A business card is usually the least electronic object in anyone’s wallet. Wilson Harper’s version isn’t — tap it against the back of an NFC-capable phone and 21 LEDs animate across the card’s surface, powered entirely by energy pulled wirelessly out of the phone’s own NFC field. There’s no battery on the card at all.
Turning a phone’s own field into the power supply
NFC readers already broadcast a small radio field to talk to tags; Harper’s card just harvests that field as power instead of only using it for data. A PCB trace antenna, tuned to the phone’s NFC frequency, captures enough energy to run a small microcontroller and drive 21 Charlieplexed LEDs — a wiring technique that lets a comparatively small number of microcontroller pins independently address a much larger grid of LEDs, which matters a lot on a power budget this thin. The whole thing is built onto a PCB cut to standard business-card dimensions, so the electronics disappear into an object that still functions as, and looks like, an ordinary card.
The antenna itself was the harder design problem — Harper describes tuning the trace’s inductance by feeding STM’s own antenna inductance calculation tools into an LLM script wired directly into KiCad, automating a piece of RF design that’s normally a fussy manual tuning process for hobbyist PCB work.
A small, complete demonstration of a bigger idea
There’s no practical necessity being solved here — a business card doesn’t need LEDs, animated or otherwise. What makes it worth covering is how completely it demonstrates energy harvesting as a legitimate power source for small interactive electronics: no coin cell, no charging port, no battery to replace or dispose of, just ambient RF energy that’s already there every time someone’s phone gets close enough to read the card. It’s the kind of project whose real audience isn’t the card’s actual business use — it’s every hobbyist who reaches for a battery by default and hasn’t stopped to ask whether the thing they’re building even needs one.