TrackWeight: Turn your MacBook's trackpad into a digital weighing scale

Original Article ↗ Hacker News Discussion ↗

Reactions to the idea

  • Many find the project delightful, “real Hacker News” material and a great classroom example of creative hardware hacking.
  • Others see it as a fun Rube Goldberg–style demo: cool to know it’s possible, but they’d still use a cheap dedicated digital scale.
  • A few note niche practical value (e.g., backpacking with a MacBook so you don’t also pack a kitchen scale), but overall it’s treated more as a novelty.

Usability & workflow

  • The requirement to keep a finger in contact with the trackpad while weighing is widely viewed as awkward.
  • People share tricks to satisfy capacitive sensing: hovering a finger, using wet sponges, foil shims, conductive foam, touchscreen stylus “nubs,” or even vegetables as capacitive styluses.
  • There’s some confusion and discussion about capacitive sensing needing a “human-sized” ground mass vs. how gloves and capacitive pens work.

Accuracy, range, and calibration

  • Several ask about precision and expected weight range; clear answers are limited.
  • One user reports readings up to ~7.3 kg by pressing as hard as possible, but warns against risking trackpad damage.
  • For objects, multiple reports say measurements are extremely inconsistent (same item giving wildly different values), suggesting the exposed API is tuned for finger force, not static loads.
  • Others suggest standard load-cell style calibration (2–3 known points) could improve results, and speculate Apple calibrates trackpads for consistent “feel,” not for weighing.

Private APIs & distribution

  • The app uses a wrapper over a macOS private framework (MultitouchSupport) to access raw pressure data not available in public APIs.
  • Commenters explain that private frameworks ship on macOS without headers; developers can reverse engineer headers and wrap them, but such use is barred from the App Store and possibly problematic for notarization.
  • Several people want a downloadable binary or .dmg; others note it’s more a tech demo meant to be built in Xcode by Mac developers.

Related sensor hacks & nostalgia

  • The thread recalls earlier “phone as scale” Web apps, barometer-based DIY scales, and older Mac/ThinkPad hacks using motion sensors for seismographs or gesture control.
  • There’s substantial nostalgia for iPhone 3D Touch, both as a weighing trick and as a superior UX mechanism that Apple removed.