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Need assistance with the electric motor aspect for a retractable projection screen. Answered

I want to build my own motorized movie screen and have just about everything thought out except for how to set up the drive system.  Mainly, I'm trying to figure out how to set up the electric motor system so that when you stop lowering the screen it stops at that point instead of drifting a little bit as electric motors tend to do.  Also, a good suggestion for the size of a motor would be helpful.  I don't know what the final weight of the screen will be, but it'll have some heft to it.  Appreciate any information on the subject or resources.  Thanks!

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steveastroukBest Answer (author)2012-07-17

The fastest stop you can get practically is to short the terminals of the motor, when it is turning off. Here's a circuit (with limit switches) that does that.

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rickharris (author)2012-07-16

Slow the motor down using a worm gear - The weight of the screen won't drive the motor that way

https://www.google.co.uk/search?q=worm+gear&hl=en&safe=images&prmd=imvnsb&tbm=isch&tbo=u&source=univ&sa=X&ei=Ux4EUPaPNcf80QX3ss2BBw&sqi=2&ved=0CG8QsAQ&biw=1920&bih=898

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RavensCraft (author)2012-07-15

Use a limit switch with the motor.
Adjust the position of the limit switch to compensate for motor drift.
The limit switch in the correct position would then shut off the motor before the screen is completely down and the motor would then drift just enough to
fully extend the screen, but not over extend it. You just have to play with the limit switch position until you find just the right spot to mount it.

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geckostudios (author)RavensCraft2012-07-15

That would be simple. I would just like that instant stop or understand how that occurs. I appreciate the suggestion. I might go that way as well.

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Re-design (author)2012-07-15

Pick up an electric window unit out of a large car or pickup. I've used them for similar things in the past and have gotten the for as little as$3 on ebay. They are geared so the weight of screen won't spin them. The work on 12 volts and may pull 5 or so amps under heavy load.

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geckostudios (author)Re-design2012-07-15

That's a really neat idea. I like that. I'll have to consider it.

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canucksgirl (author)2012-07-15

If you need something really heavy duty, how about a garage door motor? They are designed to start and stop at a specific point (IIRC, its determined at the motor).

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