The first ever flap for a split flap display I made was cut from a blank access card. I’ve still got it on a shelf here. The sound wasn’t great (more on that later), but it got me to the first idea of what a split flap display, one that can be built at home, might sound like.
I was once working for a manager who insisted that good decisions can only ever be those that are informed. Boring, pedantic, but true. So my first quest was to spend some money on purchasing an “O.G.” split flap display, and I got one from Bernd. I do not know where Bernd has those units from, but they’re authentic and in a good overall condition. I bought three – two to keep, one to slaughter.
The units arrived and they’re massive – at least in terms of weight, and also in terms of build quality. Something that’s built to last. And the flaps? Delicate. Super thin. Flexible. The polar opposite of a blank access card really. I’m not a material scientist – or a scientist, really – but my guess is that the flaps are made from PVC, with tiny holes cut or punched to have them mounted on the central drum.
Since I didn’t feel like going down the rabbit hole of driving the motor that they come with, I opted for the cheaper option: just using a screwdriver to actuate the drum. There’s a receptable for a hex driver on the side to do just that.
And if you imagine it was just me sitting there, with a screwdriver playing a human version of a motor and being entirely content with where I ended up in life: Spot on. That was a very wholesome moment.
What was next? Flex. Learning that a significant detail in what makes them work so reliably is not only the precise geometrics of where the drum aligns with the viewing window – that’s of course important. It’s that there’s a tiny amount of flex in the flaps just before they “turn over”, and that does two things: It ensures that they are always in the same position when a letter is viewed, but that also the “turn over” is happening both at the same position, and does so independent of the momentum of the drum. To illustrate that: In a display that purely depends on gravity for the flaps to fall down, you need something to provide at least some momentum, otherwise your flaps might be stuck in the wrong position. Usually, that momentum is provided by the drum moving, but especially for single character changes, that is just not a lot of momentum. And there’s a certain amount of friction that always needs to be overcome.
The magic solution here is to have flaps that can bend a tiny bit, and to rely on that in the overall geometry to have them flex, deform, just a bit before turning over. That way, there’s no outside momentum required any more, as the energy in the flex itself will ensure that the flap will turn over once it’s mechanical possible for this to happen. Magic – and so unexpected.
I made a video in an attempt to try and capture exactly that bend, that moment – and I’m sure you can spot it in the last clip.
Sound
One of the underrated features of split flap displays is that you heard when information changed: the characteristic rattle was a sure indicator that checking the departures board again might be a good idea.
There’s three things that influence the sound a display makes: First the sound generated between a flap and the case or guide inside the display. There’s always some friction, but ideally, this component shouldn’t be noticeable – unless you get something wrong (speaking from experience here).
The second is the sound that’s emitted from flaps interfacing other flaps. The significant moment here is when a flap falls down and the flaps that are already on the lower side basically brake the fall. This is creating the characteristic sound.
The third is the motor and mechanical system. I can’t judge this for my OMEGA-Module, but my assumption is that it wasn’t crazy loud – confirmed by standing beneath some operational displays and never noticing a motor sound at all.
Since the dominant sound is from the flaps interfacing each other when turning over, it’s only natural that the sound gets softer and less aggressive if you make the flaps lighter. After quite some experimentation, I ended up with using 0.4mm PVC as the carrier material, with the letters being screen printed.
And the good news? They sound great, and they also have very agreeable mechanical properties for the dynamic flex that is just so important for the whole module to work reliably.
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