Want To Learn Binary? Draw Space Invaders!

This was the week that I accidentally taught my nearly ten-year-old son binary. And I didn’t do it on purpose, I swear.

It all started innocently enough. He had a week vacation, and on one of those days, we booked him a day-course for kids at our local FabLab. It was sold as a “learn to solder” class, and the project they made was basically a MiniPOV: eight LEDs driven by a museum-piece AVR ATtiny2313. Blinking lights make a pattern in your persistence of vision as you swipe it back and forth.

The default pattern was a heart, which is nice enough. But he wanted to get his own designs in there, and of course he knows that I know how to flash the thing with new code. So I got him to solder on an ISP header and start drawing patterns on grids of graph paper while I got the toolchain working and updated some of the 2000’s-era code so it would compile.

There’s absolutely no simpler way to get your head around binary than to light up a row of LEDs, and transcribing the columns of his fresh pixel art into ones and zeros was just the motivation he needed. We converted the first couple rows into their decimal equivalents, but it was getting close to dinner time, so we cheesed out with the modern 0b00110100 format for the rest. This all happened quite organically; “unintentional parenting” is what we call it.

While we were eating dinner, I got the strangest sense of deja vu. When I was around ten or eleven, my own father told me about the custom fonts for the Okidata 24-pin printer at his lab, because he needed me out of his hair for a while, and I set out to encode all of the Hobbit runes for it. (No comment.) He must have handed me a piece of graph paper explained how it goes, and we had a working rune font by evening. That was probably how I learned about binary as well.

Want to teach someone binary? Give them a persistence of vision toy, or a dot-matrix printer.

(Art is from a much older POV project: Trakr POV — a hack of an old kids’ toy to make a long-exposure POV image. But it looks cool, and it gets the point across.)

Tiny POV Turns Right Round For Volumetric Fun

Just when you think the POV thing has run out of gas, along comes [mitxela] to liven things up. In this, he’s taken the whole persistence of vision display concept and literally spun up something very cool: a tiny volumetric “electric candle” display.

As he relates the story, the idea came upon him on a night out at the pub, which somehow led to the idea of an electric candle. Something on the scale of a tea light would fit [mitxela]’s fascination with very small and very interesting circuits, so it was off to the races. Everything needed — motor, LIR2450 coin cell, RP2040, and the vertical matrix of LEDs — fits into the footprint of the motor, which was salvaged from a CD drive. To avoid the necessity of finding or building a tiny slip-ring, he instead fixed everything to the back of the motor and attached its shaft to a Delrin baseplate.

The 8×10 array of surface-mount LEDs stands atop the RP2040 with the help of some enameled magnet wire, itself a minor bit of circuit sculpture. There’s also a 3D-printed holder for a phototransistor and IR LED, which form a sensor to trigger the display; you can see [mitxela] using a finger to turn the display off and move it back and forth. It goes without saying that these things always look better in person than they do in stills or even on video, but we still think it looks fantastic. There’s also a deep dive into generating volumetric data in the write-up, as well as an unexpected foray into the fluid dynamics calculations needed to create a realistic flame effect for the candle.

All in all, this is a fantastic if somewhat fragile project. We love the idea of putting this in a glass enclosure to make it look a little like a Nixie tube, too.

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Spinning CRT Makes A 360 Degree Audio Oscilloscope

A question for you: if the cathode ray tube had never been invented, what would an oscilloscope look like? We’re not sure ourselves, but it seems like something similar to this mechanical tachyscope display might worked, at least up to a point.

What’s ironic about this scenario is that the tachyscope [Daniel Ross] built actually uses a CRT from a defunct camcorder viewfinder as the light-up bit of what amounts to a large POV display. The CRT’s horizontal coil is disconnected while the vertical coil is attached to the output of a TEA205B audio amplifier. The CRT, its drive electronics, and the amp are mounted to a motorized plastic platter along with a wireless baby monitor, to send audio to the CRT without the need for slip rings — although a Bluetooth module appears to be used for that job in the video below.

Speaking of slip rings, you’d expect one to make an appearance here to transfer power to the platter. [Daniel] used a slip ring for his previous steampunk tachyscope, but this time out he chose a hand-wound air core transformer, with a stationary primary coil and secondary coil mounted on the platter. With a MOSFET exciter on the primary and a bridge rectifier on the secondary, he’s able to get the 12 volts needed to power everything on the platform.

Like most POV displays, this one probably looks better in person than it does in video. But it’s still pretty cool, with the audio waveforms sort of floating in midair as the CRT whizzes around. [Daniel] obviously put a lot of work into this, not least with the balancing necessary to get this running smoothly, so hats off for the effort.

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Laser Projector Built From An Old Hard Drive

Spinning hard drives are being phased out of most consumer-grade computers in favor of faster technology like solid-state drives and their various interfaces. But there’s still millions of them in circulation that will eventually get pulled from service — so what do we do with them? If you’ve got one that would otherwise be going in the garbage, they can be turned into some other interesting devices like this laser text projector.

Even the slowest drives spin at around 5000 RPM, which is perfect for this type of application. The device works by mounting twelve mirrors, each at a slightly different angle, on a drum which is spun by the drive’s motor. Bouncing a laser off of the spinning drum results in a projection of twelve horizontal lines. By rapidly switching the laser on and off depending on which mirror it’s pointing at, the length of each line can be controlled.

Thanks to persistence of vision, that allows you to show text on the surface that the laser is projected on. At speeds this high, it took [Ben] of Ben Makes Everything quite a few iterations to get it to a usable space. From sensors that were too slow to lasers not bright enough to 3D prints that were not accurate enough, he goes through the design of his build and the process in excellent detail.

After solving all of the problems including building his own constant-current laser power supply, and burning up a few laser diodes in the process, [Ben] has a laser projector capable of displaying readable text at a great distance which is also portable, running on a 12 V power supply. There are some possible areas of improvement that he notes as well, such as an unbalanced 3D printed part causing a bit of a wobble and the Arduino controller not being fast enough for more text. But it’s an impressive project nonetheless, similar to a two-mirror version we saw some time ago but with the ability to display text as well.

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A New Spin On 360 Degree Displays

Back in 2018, [Salah] created a prototype display that seems to defy logic using little more than a Pringles can and a fast motor. While not volumetric, this hack does show the same 2D image from any vantage point in 360 degrees around it.

How can cardboard create this effect? Somewhat like a zoetrope uses slits to create a shutter effect, this display uses a thin slit to limit the view of the image within to one narrow vertical slice at a time. When moving fast enough, Persistence of Vision kicks in to assemble these slices into a complete image. What we think is so cool about this hack is that the effect is the same from any angle and by multiple viewers simultaneously.

The project page and video demonstration after the break are light on details, though the idea is so simple as to not require additional explanation. We assume the bright LED seen in the video below was added to overcome the relatively dim appearance of the image when viewed through the narrow slit and isn’t strictly required.

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3D Zoetrope Uses Illusion To Double The Frames

Although film and animation have come quite a long way, there’s still something magical about that grandaddy of them all, the zoetrope. Thanks to persistence of vision, our eyes are fooled into seeing movement where there is none, only carefully laid-out still pictures strobing under the right lighting.

After four months of research, CAD, prototyping, and programming, [Harrison McIntyre] has built a 3D zoetrope that brings a gif to glorious physical life (video, embedded below). All the image pieces are printed and move under a fancy backlight that [Harrison] borrowed from work. It works essentially the same as a 2D zoetrope, as long as you get the spacing juuuuust right. 360° divided by 20 frames comes out to 18° per frame. So a motor spins the disk around, and every 18°, the light pulses for one millisecond and then turns off until the next frame is in position.

The really interesting thing is that there are actually more than 20 frames at play here. If you follow a single character through the loop, it takes 46 frames to complete the animation thanks to something 3D zoetrope pioneer [Kevin Holmes] dubbed ‘animation multiplexing‘, which in [Harrison]’s example, is easily explained as a relay race in which all runners run their section at the same time, creating the illusion of constant motion.

There’s more than one way to use a 3D printer to create a zoetrope, and we doubt we would have ever thought of this one that squashes four dimensions into three.

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Wiggling Screen And DLP Power This Volumetric POV Display

It seems like the world is ready for a true 3D display. We’ve seen them in sci-fi for decades now, with the ability to view a scene from any angle and inspect it up close. They’ve remained elusive, but that might just be changing thanks to this open-source persistence-of-vision volumetric display.

If the VVD, as it has been named by its creator [Madaeon], looks somewhat familiar, perhaps it’s because editor-in-chief [Mike Szczys] ran into it back in 2019 at Maker Faire Rome. It looks like it has progressed quite a bit since then, but the basic idea is still the same. A thin, flexible membrane, which is stretched across a frame, is attached to articulated arms. The membrane can move up and down rapidly, fast enough that a 1,000-fps high-speed camera is needed to see it move. That allows you to see the magic in action; a digital light processor (DLP) module projects slices of a 3D image onto the sheet, sending the correct image out for each vertical position of the membrane. Carefully coordinating the images creates the POV illusion of a solid image floating in space, which can be observed from any angle, requires no special glasses, and can even be viewed by groups.

With displays like this, we’re used to issuing the caveat that “it no doubt looks better in person”, but we have to say in the GIFs and videos included the VVD looks pretty darn good. We think this is a natural for inclusion in the 2021 Hackaday Prize, and we’re pleased to see that it made it to the semi-finals of the “Rethink Displays” round.