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Propellor clocks are “just a matter of engineering” – ie the principles are well understood, “you just have to do it”. The gap between knowing how and doing can be large 🙂 – but following some of the examples below will make it easier.

Basic process

  • Desired display information is converted to a dot mapped display format.
  • A rotor is spun horizontally or vertically.
  • A line of display dots (usually LEDs) are arranged in a line.
  • Spin speed is known or calculated.
  • Starting position per rotation is determined (sensor of some sort usually)
  • Lines of dot data are output at a rate based on rotational speed and desired display length.
  • All the rest is “engineering” 🙂

Getting power “across the gap” may be done with a motor winding used as a power pickup (Bob Blick), induction between two coils, solar panel, brushes and slip rings, separate alternator (perhaps with a bob-weight positioned “stator” **), or …

Information transfer can be by having it on the rotor already (clock etc), short range RF, optical, contacts (less desirable), capacitive, … .

For external feed of dot data per rotation (as asked about) an eg 10 x (5×7) dot display = 350 dots at say 30 Hz rate x say 1/3 of an arc illuminated the data transfer rate = 350 dots x 30 Hz x 3 ~= 32 kbps. A more complex dot mapped display may need data rates of up to about 100 kbps. Such rates are certainly achievable but potentially ‘annoying’ to implement. The low cost of microcontrollers means that even if most processing is done external to the rotor, data speed can be much reduced by only feeding a “frame buffer” (one display of information) as required. A second buffer could be built while the current one is displaying. It may be that an acceptable compromise would be partitioning the task so that the rotor processor had all display data and implemented effects such as scrolling, flashing etc while the remote processor looked after data acquisition and management.


One of the best known DIY versions, for which full construction details are available is “Bob Blick’s propellor Clock”. This was based on earlier versions by other people and in turn many people have adapted Bob’s design.

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