slightly OT, need some electronic circuit help

[Michael Torrie]

I know many of you guys are electronics geniuses as well as linux gurus.
 I need a very simple circuit and despite searching the internet I
haven't found any real good recipes, except one rough schematic that I
will present here.

Basically I have a simple linear actuator (12 V) with a built-in
potentiometer for position feedback.  My needs are pretty simple.  I
just need to be able to control the actuator to attain 3 positions:
extended, middle, and retracted.  I only need rough resolution, so I
don't need a full PID circuit, I don't need the actuator to worry about
overshooting and homing in like a normal servo does.  I don't need PWM
to drive the motor.  I just need the thing to move and stop.

Here is a link to a simple circuit that does exactly what I need (a
switch and resisters can replace the POT):

http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU4406

However it does not specify what sort of op amps to use.  I am unsure
how to translate the schematic into a working circuit with real parts.
Also I don't know how to choose the resistor values.

Can anyone give me advice here?

thanks.

Michael