The head Mathematician at Maths World London has a desk toy made by Karl Lautman called Primer. This fun little device displays prime numbers on a mechanical counter similar to an old car odometer. As it moves you can see and hear it increment to the next prime. He wanted to replicate this device on a larger scale. The main challenge was to find a large mechanical display so we would have lots of noise and motion as it jumped from prime to prime. We settled on a flip dot display (because I found one on eBay) and so the next challenge was how to talk to this obsurce and 30 years old technology.
A flip dot display is a grid of dots which can flip over and are painted yellow (or sometimes white) on one side and matt black on the other. To flip them there is a electromagnet underneath each one and each dot is magnetised. So if you flip the polarity of the electromagnet underneath, you flip the dot. Throw in some electronics to manage all the electromagnets (which are just a coil of wire) and you can set the state of each dot to display a message or whatever you want.
These display were popular on buses before LED technology was as prevalent. Buses need a way to change the displayed destination but only very occassionally. So this type of display works well for clear high contrast letter than can be changed if needed. If you remove power from the display the message will remain, similar to a modern e-ink display.
Luckily these displays are popular nowadays for art exhibits and other installations, so I was able to find others online who had reverse engineered the elctronics. This helped use a microcontroller to start flipping dots. With this, some programming and a few additional electrical components the sign was alive!
To operate the sign there is a button below it which lights up when it is ready to move to the next prime. Press the button and the sign will increment to the next prime to highlight the random gaps between each prime. Once at a prime the computer inside will calculate the next one by checking each number until it reaches another prime.
The sign can display 10 digits. The largest 10 digit prime is 9,999,999,967 which is the 455,052,511th prime (the 455,052,512th prime is 10,000,000,019). If you assume about 5 seconds per prime - it will take just over 72 years to hit the 10 digit limit.