[sdiy] Schematics for Light Activated Switches? (PhotoDiode / PhotoTransistor Question)

[harrybissell]

OK I guess I'll go for that...

Phototransistor is a transistor where light provides the base current.

If you run the transistor with a grounded emitter (collector load) its
inverting
and has gain just like the normal transistor.

Some have an actual base lead, which lets you add a resistor to ground, you can

trade gain (lower) for speed (faster).   Those photons that reach the base have
no
path to leave except through the base-emitter junction, or through the
resistor.

Some packages are clear, some have black plastic that limits the range to near
IR
(almost all of them will go only slightly into the non-visible IR range).

Photodiodes can be run two ways...

Photovoltaic mode (they produce electricity... about 0.7V per junction) and are

pretty slow.

Reverse bias mode... you run them into a current to voltage converter (an
opamp)...
they have a very wide range and very good speed... at the expense of extra
circuitry.

Most applications suffer from ambient light. You can modulate the source and
decode
the AC output from the photodiode/transistor to get better performance...  or
carefully
shield the cell.

I was able to get about a two foot gap with a single narrow beam LED and
phototransistor combo.   You could illuminate with laser diodes and do much
better.

If you want a voltage proportional to illumination, use the reverse biased
photodiode!

H^) harry

Joel Kirchartz wrote:

> I'm trying to build a light-activated "keyboard" of sorts  somewhat akin to
> the http://www.oddmusic.com/gallery/om21300.html LightHarp project.
>
> The problem is I've never worked with PhotoDiodes or PhotoTransistors can
> anybody give me some insight into what each of these components do?
>
> any help is greatly appreciated.
>
> ~Kirch