Re: A LEGO double-throw switch?

In lugnet.robotics, Brian Davis wrote:
>    So I've got an application where a forklift loads a crate onto a conveyor
> belt. I could then start the conveyor belt up by having the forklift throw a
> polarity switch. What I would *like* is that when the crate gets to the other
> end, it somehow stops the conveyor motor again, (here's the tricky part),
> leaving the system in a state where the next forklift delivery can re-trigger
> the whole system from the "front end", as it were.

The following design will turn a polarity switch into a two-way splitter.
<http://www.brickshelf.com/cgi-bin/gallery.cgi?f=214>
That is, the switch controls whether the input will go to the left output or the
right.

Use two of these, one at each ends of the conveyor belt, to make a three-way
switch.

In total, it takes two polarity switches, plus a bunch of connector wires.

Cheers,
Hao-yang Wang