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)?

Brian -


Since I am a spark head...let me just say....this would be interesting.

Presumably, you are moving a polarity switch from one "on" position to the next
when you hit it...because going to an "off" position would disable the system.

Basically, the polarity switches do not work like your standard "two light
switch" (properly called a "3-way switch") system.  The three way switches on
your wall are always "on"  (there is no off position and no "reverse" position).
Basically, either both switches are closing the same circuit or they aren't
(hence the light is on or off).

I think you could duplicate this (kind of) by rotating a polarity switch 180
degrees on a hit from Gus or a crate...you would obviously pass through the
reverse polarity on the way around, so you would have to be tolerant of
momentary reversing of the conveyor belt...and, I don't know how it would shut
off since the polarity switch is not a 3 wire connection...this probably gets
complicated (though I did like Kevin's single switch suggestion with Steve's
mods).

A better solution (and probably cheaper) might be to do something ala John
Barnes...use CAMs to mechanically actuate the switches on the top of a battery
box...Gus's forks could do the trick before he backs away.  Gus could rotate the
cam into the "on" position (pressing the switch) and the crate hitting the end
of the conveyor could rotate the cam another half turn to the off position...or
you could gear something that would simply move the cam to off after so much
running time (like Steve's startup timer on Reactor II).

Bryan