Battery-free NFC business card powers an MCU and 21 LEDs from a phone's RF field

A maker built a PCB business card that lights up a 21-LED animation with no onboard battery. When tapped to a smartphone, an NXP NTAG I2C Plus NFC chip harvests energy from the phone’s 13.56 MHz magnetic field and routes surplus DC to an ATtiny816 microcontroller. The LEDs are driven via Charlieplexing, which lets six GPIOs independently control 20 LEDs by exploiting the one-way current flow of diodes, with PWM used to fake simultaneous illumination. The project began as a KiCad learning exercise and grew into a full lesson in RF and power design.

The hard part was working within the NFC chip’s power budget. The datasheet caps VOUT capacitance at 220 nF to avoid a voltage-collapse oscillation, so the builder added a MOSFET-switched 10 µF smoothing cap gated by the MCU after a 120 ms inrush-limited startup, plus 100 Ω current-limiting resistors on each Charlieplex trace. The full-card antenna was tuned to roughly 2.75 µH to resonate with the chip’s internal 50 pF capacitor, deliberately kept slightly low so it could be trimmed up. A gap in the antenna loop prevents destructive eddy currents from coupling with the NFC field. He chose JLCPCB for RoHS-compliant lead-free assembly and a small QFN package he couldn’t hand-solder himself.

The design is fully open-sourced, including a Python script—written with LLM help—that generates the rectangular-spiral antenna trace, filling a gap in KiCad’s tooling. It’s a clean example of hobbyist hardware pushing energy-harvesting NFC beyond its usual role of powering only the tag’s internal logic, and a reusable reference for anyone wanting cheap, battery-free interactive PCBs.