 
Phototransistor-> X100 -> X100 -> NE567
with 330Kohm              Limiter 
feedback to base                  
to control operating point (I expect this really hurts Noise Figure! Any
comments from Phototransistor experts?)

We intended to use this atop a stepper motor, to provide headings to the
beacons. The plan was to have 3 or 4 beacons, of different frequencies, in 
each room, and tell the 567 what frequency to phase-lock to. We did not do so.
One challenge was that the available stepper had 8 degree steps!
Knowing the headings to each beacon, we would have used the surveyor 
technique of RESECTION to determine robot location. (No, I ain't going to
explain resection over the net.)