30 FPS Flip-Dot Display Uses Cool Capacitor Trick

Most people find two problems when it comes to flip-dot displays: where to buy them and how to drive them. If you’re [Pierre Muth] you level up and add the challenge of driving them fast enough to rival non-mechanical displays like LCDs. It was a success, resulting in a novel and fast way of controlling flip-dot displays.

Gorgeous stackup of the completed display. [Pierre] says soldering the 2500 components kept him sane during lockdown.
If you’re lucky, you can get a used flip-dot panel decommissioned from an old bus destination panel, or perhaps the arrivals/departures board at a train station. But it is possible to buy brand new 1×7 pixel strips which is what [Pierre] has done. These come without any kind of driving hardware; just the magnetized dots with coils that can be energized to change the state.

The problem comes in needing to reverse the polarity of the coil to achieve both set and unset states. Here [Pierre] has a very interesting idea: instead of working out a way to change the connections of the coils between source and sink, he’s using a capacitor on one side that can be driven high or low to flip the dot.

Using this technique, charging the capacitor will give enough kick to flip the dot on the display. The same will happen when discharged (flipping the dot back), with the added benefit of not using additional power since the capacitor is already charged from setting the pixel. A circuit board was designed with CMOS to control each capacitor. A PCB is mounted to the back of a 7-pixel strip, creating modules that are formed into a larger display using SPI to cascade data from one to the next. The result, as you can see after the break, does a fantastic job of playing Bad Apple on the 24×14 matrix. If you have visions of one of these on your own desk, the design files and source code are available. Buying the pixels for a display this size is surprisingly affordable at about 100 €.

We’re a bit jealous of all the fun displays [Pierre] has been working on. He previously built a 384 neon bulb display that he was showing off last Autumn.

35 thoughts on “30 FPS Flip-Dot Display Uses Cool Capacitor Trick

  1. I wouldn’t say novel – the company I worked for was doing this over 30 years ago with flip dots and flip segments and successfully installed a large flip dot scoreboard at Lords cricket ground as well as many clocks and displays using th e7 segment versions (the biggest I personnally produced software for was a conveyor load cell display in Wales that had flip digits around 36 inches tall).

    1. Interesting – do you remember what kind of capacitors were used? Nowadays it is trivial to throw 10µF ceramic capacitor on every dot. But I guess back then a huge array of electrolytic caps would be the most affordable solution?

      1. It was an array of electrolytics – the station clocks were not allowed to show the wrong time (the time source was a Patek-Phillipe atomic clock) so in the event of an error, the capacitor bank was fired on each driver board to quickly clear all segments. The conveyor load cell display needed *very* large caps just to shift the segments, I can’t say they actually accelerated the digit change so much as facilitated it – I never actually got to see it installed (no camera phones then) but the construction was by all accounts impressive, with the display on top of a building, though the equipment I coded also sent the data serially to some other system that I was not privy to, beyond the communication protocol.

          1. The one I posted the picture of isn’t ours, I got a bit over eager when I was searching. I assumed that ours was removed when the big new one on the conference stand went in but since there is a smaller dot display that evidently post dates the new display, it is possible that it wasn’t taken out straight away. Sadly I can’t find any pictures of the ground that include the area it was installled. The Lords website only shows the swish and expensive display the rendered ours obsolete (though ours never obsoleted the ancient mechanical display that it was mounted opposite :~D)

    2. I’m sure all the possible efficient ways of driving a flipdot display are well known by the commercial manufacturers. That doesn’t mean it’s not new in the hobbyist community.

      1. I used the same technique for driving bistable relays in my HAM antenna tuner. That is basically the same thing – a coil that needs to be powered both ways to switch sides.

        DRV_signal — || — mmmmm — COMMON

    1. I would say that you should feel bad for not knowing that it’s playing a fan made music video of a remix of the song Bad Apple!! from Touhou Project, which has been used to demonstrate the ability to play it on a variety of systems previously thought to be impossible or impractical to play it on. It’s true that the song isn’t playing, it would still cost royalties to include it.

      1. I doubt if written permission and/or licensing is available at all, as lots of Japanese artists don’t like hard currencies getting involved.

        Usually their implied/expressed terms are akin to BY-NC-SA or NC-ND or just (C) depending on circumstances, and in order of emphasis NC > ND > BY.

    1. No way to flip the dots partway. Now if there were 4 sided flipcube displays, that would make it possible to have black and 3 shades of a color per pixel, with the cubes pivoted on one axis.

      Figure out a way to magnetically spin a cube in any direction and encase the cubes inside spheres with a clear front, perhaps with a very low viscosity oil to suspend and lube them so the inside of the spheres don’t get scratched.

  2. This is a simple half-bridge driver. It is used in a lot of things, like audio amplifiers, power supplies, and even Tesla coils.

    Nice build and a clever use nevertheless, but please call things by their name, instead of reinventing the wheel over and over again for every application.

    You wouldn’t call a shift register a ‘cool flip-flop trick’.

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