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

[Joel Kirchartz]

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