Yesterday Eric Bergstrom asked for ideas for his project of automating
his rewind/shut-off device.  Rather than use a solenoid I would use a
automatic door lock unit.  You can find them for less than $10 and they
provide uniform power from start to end of the stroke.

This one is available for four dollars and gives a 7/8 inch throw:
   http://www.aapexonline.com/universal-power-door-lock-actuator-motor-12-volt-8-8lb-torq-7-8.html 

 [ Here's a shortened version of the url:  http://tinyurl.com/cqq3b5k 
 [ --Jody

If I were to design a unit, I would use a cheap PIC processor and a
ULN2803 to drive the play/rewind actuator motor.  The CPU could even
provide the timing for turning the Trac on and off for the vacuum speed
control.  Put two buttons on the box to increase or decrease the motor
speed during play.  If the unit thinks it is rewinding, then the same
two buttons control the rewind speed.

These speed values could be saved in the CPU memory rather than using
two potentiometers.  Your shut-off indicator could use one of those
piezo-slap sensors mounted to the back of the top roll area.  When the
mylar on these units is moved quickly, they give out a voltage that
would go into the CPU and turn it off.  To make sure the system is
safe, I would use a relay that would latch on, when started, and that
would completely remove power following the rewind cycle, even the 12v
power source.

Pete Knobloch
Tempe, AZ