What's that Lassie? You say that Bill Noble fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sat, 6 Dec 2008 22:18:09 -0800:

it is probably a photocell - a long time ago (inthe 60s) there was an
article in popular electronics about building a robot that would follow a
line - it used a single photo cell and a light bulb, no lenses. it would
follow one side of a piece of tape - you set a threshold (it had one
transistor, a CK722, and one sensitive and expensive relay) - when the
voltage out of the photocell was greater than some amount, the relay
energized and it drove one motor, making the thing turn left, when it was
less than the threshold, the relay dropped out and that would energize the
other motor making the thing turn right.

with the addition of a lens and maybe one more transistor, this could be of
use, no?

Actually I get how you could use a photocell to sense a line. And I
get how to use that to steer something. I could build a toy car that
would follow a line. No problem.

But I don't know how to get an X Y table to move when it needs to
change directions of one or both of the axes.

On the tracer I saw, the 'eye' rotated to follow the line. The X and
Y drives were controlled by the direction that the 'eye' was pointing.
That's the part I'm not sure of. How to control two axis, forward and
backward as needed, to move in the direction that the eye is pointed.

--

Dan H.