Right. I remember trying to fit a slotted variety IR-pair with its inherent alignment...but had to use discrete parts because of fit issues - this is an HO truck. I'm sure there are all kinds of packaging option now but I built this about 20 years ago!
I would have benefitted from a 3-D printer to make a slotted wheel. As shown I fabricated a printed circuit board and used "vias" to make the holes precisely spaced around a circle. Then I had to cut/file the circuit board in a wheel.
Interesting you bring up Hall sensors which were also "novel" 20 years ago but are now mainstream. For the OP's application, there are now AC Hall sensors that can measure current (no coils or transformers). For example, Allegro makes a family of IC Hall sensors. They are just a couple bucks a piece. But you do need to surround it with a few components, mount on circuit board, etc. These sensors would have even less effect on the DCS signal...not that the 1-turn current-transformer method does either.
