Relays

[daliad100]

Your best bet is to buy relays.

http://www.technobotsonline.com/rela...dt-10a-5v.html A couple of those should do the trick, however, if you are worried about current draw then there are some larger relays avaliable.

[attachment=0:2tjxf4yn]Lifter.png[/attachment:2tjxf4yn]

If you remove the motor from a servo and wire where it was attached to the points on the diagram one relay will trigger when the servo should be rotating in one direction and the other when it should be rotating in the opposite direction due to the two diodes. Due to the wiring of the relays the lifter motor is off when the servo is in a neutral position and can be driven in both directions.

You can centre and glue the potentiometer in the servo/replace it with a pair of resistors to provide no feedback and have only directional control of the motor (fwd/off/bwd) or attach the potentiometer/a different potentiometer to the lifter arm in some way to make the motor act as a giant servo. I remember seing this done quite cleverly on a simulator chair without having to remove the potentiometer so it might be worth a look if I can fish out the link.