Yes you certainly can. What you want to do is set up a 12VDC relay that is turned on by the Digidoc when the temperature reaches its set point. A relay is basically a switch, but it is turned on or off based upon whether voltage is applied to it or not. Anyway, you would want to wire the relay so that when the digidoc is not applying any voltage to the fan (below your set temp) then the coil is open and the fans receive their power from your rheostat. But when the temperature reaches a certain point (or you force the fans to the ON position to the digidoc) then the relay is enabled, the coil is closed, and the fan receives 12V. This was discussed (well in a simpler form) at
http://www.commongrounds-pc.com/Arti...utobaybus.html