An NFC energy-harvesting PCB business card with a microcontroller

Wilson Harper set out to learn KiCad and ended up with a battery-free PCB business card that lights up 21 LEDs when tapped against a phone, powered entirely by harvested NFC energy.

The card pairs an NXP NTAG I2C Plus, which routes excess DC voltage out to external hardware, with a 3mm x 3mm ATtiny816 chosen for its QFN package and 17 GPIOs. Six of those pins drive 20 Charlieplexed LEDs plus a simple indicator LED.

The rectangular spiral antenna fills the whole card and was sized with STMicro’s inductance calculator to land near 2.75uH for resonance at 13.56MHz, generated by a KiCad Python script included in the repo since no existing plug-in could make the shape. Routing avoided a closed loop, and a ground plane, so eddy currents would not kill the antenna, and all traces are mirrored onto the back silkscreen alongside a block diagram and BOM.

Three exposed pads on the back at breadboard spacing allow programming over UPDI with a pogo pin clip, with a TVS diode protecting the MCU from wallet static. After a Pico-based programmer failed to produce any signal on the scope, an Adafruit UPDI Friend worked instantly. The main loop is bare-metal C at 1MHz to minimize flicker, the tag was written with NXP TagWriter, and thirty RoHS-compliant, credit-card-thin boards were ordered from JLCPCB. A second version is under consideration with different LED colors and maybe a button or two for arcade-style games.

See more at wilsonharper.net and content on GitHub.

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