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I'm glad you like it.

Yes, I'm sure it could be motorized to make it do both input an output, I agree, racks and pinions with three steppers is probably the most straightforward approach. It seems like there ought to be a less more surprising, lower part count way. Definitely worth thinking about.

I haven't worked out the details, but I'm envisioning one slider being advanced with a gear, like Dan's idea, but with a few missing teeth, allowing a spring to return it to the start point. Upon returning, this slider would advance the next using a ratcheting gear, and so on, down the chain.

Hi Dave --

Impractical it may be, but it's also pretty cool! I bet it would be a snap to motorize it, too -- just cut a rack into the back of each slide and use a stepper with a pinion gear to move them. Such a thing would have quite an identity crisis -- am I a display or am I an input? But that's fun too!

Wrote this up for the blog, should publish very soon. Great work, keep us posted on progress. Thanks!