LiquidCrystal Library

Learning Goals 5 min

1. Initialise an HD44780 LCD using the right library — built-in LiquidCrystal for bare wiring, or LiquidCrystal_I2C for the backpack — and call begin() + print() to display your first message.
2. Run an I²C scanner sketch once, to discover whether your backpack lives at address 0x27 or 0x3F (or something else) — and never have to guess again.
3. Print a static two-line layout: project title on row 0, sub-text on row 1, using setCursor(col, row) to switch lines.

Warm-Up 10 min

Yesterday: hardware. Today: code. With the LCD wired and the contrast adjusted, sending text to it is shockingly simple — three lines in setup() and one in loop() is the canonical Hello-World. The library hides everything about commands, addresses, busy-flags and timing. We just call lcd.print() and the words appear.

Pick your library

One of two libraries depending on yesterday's wiring choice:

• Bare 16-pin wiring → LiquidCrystal (built-in, no install).
• I²C backpack → LiquidCrystal_I2C (Tools → Manage Libraries → search "LiquidCrystal I2C" → install Frank de Brabander's version).

We'll show both versions of each sketch below. Pick the one matching your wiring; ignore the other.

New Concept · LCD object, begin(), print() 25 min

Bare wiring — minimum-viable sketch

#include <LiquidCrystal.h>

// pins: RS, EN, D4, D5, D6, D7
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup() {
  lcd.begin(16, 2);        // 16 columns, 2 rows
  lcd.print("Hello, world!");
}

void loop() {
  // nothing — display is static
}

Three new ideas:

• #include <LiquidCrystal.h> — pull in the library.
• LiquidCrystal lcd(...) — create an LCD object. The 6 pin numbers in the constructor must match how you wired RS, EN, D4–D7 yesterday.
• lcd.begin(16, 2) — tell the library this LCD has 16 columns and 2 rows. Done once in setup.
• lcd.print(...) — write text at the current cursor position. After printing, the cursor moves to the next column automatically.

I²C wiring — minimum-viable sketch

#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);   // address, cols, rows

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.print("Hello, world!");
}

void loop() {
  // nothing
}

Differences from the bare version:

• Different library name and class.
• Constructor takes the I²C address (usually 0x27 but sometimes 0x3F).
• lcd.init() instead of lcd.begin() — same idea, different name.
• lcd.backlight() — explicitly turn the backlight on (some backpacks default to off).

The rest of the API (print, setCursor, etc.) is identical between the two libraries — most of the time you can paste code from one into the other and only the setup lines change.

The I²C address scanner

Before your I²C LCD will work you need to know its address. The classic short sketch:

#include <Wire.h>

void setup() {
  Wire.begin();
  Serial.begin(9600);
  Serial.println("Scanning I2C bus...");
  for (byte addr = 1; addr < 127; addr++) {
    Wire.beginTransmission(addr);
    if (Wire.endTransmission() == 0) {
      Serial.print("Found device at 0x");
      Serial.println(addr, HEX);
    }
  }
  Serial.println("done.");
}

void loop() {}

Upload it, open Serial Monitor at 9600. You should see one line: Found device at 0x27 (or 0x3F). That's the address to put in the constructor. Run this scanner once when you set up the LCD, write the address on a sticker, then go back to the LCD sketch.

setCursor(col, row) — putting text on row 1

After begin() the cursor starts at column 0, row 0. To print on the bottom row:

lcd.setCursor(0, 0);
lcd.print("Weather Station");
lcd.setCursor(0, 1);
lcd.print("v1.0  by Adam");

The arguments are column first, row second — easy to swap by mistake. Column is 0–15; row is 0 (top) or 1 (bottom). On the 20×4 LCDs you may meet later, rows go 0–3 and columns 0–19; the same API works unchanged.

Worked Example · Project title + version 20 min

Step 1 — finish wiring (if bare)

Yesterday you wired power, contrast and backlight but skipped the 6 data pins. Wire them now:

• LCD pin 4 (RS) → Arduino D12.
• LCD pin 6 (EN) → Arduino D11.
• LCD pin 11 (D4) → Arduino D5.
• LCD pin 12 (D5) → Arduino D4.
• LCD pin 13 (D6) → Arduino D3.
• LCD pin 14 (D7) → Arduino D2.

Step 2 — scan (if I²C)

Upload the I²C scanner above. Note the address. If you don't see any device, check your SDA / SCL wires; if you see multiple, you have other I²C devices on the bus too — note them all but identify the LCD by trying each address in the next sketch.

Step 3 — the title-card sketch (bare version)

Save as title-card.ino:

#include <LiquidCrystal.h>

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup() {
  lcd.begin(16, 2);
  lcd.setCursor(0, 0);
  lcd.print("Weather Station");
  lcd.setCursor(0, 1);
  lcd.print("v1.0  by Adam");
}

void loop() {
  // intentionally empty — title card is static
}

Step 3 — the title-card sketch (I²C version)

#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);   // replace 0x27 if scanner showed otherwise

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("Weather Station");
  lcd.setCursor(0, 1);
  lcd.print("v1.0  by Adam");
}

void loop() {
}

Step 4 — upload, watch the cold-boot

Upload. The LCD should:

1. Show a brief flash of dark rectangles (the pre-init test pattern).
2. Clear.
3. Show your two-line title card crisply.

If you see scrambled text (e.g. half-characters, garbage), one of the data wires is wrong — recheck the order RS, EN, D4, D5, D6, D7. The library expects them in that order.

Step 5 — change the text, re-upload

Replace "Weather Station" with your own name; replace the version line with today's date. Re-upload. The new text appears immediately. Note: the OLD text doesn't disappear from the parts you didn't write over — if your new text is shorter than the old, ghost characters remain. We'll fix this with lcd.clear() tomorrow.

Try It Yourself 15 min

String name = "Aisha";
int col = (16 - name.length()) / 2;
lcd.setCursor(col, 0);
lcd.print(name);

lcd.setCursor(0, 0);
for (int i = 0; i < 16; i++) {
  lcd.print(i % 10);
}

lcd.setCursor(15, 0);
lcd.autoscroll();
for (int i = 0; i < 100; i++) {
  lcd.print(i % 10);
  delay(200);
}
lcd.noAutoscroll();

Mini-Challenge · Personal welcome screen 15 min

Design and code a 2-line welcome screen for your Arduino kit. It should feel like a boot screen — informative, attractive, finite. Requirements:

1. The top row identifies the device (e.g. Aisha's Toolkit, or a project name from L2 so far).
2. The bottom row shows a version + date (e.g. v0.3 2026-05).
3. Centre at least one of the two lines.
4. Use only characters that fit on a standard LCD — basic ASCII letters, numbers, space, and the punctuation : - . / _. (Skip emoji for now; we'll learn custom characters in L02-30.)
5. The sketch should not flicker in loop(). All printing happens in setup(); loop() is empty.

It's done when:

• The screen is fully readable from across a normal classroom (3–4 m).
• The text doesn't shimmer, flicker, or rewrite itself.
• You can photograph it and the image clearly shows both lines.
• If you change the name or the version, only that one line of code changes — no shotgun edits.

Recap 5 min

The LiquidCrystal library makes the HD44780 trivial to talk to: include, construct, begin(), print(). The I²C version (LiquidCrystal_I2C) is almost identical — different constructor, init() + backlight() instead of begin(), but the same print and setCursor. Today's key insight: print static text in setup(), leave loop() empty. We didn't handle changing values yet — tomorrow we learn about clear() and cursor positioning so we can refresh just the parts that change without ghosting.

LiquidCrystal library

The built-in Arduino library for HD44780 character LCDs wired with parallel pins. No install needed.

LiquidCrystal_I2C

Third-party library for the I²C-backpack version of the same LCD. Install via Library Manager.

begin(cols, rows) / init()

One-shot initialisation in setup(). Tells the library the panel dimensions and configures the chip. The bare version uses begin(16, 2); the I²C version uses init().

print(value)

Write text at the current cursor position. Accepts strings, numbers, characters — anything Serial.print accepts.

setCursor(col, row)

Move the cursor to column col (0–15) and row row (0 or 1). Column first, row second — easy to swap.

backlight() / noBacklight()

I²C library only. Turns the backlight on/off via the PCF8574. Some boards default to off until you call this.

I²C scanner

A short utility sketch that asks every possible I²C address (1–127) if anything responds. Used once to discover an unknown device's address.

0x27 / 0x3F

The two most common addresses for an LCD I²C backpack. Hex notation; written in C++ as 0x27 = 39 decimal.

Static display

Content that doesn't change after boot — print once in setup, empty loop. Avoids flicker and saves I²C/parallel bandwidth.

Homework 5 min

Three title screens. Design and code 3 different welcome screens — one per project you've already built in L2:

1. Weather Station (L02-20).
2. Smart Bin Lid (L02-26).
3. Personal Thermometer (L02-12).

For each, the LCD should display the project's name on row 0 and a one-line tagline on row 1 (e.g. Smart Bin Lid / Hands-free v1). Centre or align as you like.

Combine all three into one sketch using a button: pressing a momentary button on D8 cycles through the three screens. (Sneak peek of state machines on an LCD — we'll come back to button + screen interaction in L02-32.) Or, if button-handling isn't fresh, just submit three separate sketches.

Bring back next class:

• Your three title-screen designs (photos or text mockups).
• Your hw-l02-28.ino sketch (one with the button cycle, or three separate sketches).
• Note which library you used (LiquidCrystal or LiquidCrystal_I2C) and the address if I²C.