One thing to keep in mind is that silicon-based photo
cells work best at near infrared to red and not so well toward
blue and violet. The common photo cells such as what Radio Shack
sold are apt to be cadmium sulphide cells which are excellent
for mid-range visible light so they would probably be the best
choice for a general light detector.

	These cells are roughly the diameter of a US Penny or
Nickel and they have a clear plastic cap over them which can
sometimes fall off.

	The cells are a bit temperature sensitive so will not
always show the same resistance value for the same amount of
light but they are very good at telling you if there is light,
not so good if the light is red.

	I've seen some other cadmium sulphide cells that are
smaller. They look like a disk with two leads coming out of one
side.

	By the way, if you have any photo diodes lying around,
those won't work in this type of circuit because they never drop
to a low resistance due to the fact that they use the minority
charge carriers. They are good and fast for receiving data or
carriers over a beam of light but you must use a current
amplifier to read the few micro-amps of current drawn by a photo
diode.

Martin

Butch Bussen writes:
> I hope this isn't to many lines.
>       It is easily sensative enough to tell whether a led is lit
> or not.  We had to come up with a way to keep other light sources
> from shining on the photocell.  Some leds are quite close
> together and I need to know which one is on.  The solution was to
> use a piece of heatshrink tubing, preferably black.  It should be
> just the right diameter to slip over the photocell and about an
> inch long.