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Ok, last year I tried to make a simple prop at the last minute that would raise and lower a skull from behind a bush on the side of my yard. A head popper, if you will, but with a very long "throw".

I tried to use a little window opener motor to turn a crank, but it just stalled out. :p

I thought this year I would use the motor to just wind a cord and use a pulley system, but I need a way to reverse the direction of the motor when it reached a certain length.

Does anyone know of a cheap switch that will do what I want? Maybe I need a catch on the rope at a certain point that hooks the switch. I've seen props like ghosts that follow a string and reverse direction at each side... (I think I even have one in my basement)
 

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A relay is a gizmo that lets you switch a large current with a smaller one - basically just an electrically controlled switch.



in this image, the relays are energised by connecting power to pin 85 and ground to pin 86. When the relay is energised there is a connection between pins 30 and 87, and when it's disengaged pins 30 and 87a are connected (these are common automotive relays.)

This circuit works because both relays "rest" at ground. When one relay is energised, power is sent to the motor through that relay while the other relay provides the connection to ground. When the other relay is energised the same thing happens, but the polarity is reversed and the motor turns the other way.
 

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An easy description of a relay switch:

Imagine those giant switches in Frankensteins laboratory (they're called knife switches), You have gazillion amps going through them to bring your monster to life. BUT, You probably wouldn't want to throw that switch with your bare hands since you'll end up as burnt toast. SOO you get an electro-magnet to throw the switch for you. Add a small amout of power to the magenet that will pull the big switch together to make contact for you. You never have to touch the high voltage or High amps. Let the magnet do it for you. That's a relay.
 

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I suspect if you used either a "double throw double pole" (DPDT) switch or relay, this would do what you want. You would run + and - power to the center poles, then have + and - at one pole then - and + at the other. That would give forward, stop, reverse for a DC motor.

What would really seem to be neat is a way to crank the skull down against a spring tension, then let it 'POP' all at once - like a jack in the box. Would take a bit more time to design something workable, but overall a neat effect.
 
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