obtical movement sensor (general)

[noskule]

Hi List

I did ask yesterday the question about the trackball . .this is working so far. Now i thought it would be (relativly) simple to build a obtical movement sensor to connect like a  digital pot with this part http://www2.snm-hgkz.ch/~b/0108.gif. So far i understand the principle of digital pots i need 2 of those witch have to be shiftly placed for detection of direction of movement.

Then i need a comparator like thishttp://focus.ti.com/lit/ds/symlink/lp339.pdf

so my question:

• is this generaly right ?
  
• what size must the distance between the sensors have?
  
• witch comparator i have to take ?
  
• is there somthing what i forgot ?
  

thanx nos

[noskule]

. . .. i have one more question . ..

on the sensor : one the |<- is the led ore diode ..  .the other one is the sensor . . .is this  a variable resistor .. . ?

what dos the comparator exactly: define the point of  change betwenn 0 and 5V on the output of the comparator?

. . .. ..

do i need other parts to make the circuit complet?

.. ... .