This is the circuit I was going for originally...the LED is just an indicator to show that it's working and aligned:
I found a stereo option, but with a simpler circuit, just a simple pair of phototransistors in parallel connected to each side of a stereo jack. I think a combination of the two is probably best...I can do without the LED, but I think the capacitor will improve the graph slightly (and protect the transistors somewhat). But I would need to get a USB sound interface with a stereo line in. I'm seriously regretting selling my oscilloscope!
I think a Schmitt trigger would help out the graph no end. It would iron out the various rise/fall times into clean voltage switches at fixed times. But then it might iron out shutter bounce... Probably I'm overthinking it at this point.
Originally posted by cameracravings
Your laptop should have input for a stereo microphone.
It might well do, I just assumed the mic input was mono. I shall investigate.