chalkboard clock

A blackboard is mounted as usual. Affixed beside it is an articulated robot arm. A clock showing the current time is drawn on the blackboard in ordinary white chalk. Every minute the robot arm erases the minute hand and draws a new one.

This is basically like a plotter, but it should use a hinged radial arm rather than a beam arrangement. The arm has a chalk holder and also an eraser holder. Perhaps there should be two arms, one on each side of the board, one with the chalk and one with the eraser. A lot of the design success of this idea comes from the effectiveness of the eraser. Perhaps the eraser needs to be some kind of continuos roll of disposable tissue, or there should be an eraser cleaning subsystem mounted behind the blackboard.

I think that there should also be some kind of camera on a stalk, monitoring the board. That way, when people grab some of the (supplied) chalk and write graffiti or whatever on the board, the robot arm can, at the end of the current minute, erase the graffiti, and re-write any erased numbers. The hour hand would be erased and re-written every ten minutes or as needed.

The whole system should be as self-sustaining as possible, requiring only a supply of chalk (and possibly eraser cleaning fluid) and a power supply to run the electronics and mechanism.