diff --git a/sketches/led_matrix_demo/README.md b/sketches/led_matrix_demo/README.md
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+# LED Matrix Demo
+
+En selvkørende demo til den indbyggede 12×8 LED-matrix på Arduino UNO R4 WiFi.
+
+Demoen gentager følgende effekter:
+
+1. Scanner frem og tilbage
+2. Digital regn
+3. Udvidende ringe
+4. Bevægelig bølge
+5. Hoppende pixel med hale
+6. Tilfældige glimt
+7. Pulserende hjerte
+
+Sketchen bruger `Arduino_LED_Matrix`, som følger med Arduino Renesas UNO-kernen.
+Der kræves ingen ekstra komponenter eller biblioteker.
+
+## Kompilering
+
+```powershell
+arduino-cli compile --fqbn arduino:renesas_uno:unor4wifi sketches/led_matrix_demo
+```
+
+## Upload
+
+Upload først efter godkendelse:
+
+```powershell
+arduino-cli upload --port COM5 --fqbn arduino:renesas_uno:unor4wifi sketches/led_matrix_demo
+```
+
diff --git a/sketches/led_matrix_demo/led_matrix_demo.ino b/sketches/led_matrix_demo/led_matrix_demo.ino
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+#include <Arduino_LED_Matrix.h>
+
+// Arduino UNO R4 WiFi LED matrix demo (12 columns x 8 rows).
+// The effects run continuously and require no additional hardware.
+
+constexpr uint8_t ROWS = 8;
+constexpr uint8_t COLS = 12;
+
+ArduinoLEDMatrix matrix;
+uint8_t frame[ROWS][COLS] = {};
+
+void clearFrame() {
+  memset(frame, 0, sizeof(frame));
+}
+
+void setPixel(int x, int y, bool on = true) {
+  if (x >= 0 && x < COLS && y >= 0 && y < ROWS) {
+    frame[y][x] = on ? 1 : 0;
+  }
+}
+
+void showFrame(unsigned long durationMs) {
+  matrix.renderBitmap(frame, ROWS, COLS);
+  delay(durationMs);
+}
+
+void pauseBetweenEffects() {
+  clearFrame();
+  showFrame(250);
+}
+
+void scannerEffect() {
+  for (int x = -2; x < COLS + 2; ++x) {
+    clearFrame();
+
+    for (int y = 0; y < ROWS; ++y) {
+      setPixel(x, y);
+      if ((y & 1) == 0) {
+        setPixel(x - 1, y);
+      }
+    }
+
+    showFrame(65);
+  }
+
+  for (int x = COLS + 1; x >= -2; --x) {
+    clearFrame();
+
+    for (int y = 0; y < ROWS; ++y) {
+      setPixel(x, y);
+      if ((y & 1) != 0) {
+        setPixel(x + 1, y);
+      }
+    }
+
+    showFrame(65);
+  }
+}
+
+void rainEffect() {
+  int8_t drops[COLS];
+
+  for (uint8_t x = 0; x < COLS; ++x) {
+    drops[x] = random(-10, 1);
+  }
+
+  for (uint8_t step = 0; step < 75; ++step) {
+    clearFrame();
+
+    for (uint8_t x = 0; x < COLS; ++x) {
+      setPixel(x, drops[x]);
+      setPixel(x, drops[x] - 2);
+
+      ++drops[x];
+      if (drops[x] > ROWS + random(0, 5)) {
+        drops[x] = random(-8, -1);
+      }
+    }
+
+    showFrame(65);
+  }
+}
+
+void rippleEffect() {
+  for (uint8_t repeat = 0; repeat < 3; ++repeat) {
+    for (int radius = 0; radius <= 13; radius += 2) {
+      clearFrame();
+
+      for (int y = 0; y < ROWS; ++y) {
+        for (int x = 0; x < COLS; ++x) {
+          // Coordinates are doubled so the center can lie between LEDs.
+          const int dx = abs(2 * x - (COLS - 1));
+          const int dy = abs(2 * y - (ROWS - 1));
+          const int distance = max(dx, dy);
+
+          if (abs(distance - radius) <= 1) {
+            setPixel(x, y);
+          }
+        }
+      }
+
+      showFrame(110);
+    }
+  }
+}
+
+void waveEffect() {
+  const uint8_t wave[COLS] = {3, 2, 1, 1, 2, 3, 5, 6, 6, 5, 4, 3};
+
+  for (uint8_t phase = 0; phase < COLS * 5; ++phase) {
+    clearFrame();
+
+    for (uint8_t x = 0; x < COLS; ++x) {
+      setPixel(x, wave[(x + phase) % COLS]);
+    }
+
+    showFrame(75);
+  }
+}
+
+void bouncingPixelEffect() {
+  int x = 1;
+  int y = 1;
+  int dx = 1;
+  int dy = 1;
+  int previousX = x;
+  int previousY = y;
+  int olderX = x;
+  int olderY = y;
+
+  for (uint8_t step = 0; step < 70; ++step) {
+    clearFrame();
+    setPixel(x, y);
+    setPixel(previousX, previousY);
+    setPixel(olderX, olderY);
+    showFrame(55);
+
+    olderX = previousX;
+    olderY = previousY;
+    previousX = x;
+    previousY = y;
+
+    x += dx;
+    y += dy;
+
+    if (x <= 0 || x >= COLS - 1) {
+      dx = -dx;
+      x = constrain(x, 0, COLS - 1);
+    }
+    if (y <= 0 || y >= ROWS - 1) {
+      dy = -dy;
+      y = constrain(y, 0, ROWS - 1);
+    }
+  }
+}
+
+void sparkleEffect() {
+  for (uint8_t step = 0; step < 45; ++step) {
+    clearFrame();
+
+    for (uint8_t sparkle = 0; sparkle < 16; ++sparkle) {
+      setPixel(random(COLS), random(ROWS));
+    }
+
+    showFrame(80);
+  }
+}
+
+void heartFinale() {
+  uint8_t smallHeart[ROWS][COLS] = {
+    {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+    {0, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 0},
+    {0, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 0},
+    {0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0},
+    {0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0},
+    {0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0},
+    {0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0},
+    {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
+  };
+
+  uint8_t largeHeart[ROWS][COLS] = {
+    {0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0},
+    {0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0},
+    {1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 1},
+    {1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1},
+    {0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0},
+    {0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0},
+    {0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0},
+    {0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0}
+  };
+
+  for (uint8_t pulse = 0; pulse < 4; ++pulse) {
+    matrix.renderBitmap(smallHeart, ROWS, COLS);
+    delay(220);
+    matrix.renderBitmap(largeHeart, ROWS, COLS);
+    delay(320);
+  }
+}
+
+void setup() {
+  matrix.begin();
+  randomSeed(analogRead(A0));
+  clearFrame();
+  showFrame(400);
+}
+
+void loop() {
+  scannerEffect();
+  pauseBetweenEffects();
+
+  rainEffect();
+  pauseBetweenEffects();
+
+  rippleEffect();
+  pauseBetweenEffects();
+
+  waveEffect();
+  pauseBetweenEffects();
+
+  bouncingPixelEffect();
+  pauseBetweenEffects();
+
+  sparkleEffect();
+  pauseBetweenEffects();
+
+  heartFinale();
+  pauseBetweenEffects();
+}
+