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)?

We can use two polarity reversors to create a three way switch.  Set two
polarity reversors side by side, linked by an axle.  Set it up so that when one
reversor is on (no matter the polarity), the other is off.

If we drive 9V into both switches, and we rotate the reversor we get this
sequence:

 S1  S2
  +   0
  0   -
  -   0
  0   +

This looks very much like a three way switch.

If we then use a second pair of polarity reversors ganged together into another
three way switch we can combine them together:


9V---->S1--------->S3-----+ 
  +--->S2--------->S4-----+---Motor

S1 S2 S3 S4
 +  0  0  - Motor off

 0  -  0  - Motor forward
 0  -  -  0 Motor off

 -  0  -  0 Motor forward
 -  0  0  + Motor off

 0  +  0  + Motor forward
 0  +  +  0 Motor off

 +  0  +  0 Motor forward
 +  0  0  - Motor off

This looks like it works great.  It solves the motor reversal issue, the
non-LEGO issue, and the dead zone issue, as long as the switches start in the
correct orientation with respect to each other.

Kev