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I've been trying to recreate the Useless Machine found here: http://www.youtube.com/watch?v=QdKtgfmfA7s&feature=c4-overview&playnext=1&list=TLp4R5S7LPESk

In the description, there's a link to a brickshelf gallery that has detailed pictures of the insides of the machine: http://www.brickshelf.com/cgi-bin/gallery.cgi?f=416984

I think I've figured out most of the mechanism, but can't for the life of me figure out what motor is being used, and how the motor stops running once the arm is sufficiently retracted. There doesn't seem to be another switch hiding at the bottom... Those two things have just been bugging me for a while.

First post here, I'm only just getting into LEGO, thanks for the help!

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The motor appears to be a very standard 9V motor, presumably using a conversion cable as the switch is a PF one.

As for how it stops, it's not that complicated: when activated,the mechanism pushes the switch back to its "off" position, which is what makes the motor stop. You'll probably think the motor direction is reversed and that's what makes the arm go back in the box, but that's not the case: the motor is actually off and what makes the arm go back is simply its own weight.

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It did turn out to be a 9V motor, thanks! I've now built the structure itself (everything in this image: brickshelf.com/cgi-bin/gallery.cgi?i=4299452 ), and now I'm not too sure about whether or not the arm's weight is enough to make it close itself, since even after trying to loosen it up a bit, it definitely doesn't seem to be able to close itself. The arm reaches out and shuts the machine down just fine- it just doesn't retract. I'd rather not add weights to it, or anything like that, and now I'm curious as to how he got it to work... –  Iroh Jul 17 '13 at 18:09
    
Note that there are actually two versions of that motor (71427 until 2002, then 43362). Not sure if it would make a difference, but since the mechanism has to make the motor rotate and the newer one (43362) has more internal friction, maybe it would work better with the older version. –  Joubarc Jul 17 '13 at 18:52

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