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.
>    Oh, did I mention I wanted to do this with just polarity switches, no RCX or
> other "intelligent" systems in the loop?
>    The picture I've got in my mind is something like a light with two wall
> switches - change the state of either switch, it changes the state of the light.
> I'm thinking I can do it with a looong string from the terminal end back to a
> polarity switch at the head of the line, but it seems... clunky. Is there any
> electrical solution? Can I reproduce my household wiring in LEGO (I promise I'll
> keep the voltage... a *little* lower)?

Hi Brian,
  How will the package arriving at the other end "throw a switch?".  Is there
enough friction between the package and the belt?  Will there be a mechanism in
the belt itself to "go halfway and stop"?

  If you only used one polarity switch, and had both ends control it, I could
see how this might be done.  The forklift rotates the switch on, by turning the
switch a quarter turn.  When the package goes to the other end, a special link
in the conveyor belt rotates the switch the opposite direction from the forklift
to turn itself off.

  The conveyor would have two special links that turn the conveyor off.  The
package would just go for the ride, playing no role in actually switching the
conveyor off.  

  1. The initial conditions make sure the conveyor belt is such that it appears
to just have turned the conveyor motor off. 

  2. The forklift drops the package on the belt, and turns the motor on.

  3. The conveyor goes until it turns itself off.  The package happens to be at
the other end.

  Does this make sense?

Kevin