In brief: We've seen plenty of tiny computers described as being credit card-sized, but that usually means they have roughly the same footprint as a payment card – rather than actually fitting inside a wallet. One Redditor decided to tuen the phrase into a literal one, building a functioning, battery-powered computer that is around 1mm (0.039 inches) thick.
For years, Raspberry Pi boards and similar devices have been marketed as credit card-sized computers. But while that description is fair in terms of length and width, it's not so accurate when it comes to thickness, ports, headers, and the general likelihood of one sitting comfortably next to your Visa.
Reddit user krauseler wanted to find out what would it take to build a computer that really is the size of a credit card. The answer, unsurprisingly, was months of tinkering, a lot of compromises, and a ton of engineering headaches.
I built a fully self-powered computer in actual credit-card size (~1mm thick)
by u/krauseler in electronics
The resulting prototype is fragile, but it works. It's built around an ESP32-C3FH4 SoC with Wi-Fi and Bluetooth LE, and includes NFC read/write support, a 1.54-inch 200 x 200 e-paper display, an accelerometer, and an ultra-thin LiPo battery with charging circuitry and power path management.
The creator admits that calling it a computer "might be a little overstatement," though it technically fits the definition. It's not going to replace your laptop, your Raspberry Pi, or even most microcontroller boards, but that's hardly the point. The achievement is making all of this work inside a form factor where even a connector can become too bulky.
According to krauseler, sourcing small enough parts was not the most difficult part of the build. Mechanical stability was the bigger problem, especially solder fatigue, pressure distribution, and strain on delicate materials.
At this scale, parts that seem thin enough on paper can stop being thin enough once they are assembled into a working device.
One particularly painful compromise involved the display connection. The creator said FPC connectors were basically unusable for the design, forcing them to solder individual wires to 0.5mm-pitch pads by hand. They also etched their own custom flexPCB as part of the build process.
The card is currently powered by its internal battery, though commenters pointed out that "self-powered" is not quite the right term unless it harvests energy from solar, RF, or another external source. Krauseler acknowledged the point and suggested "untethered" might be more accurate.
Future plans could include GPIO pins, caseless USB-C, a microSD slot, and possibly wireless charging to remove another mechanical weak point. The latter sounds especially sensible for something that looks like it should survive in a wallet, even if sitting on it would probably still be a very bad idea.
Make sure to check out Krausler's GitHub Repo if you want more details about the build.