A Turntable For Model Railroads

Way back when, before diesel-electric locomotives were a thing, trains weren’t really able to go backwards too well. Also it’s sometimes necessary to turn carriages around in a small space. For that, the railway turntable was invented. If you want to implement one on a model layout, this project from DIY & Digital Railworld is for you.

The project is at an early stage – thus far, laying out how to set up an Arduino Uno using a potentiometer to control the speed of a stepper motor, which rotates the turntable. The turntable itself is a 3D printed part sourced from Thingiverse, designed to suit the specific stepper motor used.

This has the easy part sorted – rotating a piece of track through 360 degrees to orient a train properly. However, there’s significant work ahead. Power needs to be hooked up to the rails, and a system for accurately aligning the turntable with outgoing tracks needs to be devised. This is particularly relevant for N-gauge setups, where tolerances are everything.

We’d love to know how you’d tackle the various issues to build a working model turntable in the comments. We’ve seen some serious model railroad builds before around these parts. Video after the break.

 

9 thoughts on “A Turntable For Model Railroads

  1. Alignment should be easy. Put a dimple in a spot on the turntable, sized for a roller tipped microswitch lever to fit partially into. Have one switch for each track position, mounted so the switches can be adjusted in/out and sideways.
    Sufficiently weighting the turntable and using a motor with more torque should make it move smoothly across the switches without any jerking as the rollers go in and out of the dip.

    The most dead simple method of selecting the position would be to have the edge of the turntable holding all but one switch in its tripped position when stopped at one of the track positions.

    I’d have to think and draw to figure out a circuit. I *know* there are ways to make this work wiring the switches as normally open or normally closed. I’m thinking it’d be easiest to wire all the switches in series, normally closed, so that when the table moves to the desired position the circuit opens and power to the motor is cut. That has the issue of *temporarily* closing the current position switch to make the table move off that switch.

    There’s probably a way to use both the NO and NC positions of the switches and parallel connections to each switch so that the connection state of the current position switch is inverted until the physical switch state changes. then from there the table just free runs past all the *unselected* switches until releasing the lever of the *selected* switch, causing power to the motor to be cut.

    To keep it all electro-mechanical you’d want to use a ‘radio button’ switch, like those radio station or fan speed selectors that only allow one button at a time to be pushed in.

    Fancy solid state electronics could be used, without involving any sort of microcontroller.

  2. I still admire the low-tech approach featured in Model Railroader magazine decades ago: use a 1/4″ phono jack as a pivot, and align the bridge to the approach tracks by hand.

  3. Here’s a thought: use two circular ‘fences’ for optical interrupter switches. One would have very narrow gaps while the other has fairly wide ones, both centred on each track connection. How it would function is that both switches are low, the motor turns at 100% speed. When the second (wide gap) switch goes high, the turntable is close to alignment with a track connection and the speed is cut to 50%(?). Once both are high, the motor is stopped. UNLESS the ‘Connection Select’ (or w/e) button is held down. In that case, the motor doesn’t slow down as the turntable rotates past track connections. (I’m sure that one of y’all with discrete component kung fu can come up with an analogue circuit that only needs a three-position toggle switch that springs to centre so that you only need to hold the switch in a particular direction until the turntable has rotated close to the desired track connector.) (More ‘fences’ + optical interrupter switches = more slowdown options.) Does that make sense?

  4. I modded my Kato Unitrack turntable with some optocouplers driven by a function-only mobile decoder. Since I’m using the logic in the Kato controller, I just throw a switch on my DCC throttle, then kill it when I’m nearly to the track I want, and the Kato controller finishes the job. For polarity on the table rails, I have a Digitrax digital auto reverser, so nothing to do there, just turn the table CW/CCW by throttle, and back off when it’s close, then drive off. It’s a fairly easy DCC conversion that works very nicely. I still need to mount the Kato board and my mod-add-on opto board in a roundhouse ceiling, but the functionality is just what I wanted. I like it much better than the stationary Kato turntable controller!!

  5. when it comes to alignment, backlash is going to be enemy number 1. I would suggest a physical detent that a spring loaded ball can click into to make sure that there is absolutely proper alignment. Then i would use a drive mechanism with enough torque to get the ball past the detent, that way you don’t have to be too accurate about position sensing. So the motor would get the spring loaded ball detent past its secure location and only need to rotate to the desired position with an accuracy as big as the detent engagement area is.

  6. Btw, steam engines had no problem going either direction. Turntables were for putting rolling stock into Roundhouses. More compact than having a separate yard full of switches. Be able to 180 was a bonus.

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