UPDATE
File turntableNM.txt added. this an Arduino sketch. Copy and paste into a totally blank sketch. 3 libraries are required: stepper.h, EEPROM.h and encoder.h,
The stepper motor is 28BYJ-48 with driver board, rotary encoder is the very popular cheap one on the usual auction sites.The motor needs a 1 amp power supply at 5V DC.The same supply can also power vcc on Arduino but this is not recommended when used with USB connected.

NOTE this sketch has not yet been tested or debugged. It will be mid Aug 2020 before I can do this.

This is still WORK IN PROGRESS

Thingiverse won't accept the illustrations
NEW TURNTABLE

This is a stepper motor controlled model turntable
the bed is 100mm long, designed for 009 but will fit short N gauge locos (loose the fences)
The Arduino code will follow in the coming weeks.
It is Primarily for DCC++ JMRI CMRI control but I will include manual button control for up to 6 rays (with forward & reverse positions for each.
I have a prototype working manually - but not as part of a layout. It rotates smoothly and has very little backlash. so it's just a matter of software..

Other Materials required:
009/N track (100mm)
28BYJ48 Servo & Driver
3 or 3.5mm screws & nuts
Copper foil (guitar scratch-plate hum screening type or thin copper sheet) 30mmx30mm.
2 Electric guitar strings (top and bottom E)
Short piece of 2.5mm Solid copper wire (UK socket cable) 50mm
Arduino
C/MRI library for Arduino (for JMRI control)
Rotary
handful of push switches LEDs/ resistors etc
Ultra mini microswitch 125V 5A
5V Power supply
hook-up cable, tools & accessories
2 axles 6mm 009/N gauge wheels (solid metal type but others may work)

Printing
print 3 spacers, 2 handrails and 1 of everything else. I used ABS with supports no Raft on Acetone sprayed, ABS wiped, Kapton covered glass in an old covered CTC dual head

Assembly
Remove all the supports
Prep the tub
paint before assembly and handrails
1countersink the 3 holes in the base to suit your screws.Screws need to be long enough to pass through tub, spacer and mount with nut.
sitck a piece of copper foil over sentre, knife round the edges and push through the holes to make 2 semi circle tracks. Add more tape strips from each track to the back and solder solder connection wires, one to each semi circle. Glue down wires as strain relief
fill the rest of the holes with CA gel glue and kicker spray. Get the foil and glue as smooth as possible. Check that the 2 semi circle tracks are not shorted.Use grid covers to mask screw heads.
Assemble Bridge
Strip approx 30mm 2.5mmSQ solid copper cable Straighten it.
using a file or Dremel form shoulder in the wire stepping down to approx 1mm diameter forming a step taper. there should be about 4mm of 1mm diameter wire on the end Round the 1mm end over. Using flat sidecutters cut about 3mm of 2,5 so there is a 7mm length 2.5 flat ended stepped down to 1.0mm round ended. See TTPin.JPG included above. When finished, make another.
make 2 small springs from a length of thin guitar E string a slow drill and a 1mm drill bit as a former. about 5mm of close wound spring Pull it out to about 15mm long, trim and make another.
glue outer track bed to bridge on one side. drill 3 small holes from under the bridge to mount the other side. This will be obvious. Get this exact or the turntable won't line up.use a length of track to get the sleeper spacing right. Don't fit the centre bed yet. The 4 small holes in the bridge should be directly under the rails and the large hole centre.
Pull off 4 wheels from the axles and fit flanges innermost using thick wire as axles.
drop the tapered copper pins taper down through the top face of 2 diagonal small holes
3.5mm screw through centre of bridge tighten a lock nut behind and trim to 20mm long protruding from underside of bridge. Add another locknut. 3.5mm sc
clean out coupling. insert 3.5mm nut into square radial hole.align with axial hole
ensure motor spindle is a snug fit in coupling then assemble the motor snd mount. Screw bridge assembly through tub into motor mount until wheels run smoothly on rail. tighten locknut, add handrails.
The timber boards relief in decking will be concealed by squish.
blutack the centre deck in position. check for clearance.