I stumbled on a retro Dutch video from 1983 where a large four color mechanical display is shown. Each pixel consists of a cube which can be turned in order to change the pixel color between 4 colors (white, green, red and blue), a carriage moves behind the screen and slowly updates the screen by altering the rotation of the cubes. Ah this retro electro-mechanical stuff, I love it.

My question is: how does the update carriage know the rotational position for each of the pixels?

Or is the rotational state not obtained but reset in some way? The pixels have small cams on their sides, but they seem similar for each color, I can't really see it properly. When the carriage passes it performs only one color-shift per line (90 degrees), and each pixel is always cycled during an update, even for cases where there is no difference between the initial color and target color of the pixel.

Other details: The displays seems to be branded "AVTEL". And seems to consists of 3 smaller displays next to each other.

Screenshot of the video showing the pixels Screenshot of "AVTEL" four-color mechanical display

  • \$\begingroup\$ how does the update carriage know the rotational position? cannot be answered by anyone that is not familiar with the device .... what are some of the ways that the update carriage would know the rotational position? can be answered \$\endgroup\$
    – jsotola
    Mar 29, 2021 at 21:49
  • \$\begingroup\$ @jsotola, obviously its 90 degrees. I've changed it. \$\endgroup\$
    – BuysDB
    Mar 29, 2021 at 21:51

2 Answers 2


There are many ways it could be done. Knowing how it was done requires prior knowledge or some who is clairvoyant with period engineering tools.

Most likely there was an array of stepper motor driven actuators like those used on 8” floppy drives each with a home position optical sensor also used in 8” floppy drives for the Track 0 and sector 0 optical sensing.

The heavy duty chain would pull the array as the computer sends the required pixels with several rows at a time in advance of clearing the position much like a inkjet printer does many rows of pixels at a time from one scanning head. except this was a full line scan of several rows of pixels that had to be gently turned to the next rotational position counted by the next display change in pixel rotations.

So 007 would appear after a minute of mechanical flips energized by some magnetic motor for the number of rotations required spinning wildly in a clicketty-clack of changes.

Game shows used bicolour similar but different pixel flip displays to indicate the words to be guessed.


If you have 3 (or maybe 2) solenoids or electromagnets per horizontal pixel you could use some passive method (lever, permanent magnet etc) to flip the cubes one row at a time with a mechanical stop at WHITE.

Then as the bar continues to move vertically, the stop is mechanically removed and the solenoids control the flipping of the cubes to the required colors one row at a time. To allow use of 2 solenoids you'd need a small mechanism per line pixel but it might be worth it. Electronics like that was expensive in 1983.

  • \$\begingroup\$ Could also be time coded, e.g. a low-mass mechanism advances a ratchet until a stop; if the solenoid is held for longer, a high-mass mechanism gets moving and resets the rotor to home position. Not sure how reliable such a mechanism is actually, or if there are practical examples of it, but (since this question is open to speculation) it would be an interesting design, and electronically efficient. Could possibly also be row/col/full panel reset (with a much larger solenoid, or maybe a geared motor and crank). \$\endgroup\$ Sep 5, 2022 at 3:13
  • 1
    \$\begingroup\$ @TimWilliams From the video there appears to be a reset that occurs 90° at a time, ending in white and taking at most 3 scan lines to complete. \$\endgroup\$ Sep 5, 2022 at 3:27

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