The Differential Absolute Position Detector is a Geoff Bunza design. Geoff published this in his column in MRH as SMA23 and documentation can be found in the article. Geoff provides design information including Gerber files, so you can easily have your own boards made, but we keep bare boards in stock and sell them in quantities as small as one, where economic orders at the board vendors usually require the purchase of several boards.
"Every so often one needs an absolute position detector, rather than a block detector. The best I’ve used employ their own light source, but older designs use Cadmium Sulphide (CdS) photocells and depend on ambient light in the room. Both have driven me crazy with their limitations. This article describes a new design, using ambient light “seen” by the detector in two places – typically between the rails and just to the side of the track. When it notices the track light level fall below the level of the second sensor, it “indicates” the presence of an obstruction (car or loco). The beauty of this “differential” measurement is that it operates over a very wide range of light levels, does not require “aiming” to a target, needs no adjustments, and can be virtually buried in ground covering materials."
We're pretty impressed with this detector and use it in our own work!