Giving a bot some rudimentary
Light sensors are one of the most commonly used sensors
on BEAMbots, normally to enable a 'bot to be phototropic
junctions are light sensitive; photodiodes are just
junctions that are designed to optimize this effect.
Photodiodes can be used two ways -- in a photovoltaic (here
it becomes a current source when illuminated -- see
solar cell), or photoconductive
To use a photodiode in its photoconductive mode, the
photodiode is reverse-biased;
the photodiode will then allow a current to flow when it is
'Most any photodiode will do for BEAMbots; I'd suggest
you select one based on what has a convenient "field of
view" (some are sensitive to light from such a broad field
that you'll need to build them "blinders" so they'll only
respond to light from a given range of directions.
has an interesting site on the technology behind photodiodes
are light-sensitive. As this is normally an undesirable
trait, most transistors
spend their working lives in opaque packages.
Phototransistors, on the other hand, are designed
specifically to take advantage of this inherent light
sensitivity. The most-common variant is an NPN
transistor with an exposed base
region. Here, light striking the base
replaces what would ordinarily be voltage applied to the
-- so, a phototransistor amplifies variations in the light
striking it. Note that phototransistors may or may not have
lead (if they do, the base
lead allows you to bias the phototransistor's light
Photoresistors, as their name suggests, are resistors
whose resistance is a function of the amount of light
falling on them. Their resistance is very high when no light
is present (up to millions of Ohms), and significantly lower
when they are illuminated (hundreds of Ohms). These are also
often called Light-dependent Resistors (LDRs) and
Cadmium-Sulfide (CDS) cells.
Light-Emitting Diodes (LEDs)
A tutorial on using LEDs is available here.
The things you'll need to consider are current drain
(general rule: small is better than big), and LED type
(regular vs. infrared vs. flashing). LEDs can be used as
photodiodes (tho' their
sensitivity is relatively low, so they're only useable this
way in very bright conditions).
For more information...
There's a very detailed page on building an
electrometer (E-field detector) with a single FET