A Credit Card-Sized Computer

If you had a fully functional, credit card-sized computer that could fit in your wallet, what would you do with it? No, seriously — we need to know. This isn’t an academic exercise; GitHub user krauseler built one, but isn’t exactly sure what to do with it. There has to be a killer app for an ultra-thin computer like this, but even if I can’t figure out what that is, I still want one.

Called the Muxcard, krauseler’s creation is a programmable computer that matches the footprint — and nearly the thickness — of a real ISO7816 smart card. We’re not talking about a chunky “credit card-sized” gadget that bulges in your wallet. The Muxcard measures 1 mm in thickness, just barely more than an actual payment card.

Inside the paper-thin package is an ESP32-C3 microcontroller, a 1.54-inch flexible E Ink display, NFC hardware, an IMU sensor, and a tiny LiPo battery. Somehow, all of it fits into a package thin enough to disappear between the cards in your wallet. Even more impressively, the prototype runs entirely standalone with no external hardware hiding off-board.

Fitting everything into such a tiny package took a lot of effort. Months were spent shaving fractions of a millimeter from every component stack-up, experimenting with flexible PCBs, and redesigning the internal structure to survive bending stress. Instead of trying to make the card rigid, the design routes mechanical forces around sensitive components using “islands” and flexible weak points that reduce strain on solder joints and traces.

Incorporating the display into the device was quite the challenge. Standard connectors were too thick and too fragile, so the prototype ended up using painstaking hand-soldered wiring directly onto the display flex cable. Powering the computer by battery may have been even harder to get right. Ultra-thin LiPo cells exist in the 0.4 mm to 1 mm range, but capacity drops dramatically as thickness decreases. The current prototype uses a tiny 30 mAh battery, though future revisions may use thinner cells spread across a larger surface area for better durability and energy density.

Despite the compromises, the card already works very well. The ESP32 sleeps at around 8 microamps with the RTC enabled, waking only to refresh the E Ink display, handle NFC tasks, or briefly activate Wi-Fi and Bluetooth. That opens the door for a broad set of applications.

Some ideas already floated by the creator include portable 2FA storage, NFC credential cloning, QR code wallets for tickets and memberships, minimalist smart home controls, or an unforgettable digital business card.

The hardware design files and sample firmware have been made publicly available for non-commercial uses. Go grab them if you’d like to build your own credit card-sized computer.