> 2" x 2" wooden length (or something lighter like
> PVC pipe), painted black and fluoro yellow on
> different sides. Use a cheap "toy" type dc motor

Nice touch using pipe and a cheapo motor. We thought
of something like it as a probable alternative to aluminium
strips and levers but like I say, nothing got made anyway.
The only concern I'd have is the torque of a very small (eg
slot car) motor, and whether it would be enough to get
something as large as pipe moving. Maybe it could be
an indirect drive, say a rubber clutch, like a CD drawer
mechanism, and a leaf switch. Two bumps on opposite
sides of the pipe (ie 180deg apart) would operate on the
same switch to tell the driver to stop

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