>> I've seen this done with a four-quadrant photodiode and an aperture
>> plate, and with four solar cells and a cruciform center baffle.

> You should only need three sensors.

Three may be conceptually OK if you are prepared to do calculations 
(digital or analog summing). But the simplest 4 sensor systems just do 
gross on/off control when cell pair exceed a certain level.

With a single axis tracker (or a 2 axis one with occasional manual 
control of the second axis) you need only two sensors with a dividing 
baffle.

You could use one sensor (tm) if you placed it on a vibrating/moving 
arm :-). Oscillate it either side of the array's pointing vector and 
note whether the energy insolation peak occurs when the arm is normal 
(ie at right angles) to the array's face. If not, adjust the array 
accordingly. The oscillatory movement COULD be arranged to be 
activated resonantly by wind and 90 degree position switching could 
use eg a reed switch or hall cell or a photo diode. It would be worth 
doing for the confusion and wonderment that it would cause informed 
observers :-). It would fail in absolutely windless conditions but 
could be designed to need only the merest smidgeon of breeze. In a 
complete calm you could "kick" it into motion with a slight burst of 
the panel motors - highly undesirable way to design a system but great 
fun.


        RM

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