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USB-Powered Infinite Dungeon Portal

Inspired by this portal by the Crafting Muse, I started thinking about portals to somewhere <woo-woo>else</woo-woo>. Naturally, my implementation ended up being over-complicated, but it lights up, so that's OK.

The idea was to create an "infinite mirror" embedded in a 3d printed doorway. The idea behind the infinite mirror is that you have a box with a mirror in the back, a half-mirrored surface in the front (like the "one-way mirrors" you get in police interrogation rooms on TV), and a light source somewhere in between. The light source reflects off of the mirrors, the reflections create reflections of their own, and so on until you create the illusion of enormous depth. Appropriate for a magic portal in a dungeon. It's rather more demanding on materials than what I normally do, but I had most of the components kicking around anyway.

For a light source, I settled on a couple of LEDs. I experimented with a bit of electroluminescent wire, which is nifty, but not bright enough. For a power source, while I could have used batteries, I thought using a USB cable would be interesting to play with and wouldn't involve consumables which would wear out and dim in the course of use, probably needing replacement at the worst possible moment.

The USB standard provides not just a data connection but a 5v power supply as well. That's how your phone can charge itself through the same connection it uses to transfer data. I've taken advantage of that fact before by wiring stuff directly to the USB connector for the sake of compactness, but this is a little more sensible. I got an old USB cable that didn't work for data any more, snipped off the end, and stripped it down to the wires. A stripped USB cable looks something like this:

There's a thick layer of electromagnetic shielding which can just be snipped off and a pair of wires for data which can likewise be cut short, since they're not being used here. I just needed some length on the red and black wires.

To connect any power supply to LEDs, you need to do some math, or at least get someone or something to do math for you. Basically, for any circuit of LEDs, you need to throw in a resistor or else you'll burn out the LEDs out pretty fast. There are any number of on-line calculators telling you what you'll need and even suggest a circuit diagram. This is one of the better sources of data on the topic I've seen with good sample values for numbers you'll be plugging in, so aided by that I used this calculator and put my two 5mm red LEDs in parallel on a 68-ohm resistor, wired that to the USB, and got them to light up nicely.

Then there's the "box" itself and its accouterments. The base doorway was one I found on Thingiverse. However, that just gave me the rudiments of a "face." The rest of the box had to be somewhat bigger to provide both room for mirror and electronics and the necessary depth. It ended up being a three part assembly for ease of printing:
  • A back piece to hold the mirror.
  • A middle piece to provide depth (an infinite mirror should be at least a half-inch deep) and somewhere to fix the lights.
  • A front piece to provide the full facing of the doorway and, given the size of the box, the adjacent wall.

The mirror for the back was a cheap 2" square from a craft store I'd had kicking around. And the half-mirror in the front was a piece from the roll of window film I got for the last infinite mirror I did. (I experimented with scraps of semi-reflective mylar bags used for shipping circuit boards and the like, and unfortunately they turned out to be not reflective enough.)

With all the pieces ready, then, it was a simple matter of painting the face, inserting the LED wire in the middle part of the box, gluing the pieces together, and plugging in.

With power off, it's just a mirror.

The whole thing is about an inch thick.

So, not too bad, I think. Some things to play with if I do a future version:
  • Sensibly put a layer of transparent plastic (say, cut out of a strawberry container) in front of the reflective film to keep it from getting scratched.
  • Sensibly solder together the connections, currently kept together by twisting wires.
  • Painting something faintly glittery on the mirror and/or film and see how that works out.
  • See what I can do with UV-emitting LEDs and fluorescent paint in the "tunnel" section. The glow might not be enough to reflect well, but it's worth an experiment.


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