diff --git a/README.md b/README.md
index 2c95537..9f8c4af 100644
--- a/README.md
+++ b/README.md
@@ -88,6 +88,8 @@ dokumentation, forbindelser og biblioteker beskrives ved den enkelte sketch.
   – fast løbetekst på matrixen
 - [`wifi_matrix_message_board`](sketches/arduino-uno-r4-wifi/wifi_matrix_message_board)
   – browserstyret løbetekst via Wi-Fi
+- [`lcd1602_i2c_demo`](sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo) – effekt-demo
+  til et eksternt 16×2 HD44780-display med PCF8574 I²C-backpack
 
 ### Arduino UNO R4 Minima
 
diff --git a/boards/arduino-uno-r4-wifi/README.md b/boards/arduino-uno-r4-wifi/README.md
index 9a0331f..ab90a42 100644
--- a/boards/arduino-uno-r4-wifi/README.md
+++ b/boards/arduino-uno-r4-wifi/README.md
@@ -51,6 +51,7 @@ kapacitiv touch-hardware, men ingen færdige touchflader på selve boardet.
 - [`led_matrix_demo`](../../sketches/arduino-uno-r4-wifi/led_matrix_demo)
 - [`led_matrix_scrolling_text`](../../sketches/arduino-uno-r4-wifi/led_matrix_scrolling_text)
 - [`wifi_matrix_message_board`](../../sketches/arduino-uno-r4-wifi/wifi_matrix_message_board)
+- [`lcd1602_i2c_demo`](../../sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo)
 
 ## Kilder
 
diff --git a/sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo/README.md b/sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo/README.md
new file mode 100644
index 0000000..c4085b4
--- /dev/null
+++ b/sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo/README.md
@@ -0,0 +1,37 @@
+# LCD1602 I²C-demo
+
+En effekt-demo til et HD44780-kompatibelt 16×2-tegndisplay med den almindelige
+PCF8574/PCF8574A I²C-backpack. Demoen viser:
+
+- animeret opstart og blink med baggrundsbelysningen;
+- brugerdefinerede hjerte-, smiley- og rakettegn;
+- vandret løbetekst;
+- en animeret 16-kanals "BYTE BEATS"-equalizer;
+- oppetid og spinner.
+
+Sketchen bruger kun `Wire`, som følger med Arduino-kernen. Den scanner selv
+PCF8574-adresseområderne `0x20`–`0x27` og `0x38`–`0x3F`, så den typiske adresse
+`0x27` behøver ikke angives manuelt.
+
+## Tilslutning
+
+| LCD-backpack | Arduino UNO R4 WiFi |
+| --- | --- |
+| GND | GND |
+| VCC | 5V |
+| SDA | SDA |
+| SCL | SCL |
+
+Brug de mærkede SDA/SCL-ben ved AREF (eller de tilsvarende A4/A5-signaler).
+Hvis baggrundsbelysningen tænder, men tegnene ikke ses, justeres det blå
+kontrast-potentiometer forsigtigt med en lille skruetrækker.
+
+## Byg og upload
+
+```powershell
+arduino-cli compile --fqbn arduino:renesas_uno:unor4wifi sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo
+arduino-cli upload -p COM5 --fqbn arduino:renesas_uno:unor4wifi sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo
+```
+
+Serial Monitor kan åbnes ved 115200 baud for at se den fundne I²C-adresse eller
+en fejlmeddelelse, hvis displayet ikke findes.
diff --git a/sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo/lcd1602_i2c_demo.ino b/sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo/lcd1602_i2c_demo.ino
new file mode 100644
index 0000000..8bba315
--- /dev/null
+++ b/sketches/arduino-uno-r4-wifi/lcd1602_i2c_demo/lcd1602_i2c_demo.ino
@@ -0,0 +1,245 @@
+#include <Wire.h>
+
+// HD44780 16x2 display with the common PCF8574 I2C backpack mapping:
+// P0=RS, P1=RW, P2=Enable, P3=backlight, P4..P7=D4..D7.
+constexpr uint8_t LCD_COLUMNS = 16;
+constexpr uint8_t LCD_ROWS = 2;
+constexpr uint8_t PIN_RS = 0x01;
+constexpr uint8_t PIN_ENABLE = 0x04;
+constexpr uint8_t PIN_BACKLIGHT = 0x08;
+
+uint8_t lcdAddress = 0;
+uint8_t backlightState = PIN_BACKLIGHT;
+
+const byte heart[8] = {
+  B00000, B01010, B11111, B11111, B01110, B00100, B00000, B00000
+};
+
+const byte smile[8] = {
+  B00000, B01010, B00000, B00000, B10001, B01110, B00000, B00000
+};
+
+const byte rocket[8] = {
+  B00100, B01110, B01110, B11111, B10101, B00100, B01010, B10001
+};
+
+const byte spinner[4][8] = {
+  {B00100, B00100, B00100, B00100, B00100, B00100, B00100, B00000},
+  {B00000, B00000, B00001, B00010, B00100, B01000, B10000, B00000},
+  {B00000, B00000, B00000, B11111, B00000, B00000, B00000, B00000},
+  {B00000, B00000, B10000, B01000, B00100, B00010, B00001, B00000}
+};
+
+void expanderWrite(uint8_t value) {
+  Wire.beginTransmission(lcdAddress);
+  Wire.write(value | backlightState);
+  Wire.endTransmission();
+}
+
+void pulseEnable(uint8_t value) {
+  expanderWrite(value | PIN_ENABLE);
+  delayMicroseconds(1);
+  expanderWrite(value & ~PIN_ENABLE);
+  delayMicroseconds(50);
+}
+
+void writeNibble(uint8_t nibble, bool dataMode) {
+  uint8_t value = (nibble & 0x0F) << 4;
+  if (dataMode) value |= PIN_RS;
+  expanderWrite(value);
+  pulseEnable(value);
+}
+
+void lcdSend(uint8_t value, bool dataMode) {
+  writeNibble(value >> 4, dataMode);
+  writeNibble(value, dataMode);
+}
+
+void lcdCommand(uint8_t command) {
+  lcdSend(command, false);
+  if (command == 0x01 || command == 0x02) delayMicroseconds(2000);
+}
+
+void lcdWrite(uint8_t value) {
+  lcdSend(value, true);
+}
+
+void lcdPrint(const char *text) {
+  while (*text) lcdWrite(*text++);
+}
+
+void lcdClear() {
+  lcdCommand(0x01);
+}
+
+void lcdSetCursor(uint8_t column, uint8_t row) {
+  static const uint8_t rowOffset[] = {0x00, 0x40};
+  lcdCommand(0x80 | (column + rowOffset[row % LCD_ROWS]));
+}
+
+void lcdCreateChar(uint8_t slot, const byte bitmap[8]) {
+  lcdCommand(0x40 | ((slot & 0x07) << 3));
+  for (uint8_t row = 0; row < 8; ++row) lcdWrite(bitmap[row]);
+}
+
+void lcdInit() {
+  delay(50);
+  writeNibble(0x03, false);
+  delayMicroseconds(4500);
+  writeNibble(0x03, false);
+  delayMicroseconds(4500);
+  writeNibble(0x03, false);
+  delayMicroseconds(150);
+  writeNibble(0x02, false);
+
+  lcdCommand(0x28);  // 4-bit, two lines, 5x8 font
+  lcdCommand(0x08);  // Display off
+  lcdClear();
+  lcdCommand(0x06);  // Cursor moves right
+  lcdCommand(0x0C);  // Display on, cursor off
+}
+
+uint8_t findDisplay() {
+  // PCF8574 and PCF8574A address ranges.
+  const uint8_t ranges[][2] = {{0x20, 0x27}, {0x38, 0x3F}};
+  for (const auto &range : ranges) {
+    for (uint8_t address = range[0]; address <= range[1]; ++address) {
+      Wire.beginTransmission(address);
+      if (Wire.endTransmission() == 0) return address;
+    }
+  }
+  return 0;
+}
+
+void printPadded(const char *text, uint8_t width = LCD_COLUMNS) {
+  uint8_t count = 0;
+  while (*text && count < width) {
+    lcdWrite(*text++);
+    ++count;
+  }
+  while (count++ < width) lcdWrite(' ');
+}
+
+void bootSequence() {
+  lcdClear();
+  lcdSetCursor(0, 0);
+  lcdWrite(2);
+  lcdPrint(" R4 WiFi LAB ");
+  lcdWrite(2);
+
+  lcdSetCursor(0, 1);
+  for (uint8_t i = 0; i < LCD_COLUMNS; ++i) {
+    lcdWrite(0xFF);
+    delay(75);
+  }
+  delay(350);
+
+  for (uint8_t flash = 0; flash < 2; ++flash) {
+    backlightState = 0;
+    expanderWrite(0);
+    delay(90);
+    backlightState = PIN_BACKLIGHT;
+    expanderWrite(0);
+    delay(120);
+  }
+}
+
+void marqueeDemo() {
+  const char message[] = "   Arduino UNO R4 WiFi + LCD1602 = AWESOME!   ";
+  const int length = sizeof(message) - 1;
+
+  lcdClear();
+  lcdSetCursor(0, 0);
+  lcdWrite(0);
+  lcdPrint(" MAKER MODE ");
+  lcdWrite(0);
+
+  for (int offset = 0; offset <= length - LCD_COLUMNS; ++offset) {
+    lcdSetCursor(0, 1);
+    for (uint8_t column = 0; column < LCD_COLUMNS; ++column) {
+      lcdWrite(message[offset + column]);
+    }
+    delay(145);
+  }
+}
+
+void equalizerDemo() {
+  for (uint8_t level = 0; level < 8; ++level) {
+    byte bar[8];
+    for (uint8_t row = 0; row < 8; ++row) {
+      bar[row] = row >= (7 - level) ? B11111 : B00000;
+    }
+    lcdCreateChar(level, bar);
+  }
+
+  lcdClear();
+  lcdSetCursor(0, 0);
+  printPadded("  BYTE BEATS");
+
+  for (uint8_t frame = 0; frame < 38; ++frame) {
+    lcdSetCursor(0, 1);
+    for (uint8_t column = 0; column < LCD_COLUMNS; ++column) {
+      // A deterministic animated wave: no random seed needed.
+      uint8_t level = (column * 3 + frame * 2 + (column ^ frame)) % 8;
+      lcdWrite(level);
+    }
+    delay(95);
+  }
+
+  // Restore the icons overwritten by the equalizer characters.
+  lcdCreateChar(0, heart);
+  lcdCreateChar(1, smile);
+  lcdCreateChar(2, rocket);
+}
+
+void uptimeDemo() {
+  lcdClear();
+  lcdSetCursor(0, 0);
+  lcdPrint("SYSTEM ONLINE ");
+  lcdWrite(1);
+
+  for (uint8_t frame = 0; frame < 32; ++frame) {
+    lcdCreateChar(3, spinner[frame % 4]);
+    unsigned long seconds = millis() / 1000;
+    char line[17];
+    snprintf(line, sizeof(line), "UP %5lus     ", seconds);
+    lcdSetCursor(0, 1);
+    lcdWrite(3);
+    lcdWrite(' ');
+    printPadded(line, 14);
+    delay(125);
+  }
+}
+
+void setup() {
+  Serial.begin(115200);
+  Wire.begin();
+  Wire.setClock(100000);
+
+  lcdAddress = findDisplay();
+  if (lcdAddress == 0) {
+    Serial.println("LCD1602-demo: intet PCF8574 I2C-display fundet.");
+    Serial.println("Kontroller GND, 5V, SDA og SCL, og genstart boardet.");
+    return;
+  }
+
+  Serial.print("LCD1602 fundet paa I2C-adresse 0x");
+  Serial.println(lcdAddress, HEX);
+
+  lcdInit();
+  lcdCreateChar(0, heart);
+  lcdCreateChar(1, smile);
+  lcdCreateChar(2, rocket);
+  bootSequence();
+}
+
+void loop() {
+  if (lcdAddress == 0) {
+    delay(1000);
+    return;
+  }
+
+  marqueeDemo();
+  equalizerDemo();
+  uptimeDemo();
+}