Robin2:
justone:
Maybe print an optical encoder wheel to attach underneath the table and use a reflective sensor to detect.+1
A variation on this would be to print a strip that attaches to the edge of the turntable. It may be easier to install the photo-detector beside the turntable rather than under it. Som of the record turntables (remember them) used this technique to maintain the correct speed.
Remember that detecting the exact position will be a lot easier than making the turntable stop at a predetermined location.
...R
I have a higher end Marantz turntable that works like that. The edges of the turntable have polished reflective surfaces between black non-reflective strips. Then there is a plastic module mounted looking at the strips spinning past, that has a neon light source and a photosensor detector that is used for sensing the speed and adjusting the servo controlled platter motor. It has the added advantage that just the 60 Hz lighting from the neon light source would give you a visual display of the strips such that they would 'freeze' still when locked to proper speed, but there is an pot knob you could manipulate to cause the record to spin a little faster or slower if you wished.