Fun with Arduino!

Many of the new products I have in development are based on the Arduino platform, so I have begun a series of videos to introduce the technology.  The first two episodes are out, and there are many more to come.


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It’s been a while since I’ve updated this site.  For that I apologize… I’ve been “nose to the grindstone” on product development, and there hasn’t been much news.  I’m going to change that, though.

For starters, I’m announcing a layout electrical design service.  Whether you just need a little advice, or need a full turnkey design for all those wires and boxes under the layout, drop us a line and we will go to work.  Are all those acronyms — DCC, LCC, JMRI, C/MRI, AWG — giving you a headache?  We can ease the pain.

I also have a few products that are nearly ready for production, and the business plan continues to take shape.  Stay tuned! Good times are in the future.


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What the heck is a BDL Blank?

So, Digitrax has this wonderful block detection board called the BDL168.  It’s great for detecting trains and all that, but it has one drawback…

What if you need to remove it for some reason but you want to still be able to run trains?  Maybe your board has a fault and needs to be sent back for repair.  Maybe you want to sell your layout, but keep your expensive electronics. Or you are wiring the layout up, but haven’t purchased your BDL168(s) yet. Perhaps you have two layouts, but your budget won’t allow you to purchase more than one board.

Because the rail feeders run through the BDL168, removing it open-circuits the layout and stops everything dead.  So, what to do?  Rewire the layout?

No… just pick up one of these little guys, and plug it into the BDL168’s socket in place of the BDL168 itself. Voila! The layout is up and running while your BDL168 is somewhere else.

BDL Blank

The BDL Blank is a very simple board. It doesn’t even have any components.  Plus, it is reversible.  You can’t even install it wrong.

(Not affiliated with or endorsed by Digitrax in any way whatsoever, of course)

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Under Table Sound

Now, I’ll state right off, that this was not my invention.  But some folks might find this little circuit a useful and easy way to implement the idea.

Check out Lance Mindheim’s April 20, 2012 blog entry on under-layout sound (you’ll have to scroll down quite a ways).  The basic idea is to take a sound decoder such as a Soundtraxx Tsunami, hard-wire it directly to the DCC track bus, and replace the speaker with a circuit that allows you to plug in a set of headphones, or an amplifier and room-size speakers.

Once the sound decoder is wired up, it can be “consisted” with a non-sound DCC locomotive, and provide big-speaker sound for a tiny little locomotive.  By using headphones, the problem of stationary sound coming from a moving loco is eliminated, and the rest of the folks in the house will appreciate it as well.

Lance hand-wired his decoder using spade lugs and a terminal strip.  It’s a nice setup.

I thought that a small circuit board or two might make things a little easier for folks who want to do this.  Enter the UTS-1 and UTS2 under table sound boards.

The UTS1 board basically consolidates the wiring for the decoder-to-headphone interface and makes it easy to install.  There are convenient screw terminals for the track bus, decoder leads, and transformer leads, and a 1/4″ headphone jack for your audio output.  The board requires an external transformer, the Radio Shack 273-1380 audio isolation transformer (or equivalent).  There is no amplificaton, but the sound decoder has enough power to drive a basic set of headphones by itself.   Volume control is managed via the sound decoder’s gain setting CVs.

The UTS-2 board expands on the UTS-1 board by moving the transformer onboard, simplifying the installation.

The UTS-3 board goes to a 1/8″ stereo headphone jack, but integrates the keep-alive cap for the decoder onto the board for a neater install.

And of course, all 3 boards feature a handy M3 sized screw hole for mounting to your layout.

The output jack is stereo, but because the source audio is mono, the mono output is wired to both left and right outputs.

Now, this is just a “coming soon” announcement.  You can see the boards at OSH Park by following the links above, and you can order your own if you like (at your own risk) but I haven’t tested them yet for any problems such as component clearances, wrong-sized pin holes, and the like.

Once I get a set built up and tested, I’ll take some photos and make them available as complete boards.

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Fixed it!


If you compare this photo to the ones in yesterday’s post you’ll notice three important things.

  1. There’s a blue wire running between R5 and pin 2 of U1
  2. There’s a scratch on the PCB just above capacitor C2
  3. The LED is not on!

I’d already made a new revision of the PCB with the fixes for the problem that was keeping the LED on all the time regardless of whether the loco is present, but I really didn’t want to wait another 3 weeks to test the fix.  Fortunately the required changes are simple enough I was able to “yellow wire” the board (using a blue wire).  Sure enough, it now works, though I have some sensitivity adjustments to test out.

“Yellow wiring” is a term of art, at least at my day job, though a quick Google search did not turn up any outside references.  It originated, I am told, at IBM, where engineers would use bits of literally yellow wire to make repairs or changes to circuit boards in order to fix problems.  Supposedly they had a whole color code of wire colors that would be used to designate whether a particular fix was a “bug” fix, or a design change, or a repair, or whatever.  I’m not really sure of the details — perhaps an ex-IBM’er can chime in with more of the story.

Anyway, by the time I arrived on the scene, the term “Yellow Wire” had evolved to be in common use either as a noun, meaning the repair or modification itself, or as a verb meaning “to fix or modify a PCB”.  We do often use literally yellow wire, but it is increasingly common to use whatever wire color is at hand, or in the case of complex changes, to use different color wires where needed to help with human understanding of the wires’ purposes (red for power, black for ground, etc).  The term has been expanded to mean the whole fix, not just the wire itself. One will often hear hall talk or discussions in meetings about having to yellow wire this or that board, or how this or that yellow wire was easy or hard, or worked or failed.

In this particular case, all I needed to do was cut the trace routing the (wrong) signal to Pin 2 of U1, and re-route the correct signal from R5 over to Pin 2.  All done and tested (successfully!) in about 10 minutes.

Now I am much more confident that the new, updated PCBs I just ordered will work well!

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This is why we build and test prototypes


I got all the parts in for the prototype SensorBit-C boards, so I built up the first one to see how we did.  There is always this expectation / hope that it will just somehow work the first time you try it out. Rarely if ever does it quite turn out that way.

As you can see above, all the parts fit (well, most of them), and the LED lights up. But there are some significant problems that need to be fixed, which could have been caught at design time but aren’t always:

  • The drill holes for the two jumpers were much too small for the jumper pins I wanted to use.  This is one of the things I should have checked before using the part footprint.  Easy enough to fix, but requires a board re-spin.
  • The two trim pots I selected are very nice parts, but are really too small to be practical for a typical model railroader to work with when upside down underneath a layout.  So I will be replacing them with larger ones.
  • Likewise, the screw terminals are marginally small.  They work well, but require a very small screwdriver.  I *might* consider using a larger part that accepts a larger screwdriver.

The real problem, though, is that the LED is on all of the time, regardless of whether the circuit is detecting  a train.  You’ll note that it is on in the photo, even though there’s not even a detection coil installed.

The reason it’s not working is a design error in the schematic, where I connected the detection circuit to the time-delay circuit the wrong way.  I stared at that schematic a hundred times looking for mistakes, but this one escaped me.

Oh well. No harm, except a few dollars and a few weeks “lost”.  We’ll make the required adjustments and get it right on the next pass.  And the next round of designs will be that much more solid!

Here’s another pic of the PC board on a “standard” Altoids tin, to give you an idea of scale.  This thing is quite small indeed, I think.


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Hello world!

Welcome to WordPress. This is your first post. Edit or delete it, then start blogging!

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SensorBit-C Sneak Preview

Here’s a sneak preview of the SensorBit-C!

The SensorBit-C is a CT-coil current-based block detector board for DCC layouts.  It comes in two flavors:  the SensorBit-C with a built-in coil and the SensorBit-CW, with a connector for a remotely mounted CT coil (included).


  • Open collector output
  • Two models: Built-in or remote CT coil
  • Compact size (less than 2 square inches)
  • Low power (less than 20mA)
  • Works on 5-12V DC power supply
  • Adjustable sensitivity and release time delay (0-5 seconds)
  • 150mA sink current on detect output
  • Onboard detect LED and connection for remote LED
  • Compatible with Modular Signal System (MSS)

SensorBit-C with built-in CT coil



SensorBit-CW with remote CT coil



I’m working on a version with a high-current relay output, and another with an onboard coil but screw terminals for the track bus for easy retrofit onto existing layouts.

The SensorBit-C isn’t available just yet, but stay tuned…

In the meantime, let me know what you think!

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New Products Previewed

I’ve added preview information on a few new accessories in the Accessories section of the Products pages.  Have a look!

I’ll be adding pricing and ordering information soon…


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Welcome to Appalachian Rail Technology!

Welcome aboard!

This is a new adventure for me, combining my love of trains and my many years of interest, training and work in electronics.  I’m not quite sure where this will go, but we will find out together, eh?

I have a series of products in development for use in home, club and modular model railroad layouts of all scales.  I will be introducing them soon, as they become ready.

I am also considering offering custom PCB design and manufacturing services for small quantity boards.

And of course, I hope to have plenty of quality, informative and entertaining info here on the site to help you in your modeling journey.

– Mark

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