Be patient. I did this before I had the use of a CAD program, so the files are non-existent. And I'm going to have to return the computer for a warranty repair.
Basically, though, the IR emitters had 50 ma current through them, and the detectors (phototransistors) had somewhere from 56K to 82K collector resistors, running on 5 volts. I drilled axial holes in scrap aluminium to mount the devices. The scrap was thick enough to collimate the optical path - handy to shield stray sources and you can pass a rod through the holes to allign the mounts before fixing them in place.
I think the signal from the phototransistor went to a comparator, with a small positive bias on the - input. I used two of them because the project needed to see if the drop was on a path exactly into a vial of soil used to measure mechanical errosion from raindrop splash.