A Choker with a Screen: How a Hacker Built Glowing Jewelry for DEF CON

The author of the blog The Cat House made it to the DEF CON hacker conference for the first time and, being an introvert by nature, decided to give the people around him a reason to talk. Instead of a business card, he built a homemade choker — a neck ornament with a screen in the center and LED strips along the sides. The main requirement wasn't the functionality he was used to, but the look: a thin body, minimal bezels, a neat appearance.

The Screen and the Board

His search led to Panelook, a niche marketplace for displays, but the datasheets there cost money, so the author went straight to the manufacturer. His choice fell on a square AMOLED panel from Hello Lighting with a resolution of 480×480, rounded corners and tiny bezels — just 12 dollars apiece, but with almost no documentation. The glass turned out to be less than a millimeter thick.

At the heart of the device is an RP2350 microcontroller: its PIO block lets you drive the display at the full speed of the quad bus, and it has a bit more RAM than the RP2040. Even so, it's tight: a single frame in 16-bit color takes 450 KiB, so there was no room left for double buffering. The display took the most effort of all: no ready-made footprint for its 31-pin flex cable existed in any CAD program.

The Firmware and the Leather

As usual, the author wrote the firmware in embedded Rust with the Embassy framework. The final animation is simple, but when all the elements are drawn the frame rate drops to 30 frames per second — imperceptible to the eye. The screen is touch-sensitive: visitors were invited to draw with a finger right on the wearer's neck.

The leather strap was a saga of its own. After weeks of studying grades of leather, the engineer realized it was easier to buy a ready-made choker on AliExpress and take the strap off it: cheaper and faster than any piece of leather.

A Bomb Around the Neck

To keep the device running all day, the author strapped an 18650 battery to the back of his head — with the caveat that the protection circuit stayed inside the enclosure, while long bare wires ran to the cell. He himself ironically calls it “a little bomb around the neck.” The day before his flight there was a near-catastrophe: superglue ate through the wire insulation and exposed the copper. The damaged sections had to be urgently cut out and resoldered.

The Reception

At DEF CON and the HOPE conference that followed, the choker was received enthusiastically. Many took it for a commercial product — and got even more excited when they learned it was homemade. The battery lasted with room to spare: after a 16-hour day the voltage held at 3.7 V. The touchscreen, however, worked poorly without a connection to a computer — capacitive coupling through the body was only partly enough. The firmware and schematics, as always with this author, are published in the open.

Source: Lobsters