sensing light and outputing voltage suitable for audio
If you just use a 555 timer you won't need to amplify it, you can plug it directly into a line level input (microphone in on computer, aux in on hifi etc). By putting the photocell on the bottom of the fan and a light source above the fan you could get some interesting results. The speed of the fan would create an oscillation, also the 'brightness' (pitch of harmonic content) of the oscillator could be adjusted by changing the intensity of the overhead light source. Amos is right, you could use a very similar circuit to the one in my Posc, I'll draw up an example just give me a mo....