Flip-dot clock control board

Controlling flip-dot clock modules by AC - now you don't need complex polarity reversal circuits

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At the beginning of the project, we had 6 flip-dot blocks from a 50-year-old clock that had previously been installed at the post office. The control unit has not been preserved. The goal of the project is to learn how to manage these blocks and re-create a clock based on them.
Controlling a flip-dot pixel usually requires changing the polarity of the input voltage. In order to change the polarity over tens of pixels, a complex circuit is needed. We tried to simplify this process by using alternating current. Turning on the triacs at the moment of passage of the alternating current half-wave of the desired polarity allows us to switch the blinkers in the forward and reverse directions.

Flip-dot digital block looks like this:

The blocks has a size about 200x120mm and weight 1.7 kg

Idea of the circuit

The image is formed by blinker dots.. Each blinker is a plastic curtain controlled by an electromagnetic coil. One side of the curtain is black, the other is painted with reflective paint. When voltage is applied to the coil, the curtain opens or closes, depending on the polarity. After the tension is removed, the curtain maintains its position for months and even years. Each module has 35 blinkers, forming a 5x7 pixel image. The blinker switching diagram is shown below (for simplicity, there are only 4 coils out of 35 in the figure).

In order not to develop a complex bridge circuit for each coil to change the polarity, we decided to use alternating current. The idea of control is shown on the diagram below. By turning on the desired blinker, we switch optotriacs U3-U6, and the polarity of the signal is selected by one of the opto-relays U1-U2, connected through diodes D3-D4. All that remains is to apply alternating current to the block for one period and a half-wave of the required polarity will switch the selected blinkers:

The circuit turns out to be quite large - 35 optotriacs - but at the same time simple and cheap. The triacs are controlled through a cascade of five shift registers - one for each vertical column of blinkers. The project uses 6 flip-dot digits - 4 for displaying the clock and two for the date, as was in the original. The selection of the desired digit for updating is carried out by relay K1 on each digit, switched by mosfets. The main controller of the project is Arduino Nano board.


We have developed two types of PCB (drawn in Kikad and manufactured by PcbWave). The right board is for triacs and shift registers (14 and 2, respectively). We need three such boards. The left board is a control board for Arduino, polarity relays and connectors.

The code

The project controlled by Arduino Nano board. The code can be dowloaded from project's Github page:

The code use Arduino library for the DS3231 real-time clock (RTC) by Andrew Wickert.

Sample videos

  • 6 × Flip-dot digit module
  • 2 × Main board PCB
  • 3 × Blinker keys PCB
  • 1 × Arduino Nano
  • 35 × MOC3023 optotriac

View all 12 components

  • 1

    The control unit is assembled from five boards - three blinker key boards at the bottom and two control boards at the top, installed one above the other like a sandwich:

  • 2
    Blinker keys boards

    The purpose of the board is to switch of individual blinkers. Each board contains 14 optotriacs controlled by two shift registers. The shift registers of the three boards are chained, allowing up to 42 outputs to be controlled. Since the number of flip-dots on each digit block is only 35; seven outputs on the bottom board are not used.

  • 3
    Main board

    The main board is a heard of the control module. It contains a Arduino Nano board and two polarity switches. Most of the panel is occupied by connectors linked it to key boards and blinker blocks

View all 4 instructions

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