Persistence of vision device

It’s super hard to take any picture or video about this

A while a go I saw a neat article in Hackaday about a device that shows a picture that looks exactly the same looked from any direction. The idea is very simple: The picture is printed on a paper and placed inside the rotating cylinder. The cylinder has a narrow slit in it allowing the observer to only see a narrow section of the image at the time. When the cylinder rapidly rotates, observers brain parses the narrow sections to a whole image. Since the picture inside the cylinder rotates, it looks the same no matter which direction it’s looked at.

In the end of July there’s my favorite festival Naamat held in Muurame, Finland. The nights in July start to be dark after the midsummers endless day. The major point of the festival is the party at night at the camping area. My tent has a plastic dome on top of it, so I got the idea to replicate this device and place it on top of my tent to make it stand out in the night.

The construction

I thought it would be a one night task, but like with most of my ideas I ended up spending way more time with this that I intended. I started modeling the parts with Fusion 360. From top down, there’s the cylinder with the printed picture. The illusion works best when it’s well lit, so I made the top of the cylinder white from the inside to reflect the light back. The next part down is the rotor, where the cylinder is attached and which the motor rotates. Below that is the motor mount. The motor mount is essentially a white disc where the motor is fixed at the center. The light is provided by pieces of led strips glued to the disc. Again the white color is chosen to reflect the light inside the cylinder. After that there’s the bottom shell that has the electronics inside. The mounting on the tent dome is made by making a dome sized hole to the bottom of the shell and embedding three thread inserts to that the device can squeeze the dome on all sides.

The electronics

As for the electronics, there’s a RC ESC to control the motor speed and an Arduino mini that starts the motor with a fixed speed and then monitors the battery voltage. Not strictly necessary but I added a RC controlled switch to turn the light on and off. The rationale was that I wanted to be able to leave the device on for the whole night without destroying the lipo-battery by draining it completely. After power up, Arduino turns the light on, then waits for a few seconds for ESC to start and then it starts the motor at predefined speed. The rest of the time the Arduino is looping to check the battery voltage and when that falls below the set limit, the motor and the light is shut down. The idea is that the current draw after that is negligible and the battery can endure that for several hours.

When testing I used a meme picture that was all too familiar from the university where it usually included the text “Et vain osaa”

The problems

Boy were there a lot of those. I tried to make the cylinder as light as possible. When printing, I only used a single perimeter line with 0.6mm nozzle. To test that the idea works I printed only bare minimum parts, the cylinder and the rotor and tested just by holding the motor in my hand. When spooling up the cylinder, it immediately exploded to pieces in my hand. I re-printed the cylinder with walls twice as thick and tested again. I used a servo tester to find out the right speed. Somehow I had put the tester to full speed. Before I even had time to be scared the cylinder exploded again, fortunately missing my head.

When I got tested so much that I saw the idea worked, I printed the motor mount. Since this was supposed to be a quick and dirty project, I tried to use the parts I had in my junk box(es, there’s a lot of those boxes. And junk). I thought I’d get a perfect motor from an old camera gimbal. Those are small, have a lot of torque and rotate slowly. The only problem was that my ESC refused to work with that motor. I then tried to use 400 class RC helicopter motor, but that spins way way too fast for this purpose. I finally settled with the regular 550 class brushed motor. The same you can find from any cordless drill. This made the device a lot more taller than I wanted but it works and at least the motors are widely available. Fortunately I also found an ESC for the brushed motor. Those are becoming uncommon too.

Now that the cylinders were not exploding any more I noticed that the cylinder wobbled a lot. 3D-printing is not a tool for the tight tolerances. I thought that the mounting surface against the motor is not flat or the hole is not perpendicular to that surface. I printed a new part avoiding supports and with on purpose too tight hole. I then got my brother to flatten out the mounting surface and drill the right size hole with a lathe ensuring perpendicularity. This helped a lot but some wobbliness remains. This is likely because the slit on the cylinder, which makes the center of gravity to be somewhere else than the center of rotation. It also might act aerodynamically generating lift like a wing. The wobbliness don’t matter much but it makes the picture to appear a little fuzzy.

The final issue I only found about at the festival. I was a bit skeptical about this working at all when there but it seemed to work well and draw interest. But soon it shut down even with a freshly charged battery. When restarted, it would work for a random time and again shut down. I realized the mistake I made. The loop that checks the battery voltage only looks at a single measurement result and if it’s below the limit, it turns the device off. With ADC readings, you should always assume that they are dirty and full of measurement errors. This was later fixed by taking a rolling average over multiple measurements.

Jukka with a lathe

City of Light

There’s one and perhaps the only event that my home city Jyväskylä can be proud of, the City of Light which in every autumn fills the city with ingenious and beautiful light installations. I thought this would suit well to the theme. I printed a small adapter which attaches to the handle of my boys bicycle chariot. The device was fixed to this adapter. Against all odds it worked perfectly for the whole evening even in quite bumpy bicycle ride and in a rainy weather. It was fun to see people trying to figure out what it is and wondering it aloud but still nobody came to ask 😄