Ohm for the Holidays

We are now in the homestretch and Christmas magic is in the air! Engineers and electronics hobbyists Jeff Keacher and Sean Beever couldn’t let this hacking opportunity slip by like Santa eluding curious eyes on Christmas Eve, so they built a cool gadget that can pluck some of that magic out of thin air to spread Christmas cheer. Their device is an electronic holiday card that uses ambient RF and light energy to light up LEDs on a custom printed circuit board tree. Since no battery is required, this little card can keep the Christmas cheer going long after the big day has come and gone.

To power the device, energy is gathered from multiple sources: light-harvesting LEDs, a 2.4 GHz RF resonant circuit, or — if you are not feeling especially magical — a USB-C port. When available energy exceeds immediate requirements, the excess is stored in a supercapacitor to maintain operation during periods without external power. A PIC16LF18854 microcontroller manages the interconnection between the energy accumulation and energy consumption voltage rails.

The RF-based energy harvesting system is particularly interesting, resembling a simple 2.4 GHz crystal radio with a resonant antenna, an impedance-matching network tuned to 2.45 GHz, and a two-stage voltage multiplier made with efficient Schottky diodes to convert RF signals into usable DC power. The antenna, a carefully simulated and optimized meander antenna, achieves approximately +3.8 dBi gain and maintains good return loss across the 2.4 GHz band.

The light-harvesting mechanism on the card utilizes specially selected LEDs to capture ambient light and convert it into usable electrical energy. While LEDs are primarily used to produce light, they can also work backwards and function as small photovoltaic cells when exposed to light. The harvested light energy generates a voltage, which is then directed to the energy accumulation rail for immediate use, or stored in a supercapacitor for later consumption. The system is highly efficient, capable of operating on as little as 200 lux of light — about the brightness of a dimly lit room. This unusual design allowed the card to harvest energy without the bulk of traditional solar panels.

Another interesting feature of the card is that it can be controlled via a 2.4 GHz Wi-Fi network. This works by using amplitude-shift keying to modulate the 2.4 GHz Wi-Fi signal's power. A control web page on a phone or computer streams or pauses data to a websocket connection, creating high (data sent) or low (no data sent) signal power states. The card detects these power changes without joining the network and interprets them as binary 1s or 0s.

To decode commands, the card samples the RF signal and looks for a Barker-7 sequence for synchronization, followed by a 16-bit payload containing one of eight valid commands. The card processes the payload, executes the corresponding action, and confirms successful decoding by flashing an LED. Range depends on power: when powered via RF from a phone, the card must be nearby, but with external power (USB or supercapacitor), it works over a few feet.

On the surface, this may look like a typical PCB Christmas tree build, but on closer inspection it quickly becomes clear that this isn't your grandma's electronic card. Check out the technical write-up of the project for all the details, and if you want to build you own, be sure to take a look at the instructions as well.