I had the microfiche machine at Maker Faire. And I said that I would post some information about it here, so that’s what I’ll do.

The microfiche machine takes light (in this case, from the screen of the microfiche reader) and converts it into MIDI signals that can then be sent to devices that take MIDI input, like synthesizers or computers. We use a phototransistor on the end of an antenna to read the light level from the screen. That information is then processed by a BS2 and converted into a MIDI signal.

In addition to the phototransistor, the microfiche machine has several other inputs.

• two dials and a button to set the highest and lowest note played
• a stop/star button that stops and starts the MIDI signals without actually turning the machine off
• two buttons that are used to calibrate the highest and lowest light signals
• a dial that is used to set the key of the notes that will be played

The BS2 does not have an analog to digital converter, but it does have a function called RCTIME that can be used to detect changes in resistance by determining the time it takes for an RC (resistor-capacitor) circuit to change its voltage. This article talks about how to use the RCTIME fuction and how to construct circuits that can use it. I use the function to measure the light levels from the phototransistor, and to measure the settings of the dials, which are just potentiometers (variable resistors).
Here is the BS2 source code that I use. If you have any questions, feel free to post them.

Update: The IEEE Spectrum website has a little writeup and a video showing the microfiche machine in action.