The Casio F-91W contactless payment project by Matteo P is the kind of build that rewards a careful read well beyond the finished result, turning one of the most enduring LCD watches ever made into a functioning NFC payment device, and delivering a proper RF engineering write-up in the process.
The Casio F-91W Contactless Payment Build
The F-91W needs little introduction to anyone who has spent time around cheap, reliable electronics. It has supplied accurate, no-fuss digital timekeeping for many decades, costs very little, and has accumulated a following that keeps producing interesting hardware projects. Matteo P’s contribution takes the watch somewhere new: a fully functional contactless payment device built around a 13.56 MHz NFC card.
The approach is cleverer than simply glueing a card to the strap. Matteo P uses a specific type of 13.56 MHz NFC card whose internal architecture suits the job particularly well. As Hackaday previously covered in its own investigation of these cards, the antenna inside is a tuned circuit that spans most of the card’s area, with a smaller coupling coil sitting close to the chip itself. That separation between the main antenna loop and the chip coupling coil is the key detail Matteo P exploits.
The physical integration produces a SLA-printed front face for the watch that positions the NFC chip above the display and routes a pick-up coil around the outside edge of the watch face. The result is a wearable that looks, from a short distance, like a modestly customised F-91W, while carrying the credentials needed to tap a payment terminal.
Why the RF Design Section Is the Real Story
Hardware hackers will often skim a build log for the physical assembly steps and move on, but the most substantial part of Matteo P’s write-up is neither the printing nor the fitting. It is the deep-dive into RF coupling design. Getting NFC to work reliably at something close to resonance, with a coil that has been reshaped and repositioned to fit around a watch face rather than across a credit-card-sized substrate, is not trivial. The antenna geometry changes the inductance; the proximity of the watch’s own components and casing affects tuning; and a payment terminal expects a well-coupled, correctly resonant response. Matteo P works through these considerations methodically, making the write-up genuinely useful for anyone planning to embed NFC into an unconventional form factor.
Ensuring good coupling at close to 13.56 MHz resonance is the engineering problem the project really turns on, and the write-up addresses it directly rather than glossing over it with a “it just worked” conclusion. That honesty about the RF challenge is what elevates this from a novelty mod to something worth studying if you are considering NFC-enabling any object that was never designed to carry a radio antenna.
The NFC Forum defines the standards that govern how 13.56 MHz devices communicate and couple, and the constraints Matteo P is working within (resonant frequency, coupling coefficient, field strength at the reader) are precisely the variables those specifications care about. Understanding them is what separates a payment mod that works once on a forgiving reader from one that functions reliably in daily use.
A Wider NFC Rabbit Hole Worth Exploring
For those the project has pulled into an NFC enthusiasm spiral, Hackaday’s own one-transistor 125kHz NFC reader challenge sits at the sillier end of the same spectrum and is worth a look as a contrast: a deliberately constrained design exercise that strips the technology back to almost nothing. The gap between that minimum-component reader and Matteo P’s carefully tuned wearable payment device neatly illustrates just how much engineering headroom the NFC frequency bands contain.
The tip for the project came from John Elliot V. The build itself is Matteo P’s, and the RF write-up that accompanies it is the part to read twice before reaching for a soldering iron and a spare F-91W.
