DC motor size and automatic stop

[doyle928]

First of all I don’t have much experience in fabrication and circuits.  I can wire leds and resistors no problem, but that’s about it.

I’m trying to put together a mechanism that will slide an object forward, stop and turn off, then reverse, stop and turn off.  The plan is to use a DC motor and a DPDT switch, the problems I am having is the mechanism will be out of my view (can’t tell when it is in the desired position) and what size of motor needed for this.

The motor will be powered by 12v (car battery) and will only need to push/pull max 3lbs.  (I would like to go as cheap as possible)

What would be the simplest way to turn off the motor once it reaches the desired position? 

I can only think of using microswitches at each ends that will turn on leds, then manually turn the switch off… is there a way to automatically turn the motor off?

Thank you in advance and I apologize if me trying to explain this is confusing.

[Windell Oskay]

One type of mechanism that you might consider is a solid state relay, one is rated to handle 12 V DC at sufficient current as your motor draws.    

The input to control the solid state relay is usually (essentially) an LED, so if you can control the internal LED, you can control the motor, too.

[Iron4ge]

There are other questions here…you want the thing to move, stop, reverse, stop…but do you need it to STOP and turn off?  or just reverse direction?  You could achieve a back and forth motion with a rack and pinion assembly that wouldn’t require all the electronics.  The mechanical assembly could even include a timed pause before the return action…and might be more up your alley for design work.  But if you need the electronic control, the relay is definitely the way to go.  As for the motor, shop the local flea market for a junker 12V power drill and rip the motor out.  That should get you pretty close to your power needs.  Even better would be if you could find a 12V cordless skillsaw.  Usually, you can buy them at really low prices from flea markets, not so much from yard sales.

Another way to go would be to use a stepper motor, but the motor will be more expensive, the wiring more complex.  The payoff is that the motor can tell your electronics exactly how far it moved, and you could build a control circuit to stop the motor whenever you want.

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