Here's a discovery yet to be confirmed with further experimentation, but worth mentioning anyway.
It seems raw fibreglass PCB material is translucent or transparent to infra red at the frequency used by at least some opto interrupters. I couldn't figure out why my gadget, which used an interrupter made from PCB material, didn't work when the circuit worked well enough on the breadboard. When I stuck a piece of PCB between the test device, bingo! No reaction. The PCB lets too much light through.
Idea!! Instead of filing notches around the edge of the interrupter wheel, I'll etch a pattern in copper. (I guarantee the copper will stop IR light). Now I can make rigid, reasonably precise interrupter wheels with not a shonky filing job in sight.
How come I haven't read this before? Surely someone must have discovered it before me.
____________________________________________________ "I like to be organised. A place for everything. And everything all over the place."