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NFC Energy-Harvesting PCB Business Card with an MCU

This project details the creation of a sophisticated PCB business card that powers animated LEDs solely through NFC energy harvesting, eliminating the need for a battery. The author meticulously documents the design process, from component selection and circuit architecture to antenna tuning and PCB layout. It's popular on HN because it showcases a deep dive into practical, multi-disciplinary electrical engineering challenges, like Charlieplexing and managing power from harvested RF, resulting in a sleek and functional gadget.

36
Score
2
Comments
#4
Highest Rank
8h
on Front Page
First Seen
Aug 30, 9:00 PM
Last Seen
Aug 31, 4:00 AM
Rank Over Time
446657912

The Lowdown

Wilson Harper embarked on an ambitious project to design a business card that merges elegant aesthetics with complex engineering: an NFC energy-harvesting PCB featuring 21 animated LEDs, all powered wirelessly by a smartphone's NFC field. What started as a simple KiCad learning exercise quickly evolved into a multi-faceted challenge, pushing the boundaries of what a 'simple' business card can achieve without an onboard battery.

  • The card leverages NFC power harvesting, where the phone's magnetic field acts as a tiny wireless charger. Excess DC voltage from the NFC chip (NXP NTAG I2C Plus) is routed to power the external components.
  • The design initially used an ATtiny412 MCU and Charlieplexing for 12 LEDs but was upgraded to an ATtiny816 (in a compact QFN package) for more GPIOs, enabling 20 LEDs and professional assembly (JLCPCB) with RoHS compliance.
  • A significant design hurdle involved exceeding the NFC chip's capacitance limit on VOUT; this was cleverly overcome with a MOSFET-controlled circuit to allow a larger smoothing capacitor while managing inrush current.
  • The custom antenna's dimensions were precisely calculated using STM's tools and generated in KiCad via an LLM-assisted Python script to achieve optimal 13.56 MHz resonance.
  • PCB layout focused on aesthetic appeal and functional integrity, meticulously routing traces to avoid destructive eddy currents within the antenna loop.
  • Programming the ATtiny816 involved bare-metal C for flicker-free LED animations and using an Adafruit UPDI Friend after initial struggles with a DIY programmer.
  • The author envisions future iterations (V2) with different LED colors, interactive buttons, or even small arcade games.

This project not only delivered a highly functional and visually striking business card but also served as a profound learning experience, demonstrating that combining existing technologies in novel ways can lead to uniquely challenging and rewarding engineering feats.