Most of us have a wallet full of plastic that does exactly one thing: it identifies us or moves money. We’ve seen “smart” versions of these cards over the years—trackers that claim to be card-sized but leave a noticeable bulge in a leather slot, or prototypes from tech giants that never quite make it to a retail shelf. The gap between a “card-shaped device” and a “literal credit card” is a matter of millimeters, but in hardware engineering, those millimeters are everything.
A tinkerer known as Krauseler has bridged that gap. By leveraging the ESP32-C3 microcontroller, an e-ink display, and an NFC chip, they have developed a functional computer that adheres to the actual dimensions and thickness of a standard credit card. It is a project born not out of a specific commercial need, but from a desire to conquer extreme space constraints.
As a former software engineer, I’ve spent plenty of time optimizing code to fit into tight memory buffers, but the physical constraints of hardware are a different beast entirely. When you are designing for the ISO/IEC 7810 ID-1 standard—the global specification for credit cards—you aren’t just fighting for surface area; you’re fighting for verticality. Most “card” trackers are essentially thin boxes. Krauseler’s project is an exercise in minimalism, stripping the computer down to its absolute essentials to ensure it actually slides into a wallet without stretching the material.
The anatomy of a wallet-sized computer
The heart of the device is the ESP32-C3, a RISC-V based system-on-a-chip (SoC) from Espressif. For those not immersed in the world of microcontrollers, the ESP32 series is the gold standard for hobbyist IoT (Internet of Things) projects because it integrates Wi-Fi and Bluetooth into a tiny package. The C3 variant is particularly suited for this project due to its modest footprint and power efficiency.
To make the device useful without draining a battery in minutes, Krauseler opted for an e-ink display. Unlike the OLED or LCD screens on our phones, e-ink only requires power when the image changes. Once a piece of information is written to the screen, it remains there indefinitely without consuming a single microamp of current. This makes it the only logical choice for a device that needs to fit into a wallet where large batteries are physically impossible.
The addition of an NFC (Near Field Communication) chip transforms the device from a simple screen into an interactive tool. NFC allows the card to communicate with smartphones or readers over very short distances, effectively allowing the “computer” to act as a key, a badge, or a trigger for other smart devices.
| Component | Function | Why it was chosen |
|---|---|---|
| ESP32-C3 | Main Processor | Integrated Wi-Fi/Bluetooth, RISC-V architecture |
| E-Ink Display | Visual Output | Ultra-low power; retains image without electricity |
| NFC Chip | Connectivity | Enables contactless interaction and data exchange |
| Custom PCB | Physical Base | Designed to meet ISO credit card thickness standards |
Engineering against the bulge
The primary motivation for this project was a frustration with the current market. Many “credit card” trackers are marketed as slim, but they often feel like a thick piece of cardboard compared to a standard Visa or Mastercard. Krauseler noted on their GitHub page that most devices claiming to be credit-card sized are “still far away from actually feeling like one.”
Achieving this thickness requires a meticulous approach to component selection. In traditional electronics, components like capacitors or voltage regulators often stick up from the board. To achieve a flat profile, a designer must use surface-mount devices (SMD) with the lowest possible height profiles and potentially utilize multi-layer PCBs to route the circuitry internally rather than on the surface.
The result is a prototype that doesn’t just mimic the shape of a card but respects the ergonomics of a wallet. It is a demonstration of “extreme space constraints,” pushing the limits of how much computing power can be squeezed into a sliver of fiberglass, and silicon.
The “What Now?” problem
The most human part of this story is that while the engineering challenge has been solved, the utility challenge remains. Krauseler has successfully built the “how,” but is now crowdsourcing the “why.” The creator has shared the schematics and the build process on GitHub and Reddit, inviting the community to help define the device’s purpose.
Notice several immediate, high-value applications for a device with these specific specs:
- Hardware 2FA Authenticator: The card could generate and display time-based one-time passwords (TOTP) for secure logins, acting as a physical security key that lives in your wallet.
- Smart Home Dashboard: Using its Wi-Fi capabilities, the card could act as a remote trigger. Tapping the card against an NFC-enabled reader by your front door could trigger a “leaving home” scene, turning off lights and locking doors.
- Digital Ticket Wallet: Instead of relying on a phone battery that might die at a concert or airport, the e-ink display could show a static QR code for a ticket, while the NFC chip handles the actual scanning.
- Dynamic Business Card: The display could rotate through different contact methods or portfolio links, which can be updated over Wi-Fi.
How to build your own
For the brave and the handy, this isn’t a closed-source corporate product; it’s an open project. The schematics are available for those willing to dive into PCB design and soldering. However, a word of caution: building at this scale is not for beginners. Soldering components onto a board this thin requires a steady hand and a level of precision that goes beyond basic hobbyist kits.
The project serves as a blueprint for others to iterate upon. By providing the foundation, Krauseler has essentially created a “dev kit” in the form of a credit card. The next version might include a thinner battery, a more efficient antenna, or a different display module, depending on what the community decides the device should actually do.
The project is currently in the prototype and community-feedback stage. The next confirmed step for the developer is the integration of user-suggested features and the refinement of the firmware based on GitHub contributions. As more tinkerers attempt the build, the repository is expected to evolve with updated assembly guides and potential new use-case libraries.
What would you do with a computer that fits in your wallet? Let us know in the comments or share this story with the most obsessive tinkerer you know.
