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This clock uses a custom built 4-digit 7-segment display made from UV LEDs. In front of the display a screen is placed that consists either of phosphorescent (“glow-in-the-dark”) or photochromic material. A push button on the top lights up the UV display which then illuminates the screen for a few seconds so that it starts glowing or changes color which then slowly fades away.

This project was inspired by the awesome Glow-In-The-Dark Plot Clock by Tucker Shannon. When I rebuilt his project I gave it a little twist by replacing the glow-in-the-dark screen with one 3D printed from photochromic filament which changes color when exposed to UV light. In the meanwhile I saw that other people had the same idea (see e.g. here). Although the mechanical plotting mechanism of the clock is certainly awesome it has the disadvantage that the numbers come out a bit crooked so I was thinking of another way to make the numbers look more clean. At first I tried to replace the backlight of an LCD display with UV LEDs and then put a photochromic/glow-in-the-dark screen on top. However, it turned out that the intensity transmitted through the LCD was very low. After that I decided to build a 4-digit 7-segment display using UV LEDs to illuminate the screen which gave much better results.

DS3231 RTC module (
Arduino Nano (
UV color changing filament (
96x39x1 mm Glow-in-the-Dark sticker (
96x39x1 mm transparent plastic sheet (
MT3608 DC DC step up module (
30 pcs 5 mm UV LED (
TM1637 4-digit 7-segment display (
12x12 mm momentary push button (
3D printer
hot glue gun
soldering iron

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