue or white — and the correct resistor for that colour.

Decide which colour LED you want to use. Look up its forward voltage from the table in the Try-It-Yourself section.
Using R = (V_pin − V_F) ÷ I_desired with V_pin = 5 V and I_desired = 15 mA, calculate the resistor value you need.
Pick the closest resistor in your kit (220 Ω, 560 Ω, 1 kΩ, 4.7 kΩ or 10 kΩ).
Write down its 4-band colour code so you can prove you read it (skill from L01-06).
Wire the circuit. Upload the Blink sketch from L01-02 if needed. Confirm the new LED blinks in step with the onboard LED.

Bring back next class:

A phone photo of the working circuit with the LED lit.
Your notebook page showing the maths (R = ____), the resistor's colour bands, and which LED colour you chose.