Adding a Turntable to the L&NC, Part 2

The Roundhouse / Turntable complex at the Red Bluffs Yard is the focal point of the lower level of the L&NC and one of the more complex projects planned for the layout. In the first installment  (which is also step 4 of the build out of Module 1, lower level) I wrote about how I built the pit and bridge base, and showed the mechanism I developed for the turntable using Actobotics robotics gear.  In this installment, step 5 ( Links to step 1, steps 2 & 3) of this module build out, we’ll take a look at the buildup around the turntable, adding track and integrating it into the scene.  I’ll also talk about how I completed and wired the bridge. Along the way I took the time to install basic scenery elements while I have free access to this section of the layout.

Step 5

Finishing the Pit

The first task after installing and leveling the turntable pit was to create a rim and cover the rough opening for the  pit. After looking at a lot of pictures of prototype turntables I concluded that the rim needed to be around 4 or 5 scale feet wide. Rather than try to cut a circle out of styrene, or something like that, I elected to use a strip of cork roadbed. With the beveled edge against pit wall, and the square edge forming the outer edge of the rim, the look seemed about right.

The flexibility of n scale cork roadbed made it the perfect material for rimming the turntable pit.

The flexibility of n scale cork roadbed made it the perfect material for rimming the turntable pit.

One thing I found out in my research was that track rails are typically attached directly to the rim material (usually concrete) without ties. I decided not to model it that way because I just don’t want to get bogged down with gauging problems. So I elected to mount the ties to the rim so that they will keep the track in gauge.

Its not prototypical, but that’s OK with me because this is one of those places were functional reliability has to win out over prototypical niceties.  As you can see in the picture below, I weaved the ties of the adjoining tracks between each other to achieve the correct track placement along the rim where it meets the bridge. Preliminary testing established that I’m getting good alignment and smooth wheel transits across the gap.

Track from the Roundhouse meets the turntable.

Track from the Roundhouse meets the turntable. Ballasting is in progress so there are bits ballast everywhere at the moment!

I painted the rim concrete to match the rest of the pit.  The texture of the roadbed material is a little rougher than I anticipated. A second coat of paint smoothed it out more and left me with a surface that is a little worn from the effects of time and weather.  I’m pleased with the effect; a perfectly smooth surface just wouldn’t be right.

Finishing the Bridge

In addition to the powered track that also serves as a reversing section, the turntable bridge has an operators hut containing a warming stove with a red LED for creating a hot coals effect. The central arch–which prototypically was often a rotating connection point for the incoming power line to run the turntable–is outfitted with a simulated rotating beacon that will run whenever the turntable is in operation. Not even remotely prototypical, this little enhancement is just a way to animate and make the turntable even more interesting.  I did say at the outset of this project that I was going to throw in animations at every opportunity!

I started the deck by creating a base frame to fit over the bridge and hold a piece of flextrack (I’m doing this layout in Peco code 80 gear).

Bridge deck base frame.

Bridge deck base frame. The track has not been trimmed to its final length yet.

Starting with the base frame, I continued adding cross-members until there was one in each space between ties. Then I decked it with .030 x .080 styrene “planks” cut to various lengths from scale 8′ to scale 32′, creating a plausible planking effect..

The bridge deck before painting.

The bridge deck before painting.

The central arch is fabricated from .080 square tubing. I added etched brass X-bracing in a scale 18″ size so that it would resemble beams fabricated from plates and X-braces. I thought about trying to fabricate brace & plate beams, but felt it would be a little too difficult to pull off and make strong enough for practical use.

The rotating beacon at the top of the arch is fabricated from three red micro SMD LEDS arranged in a triangle. The magnet wires (1 common anode and 3 cathodes) are connected to an incredibly small rotating beacon simulator board that I purchased from The have a nice collection of simulator boards geared to model railroading, with everything sized for N scale (these boards will work in any scale, so don’t be deterred if you don’t do N).

The Ngineering rotating beacon simulator board with leads attached.

The Ngineering rotating beacon simulator board with leads attached.

Actually I bought 2 because I ended up screwing up one of the outputs on the first one. These boards are smaller than a dime and soldering wires to them is tricky. On my second attempt I soldered short 30 gauge solid wire leads to the board (easier than soldering magnet wire), then soldered the 40 gauge magnet wire to the leads.

To answer the obvious quesiton, I could very well have done the rotating beacon effect with an Arduino.  However, this little board is particularly good at that job, and I don’t have to use 3 PWM pins (plus the timing sensitive programming) to do the job.  I can run this effect from a single connection on a Duino Node, simply turning it on when the bridge moves and off when it stops at its intended destination.  This is good example of how one has to balance all the trade-offs when designing a system based on Arduino technology. Sometimes an external utility board gets you to the right place more efficiently than doing it from scatch.

Base and walls of the hut, and the stove.

Base and walls of the hut, and the stove.

The operator hut is made from a white metal “trackside shanty” kit by Stewart Products. I decided that the hut needed a little stove for heating, to go with the smoke jack provided by the kit. So I fabricated one from a piece of brass tubing and a styrene circle for a top. I cut an opening in the brass to serve as the front opening.  A red led mounted in the bottom of the tube (not shown) will be used to create a fire effect that you will just barely see through the open door of the hut. I’ll produce the effect with PWM on an Arduino board instead of a dedicated simulator. So the turntable bridge will have two different light animations.

With all the pieces assembled, the wiring in and everything painted, it was time to connect all the wires to the leads from the spinner and attach the deck assembly to the bridge base. If you look carefully you’ll see a black object to the right of the center — that is the beacon simulator in protective heat shrink tubing.

Attaching the deck assembly to the bridge base.

Attaching the deck assembly to the bridge base.

Are those fishing weights attached to the underside of the deck? You guessed it! The white metal hut is rather heavy (relative to the weight of the styrene); the weights are needed to balance the bridge. The white object between the girders on the left is a nylon screw with a rare earth magnet (Neodymium, available from K & J Magnetics) glued to its head, screwed into a nylon nut attached to the bridge. That is for the position sensor reed switches I’ve previously described.

The turntable fully assembled.

The turntable fully assembled.


In addition to finishing the turntable I am doing as much scenicking as possible while I have this module on its own on a work table. As I am doing this I am reaping a bonus from my modular design: the ability to take a module and place it on a work table for 360 degree access at chair height.

I'm using CadRail's layers to record information. Here I've recorded feeder positions (in red) and turnout servo positioning data.

Module 1, Lower Level


Referring to the drawing of this part of the layout above, I decided to add a small mountain in the space between the yard at the top and the  two legs of the reversing loop at the bottom.  This creates a view block that isolates the yard into its own little world.

I also decided to create a couple of roads using Woodland Scenic’s Road System paving tape and Smooth-It pavement material. The system works pretty well. I do recommend viewing their video tutorials for instruction in using the system, which you will find on the product pages.

Creating an access road at the Red Bluffs Yard.

Creating an access road at the Red Bluffs Yard.

After removing the forming tape, you are left with this:

The road at Red Bluffs with forming tape removed.

The road at Red Bluffs with forming tape removed.

You’ll see that the road includes a driveway and parking lot at the top of the picture, and another driveway at the bottom that will lead to a gravel parking lot. These are provisions for future structures I have planned –a multi-unit rooming house for railroad workers at the top and a yard office at the bottom. I also created a road on the opposite side of the module, running between the legs of he reversing loop as an access road to the roundhouse / turntable complex.  I figure the employees need some way to get to and from work!

The mountain was made from several layers of foam insulation glued together then carved to a rough shape. I glued the foam shape to the layout, then covered the whole thing in plaster cloth. Then I selected some rock molds and cast a few rocks. After gluing the rocks to the mountain  (Attach rocks with wet plaster? Forgetaboutit! Liquid Nails for Projects makes attaching hydrocal rocks to another surface a snap, with its strong tack and immediate hold), I filled holes and blended the rocks into the terrain with Sculptamold. I painted everything except the rocks the medium tan I’m using as a base color, then painted the rocks themselves with a combination of iron oxide and earth tones. I glued down some earth blend and other ground foams — and, voilà, the red bluffs were born.

The Red Bluffs

The Red Bluffs

What’s Next?

More scenicking, of course.  But it is getting to be time to go underneath the module again and install more of the electronics, including controller hardware for the 9 turnout servos and the controller package for the turntable / roundhouse.

Until then, happy railroading!


Happy Independence Day weekend to my US readers. Well its been several months where time for railroading has been limited. Top of the list, our nine-year-old beagle developed serious back/neck problems a few months ago that ultimately required surgery.

Lewis the beagle; 2 weeks after back surgery.

Lewis the beagle; 2 weeks after back surgery.

Little fellow is recovering and doing well with his rehabilitation program–exercises given to us by the specialty vet. Things like stairs are going to be a problem for him for a while, so he won’t be joining me in the layout room, as he usually does, in the near future.

Even so, I have some L&NC construction news along with updates on on-going projects.


ACS712 – How Low is Low Current

ACS712 Board

ACS712 Board

I’ve been exploring the limits of low current sensing with ACS712 sensors. Sparkfun markets a sensor board combining an ACS712 with an OP Amp that they characterize as a low current sensor. I had no luck calibrating and using that sensor; if someone reading this has used it successfully, please leave a comment explaining how you calibrate and use it.  Additional experiments with Op Amps, including creating a fairly stable instrumentation amplifier, have not been encouraging though I haven’t entirely given up and have a few possible circuit designs yet to try.

The fundamental constraint with ACS712 sensors is that their output noise inhibits current sensing below 20 mA (officially, Allegro says that you can’t detect below 20 mA without a specially designed version of the chip; which they will happily work on for a paying customer). At higher currents, the sensor works pretty much as advertised and can be used with or without filtering. At and below 20 mA it is difficult to tell the signal from the noise without some sort of mathematical filter; the RMS method I use can detect down to around 15 mA with an UNO’s 10 bit ADC.  With a Mayhew Labs 14 bit ADC, I seem to get a little more sensitivity, around 12 mA, using the RMS algorithm.

Below 12 mA noise takes over, at least so far as short term measurements go. So I’ve been looking at data collected over longer time periods and have a preliminary method that can discern a low current signal down to around 8 mA.  In essence, a low current change causes the long term output noise range to shift up or down depending on whether the current is increasing or decreasing. Capturing this range shift should improve practical detection sensitivity considerably. Its promising, but I still have kinks to work out of the system.

Is 1 mA sensitivity—the holy grail for detecting a single 10k resistor wheelset—possible with an off-the-shelf ACS712 sensor?  Probably not, and that may matter to people who want to use off-the-shelf 10k resistor wheelsets.

That does not make these inexpensive sensors useless. If you don’t mind making your own resistor wheelsets, then a single 1.5k resistor (assuming one resistor wheelset per car; two wheelsets at 3k—in parallel producing a net 1.5k resistance—one at each end of the car, would be a better idea for a bunch of reasons) will be detectable while still drawing under 1/10 watt (at 11.5 volts DCC n-scale standard; higher voltages allow for more resistance). 100 wheelsets would draw less than one Amp total current. More on that when I get the system operational.

Nevertheless, I’m curious whether a 16 bit ADC will help further resolve the low current signal. I’ll be giving that a try this summer so we will see.

Construction Progress

Laying Track-Bed at the Red Bluff Yard.

Laying Track-Bed at the Red Bluff Yard.

I finished drawing in the track plan, including some additional yard space, on the lower level modules and started in on laying roadbed and track.

Roughed In Turntable

Roughed In Turntable

Roughing in the turntable was fairly straight forward. After preliminary rough siting, I installed the roundhouse base and drew in the track center-lines. That located the turntable center (the intersection of the track center lines), enabling me to cut out an appropriately sized circle to accommodate a pit I’ve already created using a plywood base and styrene strips.

Building the Turntable Pit with Styrene Strips glued to a plywood base.

Building the Turntable Pit with Styrene Strips glued to a plywood base.

The turntable project is a major layout item in its own right and will be the subject of multiple future posts.

Laying Track for the Yard

Laying Track for the Yard

It took a few sessions, but I laid all the lower level track except for the turntable/roundhouse area (which I will do when I install the turntable) and joined it with the helix.

Tracklaying on Lower Level Done.

Track-laying on Lower Level Done.

Track Laid and Ready for Painting and Ballast.

Track Laid and Ready for Painting and Ballast.

For those interested in the nitty-gritty of track laying techniques, I used adhesives rather than mechanical fasteners for all roadbed and track. I use Liquid Nails for projects (water based) to adhere foam to wood, and anything else to foam. For the track I used DAP ALEX clear latex silicone caulk, deploying T-pins to hold the track in position while the caulk cures (just a few hours). What I like most about this method is that once cured the adhesive is completely invisible creating a nice base upon which to detail the track. Now that I’ve used clear caulk for track laying, I can’t imagine using a grey (and definitely not white) product unless the color closely matches the intended ballast color.

I don’t know if you can tell from the photograph of the whole lower level above, but the lighting system using Addressable LEDS that I’ve devised is working rather well (granted the wiring needs  tidying up; I’ll get to that eventually).


Building A Test Loop

Since the test loop project is a dry run for a couple of key methods I expect to use on the new layout, I decided to start with a little formal planning using CadRail from Sandia Software. Years ago I tried an earlier version of their product and found it difficult to use. This time I did much better. The current version, for all its CAD power, is really pretty easy to use for basic planning. I downloaded the demo, spent a few hours doing the tutorial, and was able to produce this simple plan for a 2′ x 3′ test module in short order.


Track Plan for the Test Loop.

Not much to it: a loop, a turnout and a spur. I had not purchased the product yet so I didn’t have track and structure libraries to work with; everything was done with the general features, tools and components. The color coding is to indicate wiring blocks; essential to using the test loop to work out block detection and other technical issues.

When I then laid the track I was able to replicate the plan exactly, down to the too-close clearance (to the edge of the module) at the turnout. I could have shifted the whole thing during build to improve the clearance but on this occasion the point was to reproduce track plan exactly as drawn without significant adjustments; and to see how it turned out. No surprises.

I had CadRail show the radius points for each curve on the plan (one of many view options). Locate the radius point exactly on the layout surface and, using a compass, you can draw the correct track centerline exactly where it is supposed to be. You can print a plan with grid lines and dimension measurements to help transfer the plan — if you want to sacrifice a lot of paper to the cause, you can even print in real size. Having struggled to transfer hand drawings with dubious calculations to real world materials in the past, this is a definite improvement. At $79 to download the full software and libraries, it’s a good deal  (for a bit more you can get a printed manual and CD-ROM installation disk, with a few extras to sweeten the deal. I went ahead and purchased the full monte).

Here is a view of the foundation for the loop.

test loop frame

It is a simple box frame, 2′ x 3′ x 8″, made from 1 x 2 and 2 x 2 poplar, with a 1/4″ oak plywood top (all from Lowes). In the past I’ve used common pine for benchwork. I chose hardwoods this time because I suspected they might work better for a modular system than softwoods, especially since I am trying to thin down the structure as much as possible while stiffening it as much as possible. Hardwoods are generally more dimensionally stable than low grade pines, especially in the smaller milled sizes. Furniture grade pine is good, but still not as strong as hardwoods and hard to get in my area. Hardwoods cost more, but for a small layout that extra cost is negligible. Poplar in the two sizes I’m using runs about $1 per linear foot, and is easy to work with. [Some poplar products are marketed as Sustainably Produced. I don’t know that is true with all poplar products.] The payoff is how much easier it is to keep your woodworking thin, straight and strong.

test loop frame detail

Corner Detail

I created a box frame with a top, not a fully enclosed box or box beam.  The joinery is simple lap joints with some mitered joints at the corners; nothing fancy or complicated. Brads held the frame together while the glue set (the classic Norm Abram technique.). I included a reinforcement bar under the plywood top; It is not absolutely necessary on a box this size though it did contribute to getting everything square and flattening the plywood.

I topped the plywood with 1 inch insulating foam. The resulting structure is light and amazingly stiff.  You can’t twist it easily; you’d have to push it past the breaking point to deform it. This is pretty much what I had in mind. I’m hoping that a substantial hunk of foam under the track will help suppress the mechanical sounds of locomotive mechanisms, which are not the sounds I really want to hear.

Finding a source of foam was more complicated than I thought it would be. The insulating foams sold for building purposes have changed over the years. Most are now faced with foil, paper or other additional materials. For my purposes, those extra materials are not useful. Then there is the whole problem of transporting 8′ x 4′ sheets of foam strapped to the top of the car and subject to easy damage.

Turns out that Owens Corning produces an unfaced foam product that is geared to the hobby market. foamular labelIt’s a dense foam that resists compression and holds pins very well. It comes in easy to transport and use 2′ x 2′ x 1″ squares. The only source I’ve found for it so far is The Home Depot, where it costs $5.48 per square as of this writing. I thought that was pricey at first and, compared to a full 8 x 4 sheet of building Styrofoam, is it is. But compared to hobby specialty foam products, it’s very reasonable at less than $1.50 per square foot for 1 inch thick stock; and I can get it locally.

Building with foams presents several new challenges. At the top of the list is finding adhesives that are foam-safe. Many caulks and adhesives will attack foam. Secondly, since I am joining dissimilar materials in many instances, the caulk/adhesive has to work with all materials being joined. Porous materials are usually easy; non-porous materials like most plastics are more difficult. Needless to say, I spent a fair amount of time at both The Home Depot and Lowe’s reading labels.

For adhering foam board to plywood I selected Loctite PL300 Foamboard Adhesive. This product is primarily formulated for adhering foam insulation to basement walls and the like, so once it takes hold it is strong.

Loctite PL300

I used about 2/3 of a tube to adhere two pieces of foam to the plywood; I think I can use less next time. Initial grab is mild, giving plenty of working time, but also requiring some sort of strategy to hold the foam in place until the product sets. I tried both clamping and screws: a few bugle-headed screws proved the best method for anchoring the foam while it set. I had a mild warp in one piece of foam that I easily straightened in the process. The holes can be filled or used for something else later.

roadbed trim and peel

Trim and peel test.

Liquid NailsNext I had to adhere cork roadbed to the foam surface. I bought some Liquid Nails for Projects then did a quick test to compare it to ordinary white glue. White glue will adhere to the foam, but it’s a weak bond easily broken. Liquid Nails provides a much stronger bond. As tight as the bond is, the adhesive does not dig into the foam so I could trim and peel off excess roadbed after the adhesive set without significantly damaging the foam.

Here is a view of the top, with the cork roadbed down.
test loop roadbed

Next up, laying some track, installing feeders and powering up the test loop.