Re: A LEGO double-throw switch?

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)?
>
>  
>
Yes

I'm sorry for not drawing it.. but here goes
Imagine you have a cable with ground and a cable with 9V
You need two single pole, double throw switches (I don't know what kind 
lego switches are)
For both switches, connect one throw to 9V and the other throw to ground.
Connect the load across the switches' poles
That way, each time you switch one of them, you change whether there's a 
potential difference (I don't know how to say it in English):
Switch 1    Switch 2    Voltage across load
9V               9V            0V
0V               9V            9V
9V               0V            -9V
0V               0V            0V

You could say it's like a XOR gate