Arduino Knobs

This is one of those “interim posts” I mentioned at the beginning of the year, posts where I don’t have something yet in a state where I can really talk about it, so I focus in on one detail that’s been taking a lot of my time, as a form of update. But today’s topic, rotary controls for computer-based systems, is a generally useful one, so I don’t think you’ll count this post a waste of time. At least not if you are interested in this aspect of the hobby.

A rotary control, or knob, is a control that can select a continuous range of states arranged in a circle, such as the volume knob on a stereo. Any rotary control can also be laid out as a linear one, simply by straightening out the underlying mechanism (they have to be designed that way, but often are). In schematic diagrams, a linear symbol is typically used to describe either kind, since from an electrical perspective they are identical.

In model railroading the most common application for this kind of control is a throttle. My first power pack, an ultra-cheap kit pack from Tyco, had a linear control (actually it was rotary inside the box, but the lever sticking out the side looked linear to me). Later, my first good DC power pack (my MRC 501, which you can see on my Power Pack Testing page) used a knob, albeit a simple one.

But today, I need a continuously variable control for a digital system, an Arduino to be specific. And yes, it’s for a throttle, but I’m not going to talk about the actual project I’m working on, as it’s still in the early design stages and there’s nothing much to say yet. Instead, I’m going to talk about the various options for this one control, and then go into more detail about the one I’m using, seen in the photo above attached to an AdaFruit Feather M0 Proto (a type of Arduino) for testing.

Train Power - When DC is not Direct Current

This is the second (or third if you count the previous diversion into history) in a continuing series of posts on powering model trains. The first installment covered some basics of DC motor design, and if you haven’t read it, it’s worth at least a skim before reading today’s post, since it provides a lot of background information (or read the DC Train Motors page, which has been updated considerably from the original material). Today we’ll get into how modern “DC” power packs aren’t really DC, and what aspects of DC motor behavior make that a good thing.

Most modern “DC” power packs don’t really put out DC in the sense of a fixed voltage for a given throttle position. The reason for this is that DC motors don’t really run very well at slow speeds. They work best at a fairly high rotational speed. In the early days hobbyists had to be content with models that went from “stop” to “rather quick” with no intermediate speed. This is often referred to as a “jack rabbit start”, and it’s hardly prototypical. Real locomotives weigh several hundred tons. Nothing that heavy accelerates quickly, and if it’s pulling a train that can weigh upwards of 10,000 tons it’s even slower to get up to speed.

Passenger trains (multiple unit type) tend to weigh less, and often have more and distributed power so they can accelerate faster. But there are limits to that, as passengers tend to object to being hurled backwards. And “weigh less” still means that a train will weigh 100 tons or more without passengers, and a full load of those can easily add 50 tons. That’s a lot to accelerate quickly.

I’ll get into the details below, but the short version is that modelers discovered that “pulsing”, or varying the power in a repeating manner, allowed a train to start more gently. This wasn’t without its problems, and various different approaches were tried over the years, although the earliest technique proved quite effective and continued in use for over twenty-five years. In fact many systems today use a technique known in the 1950s, but not considered “the best” at the time. It’s still a compromise, but a popular one. Today all but entry-level DC power packs produce pulsed power in some form, and it’s so commonplace that most don’t even mention it. There are exceptions, of course.


Train Power - DC Motor Basics

’m going to kick off the new year with a series of posts on some basic topics related to model railroading. Hopefully that will give me something to think about while my work on the layout is stalled, and reinvigorate my interest. I’m going to start with several about motors and control systems. Much of this recaps material I’ve covered in the past, but hopefully in a more concise and readable form.

Also, since my present modeling is all N-scale, I’m looking at things from that perspective. Much of this is equally applicable to other scales, although Z-scale motors may be somewhat different (“coreless” motor designs are more common there, and I’m not going to get into that).

While many trains today are used on Digital Command Control (DCC) systems, I’m going to focus on non-DCC systems that you’d find in a typical off-the-shelf model or a basic set, similar to the locomotive and DC power pack shown above. The same motors will work with DCC, with some additional electronics (the “DCC decoder”), so this is a good foundation regardless of the type of layout used. Future posts will cover more complex topics, but for now let’s stick to the basics: simple voltage-controlled motors and DC power.

Update: I don’t normally modify Musings after they are posted, but I have made some edits to this one to clarify sections and add some minor details overlooked in the original. These don’t alter the original in a significant way, but may make it clearer to some readers and help provide better background for later posts.