digitalWrite & HIGH/LOW

Learning Goals 5 min

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

1. Type a Blink sketch from scratch — no copying — using pinMode() in setup() and digitalWrite() with HIGH and LOW inside loop().
2. Predict what happens to the external LED if you swap HIGH ↔ LOW, or change a delay() value, before uploading.
3. Make your external LED flash in a rhythm different from the original Blink sketch — for example, a brief flicker every two seconds.

Warm-Up 10 min

Last lesson we wired an external LED to pin 13. We re-used the Blink sketch from L01-02 and watched it flash. Today we stop borrowing other people's code and write our own.

Quick-fire puzzle

Aaina is staring at the Blink sketch in the Arduino IDE. The two delay(1000) lines catch her eye. Without uploading anything, predict:

1. If she changes both delays from 1000 to 100, what will the LED look like?
2. If she changes both delays from 1000 to 5000, what will the LED look like?
3. If she changes only the first delay to 100 and leaves the second at 1000, what happens?

New Concept 20 min

The big idea — a digital pin is a switch

Each digital pin on the Arduino is a tiny switch that you control from code. When the switch is closed, the pin is at 5 V — we call this HIGH. When it is open, the pin is at 0 V — we call this LOW. A wired-up LED on that pin glows when the pin is HIGH and goes dark when it is LOW.

To use a pin as a switch you need three commands. Today's lesson is all about them.

The three functions you'll meet today

Reuse the circuit from L01-07

You don't need to rewire anything today. The exact circuit from L01-07 — external LED on pin 13 via a 220 Ω resistor, GND return through the breadboard's − rail — is the canvas for every sketch in this lesson. Keep it powered on.

If your board is already built from last lesson, leave it alone — this is the same circuit, hole for hole:

The full Blink sketch — line by line

Open the Arduino IDE. Go to File → New to start a blank sketch. Type the following yourself — do not copy-paste. The act of typing it burns the structure into your fingers.

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(1000);
  digitalWrite(13, LOW);
  delay(1000);
}

Read it slowly:

• setup() runs once when the Arduino powers on. We use it to declare that pin 13 is an OUTPUT.
• loop() runs forever, top to bottom, then starts again. Inside it we set pin 13 HIGH, wait 1 second, set it LOW, wait 1 second, repeat.
• The result: pin 13 turns on for 1 s, then off for 1 s, forever. Both the onboard L LED and your external LED blink in step.

Why it matters

Every output device you will ever drive — LEDs, buzzers, motors, relays, even your Wi-Fi router's status lights — is switched on and off using exactly this digitalWrite(pin, HIGH/LOW) pattern. Master it today and every later lesson rests on it.

Worked Example 20 min

Goal: type the Blink sketch from scratch, upload it, then change one number and watch the change happen in front of you.

Step 1 — type the sketch from scratch

Already done in the New Concept section. Save it as my-first-blink.

Step 2 — upload

1. Plug in the USB cable.
2. In the IDE: Tools → Board → Arduino UNO, then Tools → Port → (your COM port).
3. Click the round → arrow button (Upload). Wait for "Done uploading" in the status bar.
4. Your external LED on pin 13 should now blink once per second, in step with the onboard L LED. If it doesn't, check polarity and the resistor (see L01-07).

Step 3 — change one number

Change only the first delay(1000) to delay(100). Leave the second at 1000.

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(100);
  digitalWrite(13, LOW);
  delay(1000);
}

Step 4 — upload and observe

Re-upload. The LED now flashes briefly (100 ms) and stays off for a full second. It looks like a slow heartbeat. The pin's behaviour changed because the timing in the code changed, not because the wiring changed.

Step 5 — predict, then test

Before you change anything else, write down on paper what you expect to happen if you change the second delay to 2000. Then change it and check. Was your prediction right? Predicting is a skill — by L01-20 you'll be doing it automatically.

Step 6 — try the numbers before you upload

Uploading takes a few seconds, and a lesson is only 90 minutes. Set the two delays below and the LED does it straight away, at the real speed. The square wave underneath is the same information drawn as a shape: high means 5 V, low means 0 V.

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(100);
  digitalWrite(13, LOW);
  delay(1000);
}

Every task in the next section is a pair of numbers. Find them here first, then type them into the IDE once.

What changed since last lesson? The wiring is identical to L01-07. What's new is that you wrote the sketch — so you can make the LED do anything you want by adjusting two numbers.

Try It Yourself 20 min

Three small variations. For each one, predict the behaviour on paper before you upload, then check.

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(250);
  digitalWrite(13, LOW);
  delay(250);
}

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(80);
  digitalWrite(13, LOW);
  delay(2920);
}

// Fill in the two delays so the LED is on for 5 s then off for 1 s.
void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(____);
  digitalWrite(13, LOW);
  delay(____);
}

Mini-Challenge 15 min

Design your own blink pattern

Pick one of these real-world rhythms and write a sketch — at most 10 statements inside loop() — that makes the LED imitate it. The hardware is the L01-07 circuit. The only tools you have today are pinMode, digitalWrite, HIGH, LOW and delay.

1. Phone notification — two brief flashes, then a long pause. Like the LED on your phone when a WhatsApp message arrives.
2. Smoke detector low-battery — a single sharp flash every 30 seconds.
3. KTM crossing barrier — slow, urgent, on for 700 ms, off for 700 ms.
4. Stalled engine warning — long on, short off, long on, short off. Like a slow Morse "T" repeating.

It works if:

• A partner who didn't see your code can guess which of the four patterns you chose, just from watching the LED for 30 seconds.
• Your sketch compiles with no red squiggles.
• The body of loop holds at most 10 statements. Each one still gets its own line — one statement per line is the house rule, and it never bends.

Recap 5 min

A digital pin is a switch. pinMode() declares the pin is an output, digitalWrite() flips it between 5 V (HIGH) and 0 V (LOW), and delay() pauses the sketch between flips. Three functions and two numbers are all it takes to control any LED, buzzer or relay from code.

pinMode(pin, mode)

Sets a pin as OUTPUT (sends signal) or INPUT (reads signal). Lives in setup().

digitalWrite(pin, value)

Puts the pin to HIGH (5 V) or LOW (0 V). Lives in loop().

HIGH / LOW

The two states of a digital pin. Type them in capitals — they are special names the Arduino library knows.

delay(ms)

Pauses the sketch for ms milliseconds. 1000 ms = 1 second. 500 ms = half a second.

Digital output

Using a pin to send a 5 V or 0 V signal out of the chip. The whole point of today's lesson.

Homework 5 min

The "name flasher". Write a sketch that flashes the LED a number of times equal to the length of your first name in letters, then waits 3 seconds, then repeats.

• Aiman (5 letters) → 5 quick flashes, pause, repeat.
• Wei Jie (6 letters, ignoring the space) → 6 quick flashes, pause, repeat.
• Priya (5 letters) → 5 quick flashes, pause, repeat.

Use only pinMode, digitalWrite, HIGH, LOW and delay. Your sketch will be longer than 10 lines — that's fine for homework. From L01-11 we will learn the for loop, which makes patterns like this much shorter.

Bring back next class:

• The saved .ino file on a USB stick.
• A short phone video (10–15 seconds) of the LED doing your name pattern.
• A one-line answer on paper: how many times does your loop() body need to repeat the digitalWrite(HIGH)…digitalWrite(LOW) pair?