A week ago, Francesco and I bought a couple of R36S handhelds. Or at least, that's what the listing said.

What we got was the Y3506_V05_20251215 revision. It's a clone of a clone. If you've played with these cheap Chinese SBCs before, you know the drill: you flash a standard image, and the screen stays black because the driver is slightly different. It's a hardware lottery.

In our case, we had to use DTB files from this repo, and we used the ArkOS image from here.

But once the firmware is sorted, the hardware is surprisingly charming. The transparent shell is pure 90s nostalgia, and the screen is actually good for the price.

I didn't want it just for emulating Game Boy and PlayStation games, though. I wanted it to be useful on my desk.

Every time I work from Francesco's place, I'm distracted by his Braun BC03 clock. It's a Dieter Rams masterpiece. It follows the "Less, but better" philosophy that modern tech seems to have forgotten. It does one thing, and it does it with absolute clarity.

So, I wrote bc03-r36s.

When I started building this, the obvious path was to use PNGs for the face and the hands. But that felt like cheating. A Braun-inspired tool should be defined by its geometry, not by a bitmap.

The "cool" part of the code is that there isn't a single image file in the repository. Everything is drawn directly via SDL_RenderGeometry. This allows for drawing arbitrary shapes on the GPU by defining vertices, which is much cleaner than using SDL_RenderCopyEx with pre-rendered textures.

The core of the rendering is the draw_rotated_rect function. It takes a center point, dimensions, and an angle, then calculates the rotation matrix to position the vertices.

void draw_rotated_rect(SDL_Renderer *renderer, int cx, int cy, float length,
                       float width, float angle, int offset_back) {
  float half_w = width / 2.0f;
 
  // Define corners relative to the rotation point
  Pointf corners[4] = {
    {-half_w, (float)offset_back},
    {half_w, (float)offset_back},
    {half_w, -length},
    {-half_w, -length}
  };
 
  SDL_Vertex vertices[4];
  // ... apply rotation and draw
  SDL_RenderGeometry(renderer, NULL, vertices, 4, indices, 6);
}

The offset_back parameter is the secret sauce. If you look at a real BC03, the second hand doesn't start from the center; it has a small counterweight that extends backwards. By adding a 30px offset, I captured that specific analog feel.

Fractional Movement

Most digital clocks just jump every second. It looks "digital" and cheap. Real analog movements have gears. To mimic this, I didn't just use timeinfo->tm_sec. I calculate the angles using the fractional parts of the time:

float sec = timeinfo->tm_sec;
float min = timeinfo->tm_min + (sec / 60.0f);
float hr = timeinfo->tm_hour + (min / 60.0f);

This means the hour hand moves gradually as the minutes pass, and the minute hand moves as the seconds pass.

The project has four themes (Stealth, Classic, Contrast, and Nightstand), all keeping the iconic yellow second hand.

Download the latest release from the GitHub repo.