Ultrathin business card runs a fluid simulation

Original Article ↗ Hacker News Discussion ↗

Overall reaction

  • Many commenters find the card “amazing” and “unforgettable,” highlighting it as a strong, wordless demonstration of deep hardware/embedded skill.
  • Some humorously compare it to famous “prestige” business cards (American Psycho, high-end designer cards), generally concluding this one “wins” on technical impressiveness.

Learning and doing physics simulations

  • Several replies answer a question about where to start with physical simulation:
    • Suggest starting with simpler rigid-body or particle systems, then moving to cloth/soft-body/fluids.
    • Recommend resources: classic rigid-body SIGGRAPH notes, numerical methods/computational physics, statistical mechanics courses, “Nature of Code,” and short physics-simulation video series.
    • Emphasize discretizing differential equations, basic numerical integration (Euler vs Runge–Kutta), and energy-conserving schemes.
  • Some discussion of heat diffusion and gravity as simple starter problems; contrast between “physically correct” simulations and game-like “looks correct” approaches.

Manufacturing, assembly, and cost

  • Multiple commenters explain that offshore PCB fabs (JLC, PCBWay, etc.) can cheaply fabricate and assemble such boards—even in batches of ~10—for roughly $10/board including assembly.
  • LED cost is surprisingly low even for dense matrices; economies of scale push the per-LED cost down substantially.
  • There’s consensus that US manufacturing is often more expensive and sometimes lacks comparable turnkey services.
  • People note home reflow, jigs, and hobby pick‑and‑place are feasible but tedious for this LED count; most assume this was fab-assembled.

Usefulness as a business card / portfolio

  • Many see this more as a portfolio piece or high‑value trade‑show item than a mass handout; ~$10–20 per card is considered acceptable where a single deal or job is high value.
  • Several anecdotes: unusual physical CVs and memorable business cards significantly boosting callbacks and job offers.
  • Some argue it’s enough to link the project from a resume; others stress the psychological impact of physically handing over such a device.

Design and UX critiques

  • Repeated criticism of the font and silkscreen layout: text is hard to read, overlapping markings look messy, and typography undercuts the otherwise superb engineering.
  • Others defend the “Woz-like” rough aesthetic, but even fans acknowledge legibility matters for a business card.
  • The creator actively solicits font/typography advice; suggestions include clean sans-serifs and more whitespace, and updated renders with hidden vias get positive feedback.
  • Thickness is debated: much thicker than paper, thinner than a USB-C plug; some question labeling it “ultrathin.”

Hardware details: USB-C, battery, safety

  • The edge‑connector USB‑C implementation (using the PCB itself as the plug blade) is widely admired as especially clever and ultra-thin.
  • Some worry it might be mechanically fragile and requires careful unplugging.
  • Exposed coin-cell terminals raise safety questions:
    • For standard CR2032 cells, commenters claim shorting is usually low-risk due to limited current.
    • Rechargeable coin cells are acknowledged as potentially more hazardous if shorted; conformal coating is suggested as mitigation.

Interactivity and feature ideas

  • Suggestions include:
    • QR-code mode (name/contact/URL), potentially requiring a slightly larger matrix or quiet zone.
    • Displaying text, numbers, Tetris-like games, persistence-of-vision effects, and accelerometer-based controls instead of buttons.
  • The author notes:
    • The 21×21 grid was chosen with QR codes in mind but proved hard to scan.
    • Small fonts look bad with large LED spacing; scrolling text might be better.
    • They like the “no buttons” constraint and may use accelerometer clicks/double-clicks.

Software and embedded Rust

  • Commenters highlight that the firmware is written in Rust and uses modern embedded tooling, seen as a good real-world example for those struggling to get into embedded Rust.
  • There’s some surprise at how freely floating point is used, seen as evidence of how capable modern MCUs have become compared to 10–15 years ago.

Comparisons and related projects

  • Multiple references to an earlier fluid-simulation pendant that inspired this design, which offers more detailed writeups about the simulation.
  • Others mention “digital hourglass” ornaments, retro fluid games, and various playful or artistic electronics (e-ink business cards, conference badges, fluid-in-a-box toys).
  • One comment notes the particular FLIP fluid algorithm’s strengths (splashes + incompressibility) aren’t fully showcased in such a simple, low-gravity, box-shaped container.

Meta and culture

  • Some meta-discussion about the project having appeared on Reddit first, with observations that many embedded/hacking communities now gravitate there.
  • A few comments dismissively predict the card will eventually end up in the trash like any other business card; others counter that the impact before that point is what matters.