Not Your Typical POV Clock

Persistence of vision displays are fun, and a natural for clocks, but they’re getting a little Nixie-ish, aren’t they? There are only so many ways to rotate LEDs and light them up, after all. But here’s something a little different: a POP, or “persistence of phosphorescence” clock.

[Chris Mitchell] turned the POV model around for this clock and made the LEDs stationary, built into the tower that holds the slowly rotated display disk. Printed from glow-in-the-dark PLA, the disk gets charged by the strip of UV LEDs as it spins, leaving behind a ghostly dot matrix impression of the time. The disk rotates on a stepper, and the clock runs on a Nano with an RTC. The characters almost completely fade out by the time they get back to the “write head” again, making an interesting visual effect. Check it out in the video after the break.

Our only quibble is the choice to print the disk rather than cut it from sheet stock. Seems like there has to be commercially available phosphorescent plastic, or even the glow-in-the-dark paper used for this faux LED scrolling sign. But if you’ve got glowy PLA, why not use it?

23 thoughts on “Not Your Typical POV Clock

    1. So could you make the screen update faster if you sweep it with IR leds before the UV leds?

      Also wondering how to make a projection system that could sweep across a stationary display.

      1. I’m not sure of the speed at which you can “erase” this display, but I’m sure it’s somehow directly related to the strength of your IR blasters and is probably also determined by the chemical composition of the phosphorescing elements, maybe also temperature.

        Depending on how non-linear this thing is, it could be you would light up the whole thing then turn off what you don’t n eed. Or maybe light up just some parts and then turn off only the bits you want to be removed from the last frame instead of the whole thing.

        To achieve speed you might want to sweep just a “clock arm” around instead of the whole platter.

  1. One cool thing I’ve noticed with glow-in-the-dark PLA is that if you turn the lights off and watch carefully you can see the just-extruded bit glow for a few seconds as the infrared from the hot plastic and nozzle gets converted into eerie phosphor green. (At least I assume this is due to IR, but as a sort of chance observation I didn’t go too far down the rabbit hole of figuring out the mechanism of action. If anyone has a better explanation I’d be psyched to hear it).

    1. This theory is faulty. The phosphors per se can only convert shorter wavelength too longer (UV to green). This is either thermoluminescence or a similar effect like the IR indicator cards: a phosphor gets charged by white light and it’s phosphorescence (the discharge of the stored energy) is accelerated by an external influence like IR or perhaps heat.

      1. Thanks! I had originally thought (hazily remembered from chemistry class) that phosphorescent substances would only absorb shorter wavelengths but observing this had shaken my confidence in this memory. Knowing that a different mechanism must he lighting that up is actually somewhat reassuring. (The shorter wavelength only bit makes intuitive sense and fits with everything else so well).

        Heat or IRv accelerating the release of previously stored energy from absorbed white light sounds like a saner explanation than my previous guess. That is something that’s great about hackaday: you learn from your fellow geeks filling in gaps in understanding.

  2. Nice to see it done. A couple of years back I started on a project like this – bought a large wall clock from Ikea to use as the base, got 3mm UV leds from ebay and glow-in-the-dark color from a local artists shop. I got as far as painting the glass dome of the clock with a few layers of the uv-paint and also built an arm with the LEDs. But as usual I never finished the project. ;-(

  3. Nice idea, could also spin the LED strip behind a stationary disk to draw the digits or do both digital and analog. Also could easily do the cartesian to polar mapping on the nano for better effect.

  4. no problem with the idea, but the implementation grates on me. i can’t believe people do these rotating machines without using software to compensate for the curvature. there is no good reason for the font to be distorted by the circle. you can emit square characters on such a device.

    1. There’s not enough resolution in the 10 LED’s to transform the text to square. It was mostly unreadable so i didn’t bother pursuing that branch of code. Perhaps if i had used 20+ SMD LED’s that would have been possible.

      Also, the thickness of the glow disk seems to make the text blurry. a thinner disk made for sharper text but getting a thinner disk off my 3d printer was disastrous. would need to laser cut some commercial glow sheet (if exists).

      But this was all outside of the scope of my build.

  5. What a fun project! You know those candle pinwheels? (often German Christmas “pyramids”); back when lighting was incandescent we had a kids lamp which had a clear plastic cylinder around the low-wattage bulb and on the cylinder was some sort of “happy little elves” scene which then moved projected on the walls (actually nightmare material [shrug]). Anyway, how about replacing the cylinder of painted plastic with your phosphorescent material with the LED tower inside, and then the heat (uh… would leds make enough heat to spin the pinwheel? damn, might have to motorize the cylinder) would bring the time around.

  6. The maker could take it up a level and make the equivelant of tank treads out of glow-in-the-dark plastic and make a rectangular display surface that scrolls from right to left being printed on by the uv leds at the right edge. The programming would be simpler too.

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