Sam Ettinger Builds a Fluorescing Seven-Segment Display PCB, Driven by UV LEDs
Exploiting a little-known property of FR-4 PCBs, this project makes the PCB itself glow through exposure to UV light.
Maker Sam Ettinger has built a seven-segment display with a difference: It relies on the fluorescence of FR-4 when exposed to ultraviolet light.
"If you've ever played with a blacklight (or watched certain police procedurals), you've probably noticed that lots of materials fluoresce under a blacklight," Ettinger explains. "However, one material I did not expect to fluoresce was FR-4, the common core material for PCBs. It emits a blue-green that you don't see in LEDs, it's more like old VFDs or strontium aluminate."
With that fact in mind, brought to our attention by OSH Park, Ettinger set about building a seven-segment display — stripping the soldermask from both sides of the board in the familiar seven-segment pattern, then placing compact surface-mount LEDs underneath. Hooked up to an Arduino for control, the display glows in an eerie light — the fluorescence of the PCB material itself, rather than direct lighting from LEDs.
"My unscientific hunch was that the fluorescence comes from bisphenol-A compounds used in the FR-4 epoxy to sandwich layers of fiberglass together," Ettinger opines. "I think I read that BPA fluoresces. But what color? Do the other bisphenol analogs fluoresce?"
Fellow maker Paul Andrews has an alternative theory. "So I went a-searching. FR4 can have both a UV blocker and a UV fluorescer added," he writes. "The former to prevent UV etching of one side of the PCB penetrating to the other side. The latter to help with AOI (automated optical inspection) based on UV light rather than visible light. At least one link said that the UV blocker might also fluoresce."
More details on the experiment are available on Ettinger's Hackaday.io project page.
