L&NC Update; Running Lots of Turnouts

Its been a long stretch where there has been too much going on in real life and little time to write about model railroading. But I’ve been working away on the first module of the L&NC and have made lots of progress, so there’s plenty to write about.

This module has nine turnouts, presenting resource management problems that would arise in any substantial yard or staging facility. If you are familiar with the servo library then you know that you are limited to about 10 servos per microcontroller. With that many servos, your microcontroller will have few resources left to do anything else. This post will focus on a solution that problem, expanding the number of servos and other PWM devices a single microcontroller can manage.

Progress Tour

But first, a quick overview of progress to date.

Here’s the module in its current state:

Progress on the 1st Module as of June 2017. Fascia and dressing up of the edges will be the very last step; its pointless to do that while I’m disturbing things with new features and gear. Wires hanging out the pipe at the bottom of the photo are Anderson Powerpole connectors (track and master power) to the upper level.

As you can see I’ve done quite a bit of detailing. I realized early on that I need to complete all the basic scenicking and detailing of the module before moving on to the next. The big reason is having the module alone on a work table gives the best possible access, especially for electronic or animated items that require access to the underside. That’s not to say detailing will not continue after I move on to the next module, but it will mostly be passive rather than active elements.

By the way, if I were to build a room-sized layout, I’d use a modular (some call it “sectional” because you build it in sections) approach to construction even though the layout would not be portable. After laying mainline and other track spanning sections and cutting gaps between sections, I’d pull each section and do most of the remaining work in a work area under optimal conditions. When ready its just a matter of returning the section to its place in the layout and (literally) plugging it in.

Lets take a quick tour of some details so far.

Roundhouse / Turntable

If you’ve read the previous posts about the Roundhouse and the Turntable, you know these have been long term projects.

The turntable rotating beacon comes on whenever the turntable is in motion.

Then stove fire simulation inside the hut is visible through the door. It’s managed by a little PWM code that will the subject of an upcoming post.

The parking lot side of the Roundhouse has been enhanced with Rix power poles and some EZLINE power cables (which comes in two thicknesses and several colors. I use fine, and chose green–old copper–for its visibility). I fabricated a simple power connection and breaker box for the roundhouse out of a piece of styrene and a brass rod, and an EZLINE cable.

A Woodland Scenics light pole casts a pleasant white-yellow light over the parking lot in night mode. Figures, such as the worker (a Woodland Scenics prepainted figure) at the turntable end of the parking lot breath life into a scene.

Having gone to all the trouble to light the Roundhouse, I’ve started populating the space with some appropriate gear and figures.

A view of the lit Roundhouse interior.

Red Bluffs Yard

The Red Bluffs Yard area has its first structure — a fully lit Yard Office — plus a pickup truck with lighting passing by on the adjacent road.

The Red Bluffs Yard Office is fully lit for night operations. The Woodland Scenics light pole works just fine with my Duino Nodes controlled by an Arduino; treat it like any other 20 mA LED. The truck tailights in the background are from LEDS placed in the rear wheel wells, with the light allowed through tiny holes in the fenders.

Its amazing what a couple of SMD LED headlights can do for a really basic pot metal pickup truck kit from Micro Engineering.

The first of three planned scratch built Signal Bridges has been erected to control one of the approaches to the yard interchange.

And, finally, here is the underside, which is rapidly filling with gear supporting the layout above. This module, with its yard, multiple main tracks and turntable is one of the most electronically “dense” parts of the layout plan, to be exceeded only by the city scene planned for the upper level — that is going to be quite a project and I can hardly wait to finish the lower level and get started on the top!.

The underside of this module is rapidly filling with gear. Obviously overhead soldering is not an issue since I can put the module on its side. That said, I rely primarily on screw terminals and crimped fittings for connections.

PWM Drivers for Turnouts and Other Uses

Pulse Width Modulation (PWM) is used to output a timed pulse, where the output is on only part of the time. The width of the pulses — the percentage of time the pulse in the on state — is used to control external devices like servos or to vary the brightness of an LED.

Some, but not all, Arduino digital outputs are hardware PWM capable. Some of the PWM pins are SPI pins and the two serial pins, leaving only 5 or 6 PWM pins available for unrestricted use depending on the board model. If you want to make extensive use of PWM, that just won’t cut it.

PWM can also be synthesized with timed interrupts on any pin, which is how the servo library works and why it does not require you to attach to PWM pins. Unlike hardware PWM pins, PWM synthesized with interrupts represents a hidden load on your board that can affect the performance of your sketch.

External PWM Boards or “Drivers”

External PWM drivers allow  you to greatly expand the number of PWM devices a single Arduino can manage. PWM is used extensively in robotics, so PWM drivers are fairly ubiquitous and inexpensive. Aside from expanding the number of PWM devices you can control, PWM drivers allow you to off-load all of the PWM overhead and timing routines to the external device, freeing your Arduino for other tasks.

I decided to try Adafruit 16-Channel 12-bit PWM/Servo Driver for servo control and a couple of lighting applications on this module. Adafruit also sells a similar device in shield form.

Adafruit 16-Channel 12-bit PWM/Servo Driver, assembled with original terminal block (blue) that did not hold up to use. I eventually soldered leads to the underside of the board.

I chose the independent board rather than the shield because it has a number of advantages, not least of which it is chain-able with up to 61 additional boards, for a total 992 PWM outputs. A single chain of these can handle the servo needs of most club and museum sized layouts! A more modest layout could use these for both turnout servos and all lights and lighting effects, effectively centralizing and simplifying control of all connected devices. It uses the shared I2C interface for fast communication without using any regular pins on your Arduino. For more details, and a tutorial, see the Adafruit product page.

Assembling the board was straight forward, though there are a lot of pins to solder. The terminal block in the center provides independent power for the servo outputs (V+ center pin on outputs) per standard servo wiring; independent power is required by servos because of their substantial current draw. LED’s and other devices that draw their power from the PWM signal itself will not use the independent power.  Be warned: the terminal block Adafruit supplies is poor quality—substitute a better quality part or solder power leads directly to the board. The headers on the sides are for input and output, transferring both data and power to subsequent boards in a chain.

Adafruit 12 bit 16 Channel PWM Driver installed and connected to servos and lighting.

Connecting servos is just a matter of having a male->female servo extension the right size, or combining multiple extensions for longer runs. Any robotics supply store should have an assortment of extensions; as does Amazon. I have three different sizes to work with, which has worked well so far.

On the board positions 0 through 8 (1st 2 banks of four, plus the first pin of bank 3) are attached to the 9 turnout servos. Positions 9 and 10 are for headlights and taillights on the pickup truck. Using PWM I can have the headlights go between low beam and high beam, or have the taillights brighten as if the brakes have been applied. I have some thoughts about an animated animal crossing in front of the truck from time to time….

Using Adafruit’s PWM Driver Software

Adafruit’s software library for this device is available from their GitHub site. Using the software you create an object that you then use to control the board outputs:

Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver();

Creating the Adafruit_PWMServoDriver object without arguments uses the base SPI address to access the board; any different address has to be specified as an argument (and the appropriate jumpers on the board have to be closed). With multiple boards, you create a pwm object for each board using its unique SPI address.

From there, the PWM pulse is set on any output by calling the setPWM() member function:

pwm.setPWM(pin, on, off);

where pin is the board output (a number between 0 and 15), on sets the point in the cycle when the signal goes from low to high (usually 0, the beginning of the cycle, but it can be another value) and 0ff is a number between 0  and 4095 setting the point in the cycle when the signal transitions from high to low.

With the Adafruit driver board you do not use degrees to set a servo’s position. Instead we use timing “tick” values that control the signal transitions from low to high and back. There are 4096 “ticks” (12 bits of resolution) during each cycle. That turns out to be a good thing. For servos, the correct off tick values (assuming the on tick is 0) range from about 150 (the minimum or 0 degree position) to 600 (maximum position, 180 degrees).

Directly setting the cycle through ticks at 12 bits of resolution confers highly granular control and extra smooth servo motion.  Using degrees for position, as the standard servo library does, results in jerkier motion since a degree represents a lower resolution–between 8 and 9 bits–than the 12 bit resolution of the Adafruit board. For LEDS and other lighting, you can vary brightness from off to full on in 4096 steps, allowing fine control of lighting effects.

If you ask me, the smooth motion you can achieve with this board makes its $14.95 price more than worthwhile.

The only difference in your code between working with the standard servo library and the Adafruit driver, is in the object and member function you use to cause the servo to move. Every other aspect of your code and logic should remain the same.

What’s Next?

More coding, and I promise I won’t make you wait long. In the next installment I’m going to introduce you to simplified Object Oriented Programming (OOP) in C++ with the Arduino IDE. I’ll demonstrate a different way to code that, I think, improves several aspects of working with multiple turnouts, and makes the intent and flow of your code easier to understand and maintain. We’ve done it procedurally; we are going to take what we’ve learned and create some OOP code to do the work using either the Adafruit driver or the standard servo driver (Hint: we’ll use a compiler directive to select which driver gets implemented, making the object itself agnostic on the issue and universally usable around the layout).

Until then, happy railroading!

Wiring Module 1 of L&NC

After a long pause, I’ve starting in on wiring the L&NC by doing the basic wiring on module 1, lower level (each module has two levels). Module one is the largest of the three modules at 54″ long, and is the intended entry point for all the incoming power and control connections. Everything I do here is intended to set the methods and practices for the remaining modules.

Applying Lessons from the Past

Test Loop wiring. Block sensors were added in Phase 2. Phase 3 signals and lighting in progress.

Test Loop wiring. Block sensors were added in Phase 2. Phase 3 signals and lighting in progress.

My early layouts were primitive from a wiring perspective (and other perspectives…. but lets not dwell on that ….). I never did get into the suitcase connector thing, but the old layouts were wired using a single bus pair connecting to feeders every few feet.  Not much to it, so there was not much to organize. Like many layouts, the wiring was somewhat exposed and disorganized underneath.

While I was experimenting on the Test Loop, the wiring was built up in layers without a master plan.  For example, the power distribution from a central barrier strip was fine at first, but later it became necessary to create secondary distribution points to support various power needs. I ended up with wiring  less than optimally organized. On the plus side, I found that small circuit boards with banks of screw terminals are an excellent way to distribute power to to individual feeders or devices.

The other major lesson from the test loop is that it is a pain in the rail to add a major electronic component after everything else has been wired up. Major components should be sited and accounted for before interconnecting anything.  Components that move—e.g., servos, turntable mechanisms or whatever—get priority to ensure they get the placement and space they need — everything else has to adapt to their needs. In this case I will be installing the servos and the turntable early … but I’m getting ahead of myself again.

Special Issues with the L&NC

The L&NC has to be wired with a different aesthetic given that it is intended to be dissembled, moved and reassembled reliably. I am using the Digitrax Empire Builder DCC system, along with my Arduino-based independent control system and a multi-voltage power system supporting the layout — with power provided by a converted computer power supply. Computer power supplies can furnish 300 or more watts of fully regulated power at 12, 5 and 3.3 volts, perfect for every need other than track power.

L&NC Lower Level, Version 2

L&NC Lower Level

The lower level of module 1 (left-most module in the drawing) is fairly complex to wire because it contains the Red Bluffs Yard and the Roundhouse/Turntable complex. Including the feeders needed for the Roundhouse/Turntable area, 24 track feeders with current sensors have to be managed.

Yikes! That’s a lot of feeders for a 54″ x 27″ layout section! Its because of the yard, Roundhouse and turntable. Each roundhouse bay, the turntable bridge and any adjacent track segments adjoining the turntable all need individual feeders; the turntable is a reversing segment. Then each leg of the yard needs feeders, plus the base feeder for the ladder, to support occupancy detection and reliable operation. The main track loops each have two feeders. It all adds up…

So there is a lot more wiring in the L&NC than might be considered normal, and it has to be secure and well organized.

Step 1

With most track laid (I’m deferring the Roundhouse area until the turntable is installed) and the locations of all feeders and turnouts established, the first step is to place the primary distribution nodes then route, terminate and secure the main power bundle, the track bus and the LOCONET bus.

Connection Panel

Connection Panel

At the far left end of the module I placed a connection panel fabricated from styrene. There are four connections:


  1. Main power (4 conductors: 12 volt, 5 volt & 3.3 volt, plus ground);
  2. track power connection (2 conductors: Track A & Track B);
  3. LOCONET RJ12 jack (6 conductors); and
  4. Ethernet RJ45 jack (8 conductors).



My primary color coding scheme:

  • Black = Ground
  • Yellow = 12 volts DC
  • Red = 5 volts DC
  • Blue = 3.3 volts DC
  • Green = Track A (right-hand rail)
  • Violet = Track B (left-hand rail)

For the two power connections, I’m using Anderson Powerpole 15 Amp connectors to create polarized, color coded connectors. 2 connector and 4 connector brackets secure the receiving connection to the panel. I really love these connectors because they are easy to assemble (provided you invest in the crimper), come in a wide variety of colors, allowing you to assemble plugs and matching receptacles that can only be connected together one way. I get my supplies from Powerwerx.com.

Connection Panel, Inside

Connection Panel, Inside

From the panel, the wires run a few inches to a barrier strip.  Here the power is split – one branch going off to feed other parts of this level, with another branch heading to the upper level.

To supply power to the upper level I installed a short piece of PVC pipe from underneath the module, running up next to the corner post, terminating above the bottom edge of the upper level (when installed) I ran wires from the barrier strip up through the pipe, leaving 8″ or so extra cable extending from the top of the pipe, terminated with Powerpole plugs coded for the two power bundles.

Center Barrier Strip

Center Barrier Strip

To get to other areas of the lower level I ran cables from the first barrier, along the edge of the frame, using screw-in eyelets every 6 – 8 inches to channel the cables.  I terminated the cable run at a strip near the middle of the module, then ran an addition  set of cables from the middle strip to the right hand edge of the module.  Leaving 8 inches of slack, I terminated the cables with Powerpole plugs to be connected to a panel on the adjoining module.

The RJ11 jack has leads that I soldered to a six wire cable, then run that cable along the front edge of the module, opposite the power cables, to the location of a Digitrax UP5 universal panel (attached to the frame pending the addition of fascia), terminating the cable with an RJ11 plug as required to attach to theUP5. I fabricated another cable to run from the UP5 to interconnect bundle at the right side of the module.

Track power distribution block with ACS712 current sensors.

Track power distribution block with ACS712 current sensors.

The Ethernet jack on the panel is one end of a prefabricated cable available from Adafruit Industries. The other end will connect to a small Ethernet switch which will install at a later date. From there I’ll  make custom Ethernet cables (my crimper does both 6 and 8 conductor connectors) to fit where needed. Ethernet wiring will be added after I know the location of the Arduinos it will be supporting. I have an 8 port unit ready for the task.

I set up two track power distribution areas with a current sensor for each block, one on each end of the module. At this point I’m keeping the center area clear pending installation of the turntable. A third distribution area will be setup in that area to service the Roundhouse, turntable, service  and approach tracks.

Wiring phase 1

The big picture: wiring step 1 done

Now that the basic wiring is in, its time — finally! — to test the track installation, and fix what ever problems I find before moving on to the next step.  Until then, happy railroading!

Correcting track problems for flawless running.

Correcting track problems for flawless running.


Block Occupancy Detection for DC and DCC, Part 2

In Block Occupancy Detection for DC and DCC I demonstrated the use of an ACS712 sensor and an Arduino Uno to sense current draw across the rails, the basis for most block occupancy detection systems. Assuming that one has also equipped some rolling stock with resistor wheel-sets, lighting or sound decoders, so that the entire length of a train is detectable, the theory is that current draw indicates that a block is occupied — and lack of current draw indicates that a block is clear.

In reality, it may not be that simple if you have devices drawing from track power other than locomotives and rolling stock. Any background current draw on rail power can interfere with the current sensing model.

Rail Powered Accessories

The Test Loop

The Test Loop

Keen eyes will notice that the test loop has a rail powered light on the Atlas track bumper at the end of the siding. I laid the track on the test loop long before I understood block occupancy detection.  I won’t do that again as I build the L&NC.

The problem is that the bumper light draws from the track itself and thus presents a constant load on that block. The draw of the lamp forces a slightly higher threshold for determining occupancy than would otherwise be appropriate.  Static loads on monitored blocks greatly complicate current sensing.

The best practice where current sensing is used to detect block occupancy is to feed accessories from a separate power bus. To use track power (as you might with a stationary decoder running a turnout motor), draw from outside monitored blocks—from a separate branch or sub-bus—to avoid confusing the occupancy detection system.

Solving the Continuity Problem.

While current sensing works well on a DC layout, as shown in the first video demonstration, the use of current as a control mechanism means that block occupancy is undetectable at times because of normal operation. Similarly, any transient loss of DCC track power (as with a short) will throw the occupancy detection system off for the same reasons. I think of this as a continuity problem.

The solution in both cases is to monitor the track bus with an extra sensor and lock the state of the system whenever the track bus is off. The simplest algorithm, puts it a slightly different way:

if track current is flowing then check blocks and update else maintain current state

E.g. ( in C++, where “master.on” represents the master current status):

if(master.on == true) {
  for ( i = 0, i < NUMBER_BLOCKS; i++) {
    // check block sensor i and update block status

For the cost of a sensor and a few lines of code the block occupancy system will now maintain its state correctly whenever there is a total loss of track power.

Wiring for Detection

Preparing the test loop for the Arduino & ACS712 sensor based occupancy detection system was a bit of a chore, primarily because current sensing requires some rethinking of the standard approach to layout wiring.

Wiring. Block sensors were added in Phase 2. Phase 3 signals and lighting in progress.

Test Loop Wiring. Block sensors were added in Phase 2. Phase 3 signals and lighting were in progress when this picture was taken.

The test loop represents my first crack at it. Needless to say, there is a list of things that will be done differently on the L&NC.

Routing Buses

The loop has a bus structure that, except for the lack of the DCC command bus (I connect my throttle directly to the booster when it is attached to the test loop), is the prototype for the L&NC.

Test Loop Connection Panel

Test Loop Connection Panel

The bus structure begins with the connection points on one end of the layout, mounted on a connection panel made from .080 styrene sheet. Every module on the L&NC will have one of these, with matching cables at the opposite side where it adjoins the next module.

Starting from the top the first two connection groups are polarized power connections made with color coded Anderson Powerpole connectors. A “Recommended Practice” per the Ntrak Manual, these things are a reliable solution for creating polarized power connections.  Some parts are available from Amazon; I find that Powerwerx.com has the best selection of PowerPole parts and supplies. The 15 amp gear should be sufficient for most N Scale layouts (an N Scale layout requiring more than that would be something to see!). Kudos to Ntrak for finding and adopting these connectors. BTW: the crimper is expensive but indispensable if one is going to make a bunch of Powerpole connectors.

The top group is the main DC power bus in three voltages: 12 (yellow), 5 (red) and 3.3 (blue) plus ground (black). This was also where I attached a label with the loop’s assigned IP address.

The next group down is the track bus (red & black); my DC power pack and my DCC booster can connect to the loop here with a matching plug.

The next two connections are an RJ45 port for Ethernet and a USB connection to the UNO. Both are  assemblies I got from Adafruit Industries. On the L&NC, the USB port(s) for attached Arduino boards will be located in an accessible place, away from the other connection groups (which will generally be hidden).

Connection Panel Bus Connections

Panel Bus Connections

Color coded power buses are routed along the edge to a central barrier strip where feeders take off to supply power where needed. On the L&NC I’ll choose different, unique colors for the track bus and feeders to reserve black for system ground and red for system Vcc (+5 volts DC).

Suitcase connectors are really popular these days for connecting wires without soldering. If you are not going to do occupancy detection then directly connecting feeders to bus wires with suitcase connectors is a clean solution. However, once you start down the road I’m traveling–whether your solution is off-the-shelf or homemade–suitcase connectors are not helpful. Instead, I find that PC board mounted screw terminals make easy wiring connections that are reliable and easily changed when necessary. When dealing with magnet wire, which you have to use in N scale if you want to run wires through scale light poles, conduits, etc., top-of-the-line PC board terminal systems are the ONLY reliable solution (the cheap versions will drive you crazy with intermittent connectivity).

ACS712 Board

ACS712 Board

The ACS712 sensor boards come with screw terminals for the monitored load, plus 3 pin headers that take standard 3 wire servo connectors for connecting to system power and the microcontroller. Wires that connect to the central barrier strip have crimped terminals.  I mostly solder at the PC board level and, less often, to make special cables (I solder track connections too, but that is a different issue).

Power Distribution on the Test Loop

Power Distribution on the Test Loop. The board in the upper right corner is for distributing 5 volt power to the signal system via PC board terminals.

On an L&NC module, the bus will continue from the central distributions point out to the edge configured to connect to the next module. One big change in module construction on the L&NC will make wiring easier: the cross frame members will be pre-drilled with large holes. That “little” gouge you see where wires cross the frame member at the bottom of the picture taught me that lesson.

On the test loop the 12 volt bus supplies the attached UNO with its Ethernet shield; the 5 volt bus supplies all sensors and actuators attached to the UNO and is generally considered system Vcc. The 3.3 volt bus is not currently being used on the test loop (but would be great for low current lighting or animations). All share a common ground (really important!). NB: I use modified Computer power supplies that  produce all three voltages at once with a common ground. Multiple DC power supplies can be combined to the same effect; just tie their grounds together to create the common ground.

Even on the test loop, the 5 volt bus is used so much that I need to create a distribution system to get power where needed. The lesson for the L&NC is  I will need to have distribution nodes for the whole power bundle in a few strategic locations on each module. Obviously, I need to do some more detailed wiring planning in advance so I know where everything has to go.

Routing Feeders

Current sensing encourages a nodal distribution system for track power. Instead of attaching feeders to the nearest track bus, the feeders are routed to an area where the sensors have been mounted. You could mount individual sensors at the points where feeders descend from the tracks above. In some situations that may work better; in other situations, aggregating the sensors in groups is the easier and more effective method.

Current Sensors and Block Feeder Connections.

Current Sensors and Block Feeder Connections.

ACS712 sensors read only one of the two rail conductors. On the test loop sensors are attached to the red rail.

I tried to complete the rail feeder connection with a board (with terminals) across from the sensors for the black rail connection points. I also ended up using the same board to centralize the power and data connections for the sensors and the servo for the turnout. In the future I will not mix track / control system connection points in this way.

Block Occupancy Detection in Action

Here is demonstration of the progress so far, with block occupancy detection active on all 4 blocks of the test loop. The signals are programmed to respond to block occupancy and the state of the turnout (the turnout actually has three unique states: “aligned main”, “aligned divergent”, and “in motion”), allowing you to see the system responding as trains move along the track. You’ll see it work in both DC and DCC — the layout works whether a power pack or a DCC booster is plugged into the track bus port. I have yet to see a single commercial occupancy detection solution that moves as easily between DC and DCC as this one does. I added a station with internal lights (connected to the same nodal system that supports the signals) to give just a little hint of the capabilities that will be unlocked on the L&NC.

In future installments, I’ll dig into the occupancy detect code again to give more insight into how it works. I’ll also go into the details of how I’ve implemented signalling on the test loop, including some special (and inexpensive) gear I’ve designed to allow a single microcontroller to run up to 255 nodes of signals and other kinds of lighting or animation.