Bringing Flip-Dot Displays Into the 21st Century

We may often think of pixels as individually addressable units in an LED matrix or on an LCD display. However, while these are types of pixels, they are not the only examples to meet the definition. The fact of the matter is that pixels can be way more exotic than this. They don’t even need to be illuminated to produce a dynamic image.

Consider the flip-dot display, for instance. These mechanical displays physically flip dots — which have a colored side and a black side — to produce an image. With this type of display, one can get a very different effect than is possible with any type of modern digital display. But if you want to give it a try for yourself, you are likely to run into difficulties. Flip-dot displays are an old technology, and interfacing with them is not especially easy.

If you are running into trouble with your own flip-dot display, then Redditor GenerallyOkayTimes’s recent project might be of some help to you. GenerallyOkayTimes was gifted a flip-dot display and went through the work of figuring out how it works, and how it can be driven with modern hardware. That work has been written up for the benefit of other hardware hackers.

The particular board turned out to be a BUSE hybrid flip-dot and LED panel originally designed for transit signage. Flip-dot displays were popular on buses and trains because they remain perfectly readable in bright sunlight and, thanks to magnetic latching, consume no power once an image is set. Each dot is a tiny magnetic plate sitting above a coil; reversing current through the coil flips the plate to show either its light or dark face. Residual magnetism holds it in position afterward, making the display bi-stable.

This panel features a 28-column by 19-row grid — 532 individual dots in total. Instead of dedicating two wires per dot, the coils are arranged in a matrix. Each row shares a common line, and each column shares another, allowing a specific pixel to be addressed by energizing its row and column simultaneously. A multiplexer reduces the number of column connections further, and all signals are delivered through a 50-pin connector that also carries 24-volt power capable of supplying hundreds of milliamps — far beyond what a typical microcontroller GPIO can handle.

To safely drive the panel, GenerallyOkayTimes used a dedicated driver board called Flippity210. An STM32 microcontroller runs firmware that was slightly modified to improve refresh speed and enable serial control. GenerallyOkayTimes plans to pair the STM32 with an ESP8285 Wi-Fi module via I²C, though establishing reliable communication proved tricky when using the native ESP8266 RTOS SDK.

In the meantime, the board is being tested out in a series of MicroPython experiments. The display successfully ran Conway’s Game of Life and a clock, though the limited RAM of the ESP8285 made heavy HTTPS requests — such as fetching public transit schedules — impractical. Performance was also modest, with around five frames per second, prompting the builder to occasionally drive the display directly from a laptop.

If you come up with some great ideas for your own flip-dot display, be sure to let us know!