Building the Fastest Flip-Dot Display in the West

Display technology has evolved dramatically over the past several decades, but the goals have always been the same: to achieve the highest resolution, largest color palette, and fastest refresh rate possible and the lowest cost possible. Today, LCD screens are very inexpensive and offer incredible specifications. But decades ago we had flip-dot displays. Those were electromechanical devices that were useful for displaying short strings of text, and were quite popular for showing information like train departure times. Unfortunately, those usually take a couple of seconds to “refresh,” which is an eternity in the modern era. That’s why Pierre Muth set out to build the fastest flip-dot display in the West.

Flip-dot displays are made of a matrix of “pixels,” which are the dots. Those dots are actually small, physical disks that are painted a bright color on one side and a dark color (usually black) on the other. The dots are attached to low-friction pivot mounts so they can spin easily, with the pivot axis parallel to the surface of the display. Each dot is magnetized, so one half is North and one half is South. A magnetic core underneath each dot can be charged with a pulse of electricity to reverse its polarity, which causes the dot to swing around — effectively turning that pixel either on or off. The problem is that it takes a relatively large amount of current to reverse a core’s polarity and there are many dots that need to be flipped, which takes quite a long time. The purpose of Muth's project was to speed that process up as much as possible using readily-available technology.

Muth used 48 flip-dot displays from Hannio for this project, each of which has a single row of seven dots. Those were arranged to create a 24x14 matrix, for a total of 336 dots. Like a similar LED display, the idea was to drive these by wiring them as a matrix. Normally, that would mean that you need one pin on a microcontroller (or connected shift register) for every row and every column. In this case, that would be a total of 38 pins, which is far less than the 336 needed to drive each “pixel” individually. But there is an additional challenge with flip-dots: you don’t just turn them on or off. You have to flip them by reversing the polarity, which is significantly more difficult than simply setting pins to high or low in a matrix.

The solution that Muth came up with was inspired by H-bridges, which basically act like linked switches that flip the polarity going to a component of a circuit (often an electric motor). Muth took that idea and applied it to capacitors. By switching which side of the capacitor was grounded, he could control the side that the capacitor would discharge to. That discharge runs through a dot’s core to set its polarity. That required that each dot get its own capacitor, but those are cheap. Custom PCBs were fabricated to simplify the wiring. Control is handled by Microchip PIC16F microcontrollers. As you can see, Muth was able to achieve very speedy results. The dots can be flipped so fast that animations can even run smoothly at 30 FPS, which looks absolutely amazing on the high-contrast display.