BobH wrote:
....
As a wild guess, could those sensors be Cadmium Sulfide sensors? I
always thought that they would be too slow for this task, but I believe
that they are effectively photo-resistive and not polarity sensitive.
All of the CdS cells I have seen have a good sized window on them and
and a noticeable squiggly line dividing the sensor in half.

Ah, I can't see them - they're buried in an assembly with just a slit
for the wheel. Bob

--
Nota for President