Installing MRServo Switch Machines

A couple weeks back, I removed the old staging yard from my layout in the process of tearing out the old to make room for my new layout.   I didn’t need it for the new layout, so we decided to repurpose it for writing an article on complex turnout control using MRBus.  It was built ten years ago, long before MRServo was even a thought in my mind, and was consequently powered with a competitor’s large green switch machines.  I’ve gone entirely to MRServos now for the new layout, and we’re obviously going to try to show them off as part of any article we’re working on.

As we were installing them, it occurred to me that it might be nice to show folks how to reliably install MRServo, step by step.  The instruction sheet covers this all, but sometimes a picture really is worth a thousand words.  Once you get the hang of it, it’s quite easy and goes quickly.

Step 1 – Tools and Prep
You’re going to need a few basic tools, in addition to a MRServo kit.  I typically have a small set of needle-nose pliers, a small Philips-head screwdriver, and a set of strong diagonal cutters for cutting off the piano wire once MRServo is installed.  Piano wire is incredibly tough stuff, and will destroy the jaws on cutters over time (or on small cutters, almost immediately).   I’m a fan of Harbor Freight’s 8″ diagonal cutter – the jaws are hard enough they don’t take much damage, and they’re cheap so I don’t feel bad when I destroy a pair.

You’ll also probably want a 8-15 volt DC power supply so that you can test MRServo on the bench.


Step 2 – Throw Wire Installation
Once you’ve got everything ready, the first step is to bend piece of piano wire included in the MRServo kit to fit through the servo horn.  The instruction sheet has pretty good details on how to make these bends, but basically we’re going to make a u-bolt looking shape on one end.  It should be roughly a quarter inch long, with enough left on the end that you can get it back through the servo horn and fold it over.

Once the bends are made, find the single-sided servo horn in the bag of servo parts.  They’re not all exactly the same, as we source servos from several vendors, but there will be one with stout arm coming out of one side only and with 4-5 holes in it.  That’s the one you want.  Feed the formed wire through the hole nearest the pivot screw hole, and then bring it up through one of the holes further down on the arm.  I typically aim for the last or second to last hole, but it’s not really that critical.  Once it’s through  (and be careful feeding it through – the cut ends of piano wire can be very sharp), use the needle-nose to bend it over, back towards the pivot point.

step2-bend-jpg step3-horn-jpg step4-bendover-jpg

Step 3 – Centering the Servo
Inexpensive RC hobby servos aren’t exactly finely calibrated pieces of gear – no two (except by accident) will respond exactly the same to the same input command.  They’ll be close, but not exact.  So, for our use, we need to adjust things a bit.  Some other vendors have opted for calibrating on the layout, but I’ve always thought it was easier to do it on the bench instead.  Don’t worry, it’s simple, and makes installation on the layout much easier later on.

The basic step is to place the servo arm you just made on the servo (don’t screw it in yet!), and slowly and gently turn it to the ends of throw.  (Those gears in the servos are small, and can strip out if back-driven too fast.)  The idea is that, at the middle of its throw, the wire should point straight up.

Once we’ve done that, we’ll go back and validate it using the actual control board.  You’ll need 8-15 volts of good DC, same as your MRServos will need once they’re installed.  We actually have a test harness with a pushbutton that we can just jam in the terminals to make this process fast and easy, but if you don’t, just connect power to the servo control board in the VCC and GND holes, and have some capability of connecting the control input to ground.

Use the control input to throw MRServo back and forth, making sure it throws approximately the same distance on either side of “straight up” for the two different positions.  You have two adjustment options – you can take the arm off and move it a tooth on the servo shaft, or you can tweak the wire a bit to the left or right.  Most of the time, if you’ve done the manual centering correctly, this part doesn’t require adjustment.

Once you’re happy with it, find the screw in the little bag of servo bits, and screw the horn into the servo shaft.  Again, be careful not to torque the servo shaft around too much when doing this.

step6-powerup-jpg step7-calone-jpg step8-caltwo-jpg step9-screw-jpg

Step 4 – Mounting
The servos we use are lubricated with a thin grease that just floats around inside the case.  Unfortunately, that also means that some of it will leak out through the joint where the two halves of the case come together.  So, before we attach any mounting tape, we need to thoroughly clean the surface to be free of dirt and oil.

For this, all you need is isopropyl rubbing alcohol.  I used 91%, because that’s what we had on hand, but most concentrations work fine.  Remove any stickers from the sides of the servo case, place some alcohol on a paper towel, and rub both sides until clean.  Doesn’t usually take more than a few seconds of cleaning.

Once you’ve removed any gunk from the sides, attach a piece of mounting tape.  (The kit includes two pieces, but if for some reason you mess one up, it’s just 3M/Scotch #411 1″ grey mounting tape, available at many home improvement and office supply stores.)  Make sure that the tape contacts as much of the servo as possible without fouling the throw rod.  Then press it all down firmly to assure a good bond.

Center the wire on the servo to point straight up, and also center the points on your turnout (I often use bits of styrene to hold it there).  Then take off the backing paper on the servo’s mounting tape.  We’re now ready to actually stick it down.  Get under the layout, look up through the hole, and put the wire through the hole in your turnout’s throwbar.  You want the wire to run straight up, and not be applying pressure one way or the other (either side to side, the way it’ll throw the points, or lengthwise along the track). You want it to look like it’s just sort of “floating” in the throwbar hole.  Once you’re happy, stick it down firmly to get the tape fully engaged with your benchwork.

Typically then I remove anything I’m using to stick the points in place and then will gently move the servo arm from side to side, just to verify that it’s going to move the way I want it to.  Once I’m happy with that, I go find the MRServo control board, apply the other piece of double-sided tape to it, and stick it nearby.

After a little more testing, remember to use the big diagonal cutters to clip the throw wire.  (You wouldn’t believe the number of times I’ve run a test train into an unclipped throw wire…)  Wire the thing up to however you’re going to control it, and you have a reliable, low profile switch machine that will last for many years to come.

step10-clean-jpg step11-mountingtape-jpg step13-centerpoints-jpg step12-mount-jpg step14-tweak-jpg step15-tapeboard-jpg step16-tapeboard2-jpg step17-test-jpg


  1. I am recently retired and just getting back into Ho modelling, my interest is the technical side. I like the idea of using a servo as a switch machine! My question is how many servos can you hook up to a single power source; such as when you use a crossover?

  2. William – How many MRServos you can drive from a single source depends upon your DC power supply’s current rating. When the servo initially powers up, there’s often a very brief (<0.1 second) surge of demand for current. However, for normal operations, you only have to accomodate their stationary current draw - ~22mA for a MRServo-1, ~62mA for a MRServo-2, and ~92mA for a MRServo-3 plus some amount of extra capacity (~100-200mA) for each switch that's in motion at any given time. (The increasing demands for current are related to the number of relays on the MRServo board that may be energized at any given time. Each relay can draw approximately ~30mA if it's activated - the MRS-1 has none, the MRS-2 has one, and the MRS-3 has two.)

    One of the layouts I work on has approximately 18 MRServo-1s in their new yard, and they're all fed from a 2A, 9VDC power supply quite happily. Usually we figure no more than 3-4 of them are actually moving at any given time, so there's plenty of capacity.

    Also, I haven't heard back from you on our response to the question you sent us through the web store. Realizing it was a holiday weekend, we figured you might be away, but if you didn't get it or are still having problems let us know.

  3. Don’t be afraid of manufacturing your own linkage. I use a single servo to drive a crossover (two switches) and a double slip switch. Brass tubing and brass wire, piano wire, and springs from old ballpoint pens are standard in my hardware drawer….

  4. Do you have any suggestions for installing and linking MR-servo units above board (trackside) such as in hidden staging or other hidden track. Some of us older MR’s don’t like crawling around under the bench work as much as we used to. ;>)

  5. It’s honestly not something I’ve ever tried – all of mine are below the benchwork – so take my suggestions with a grain of salt as I’m just thinking off the top of my head about how I’d approach the problem.

    Probably what I’d try first is to mount the servo vertically through the subroadbed so that the shaft came straight up from the top. Then attach the servo arm as normal, and connect it to the throwbar with a spring linkage. I’m not sure exactly what to call this type of spring, but it’s the kind that forms about a 90-degree angle with a coil at the corner. They were often used with old twin coil machines to link the throwbar to machine. While looking around for their proper name, I found this blog post by an SP modeler doing something very similar to what I’m describing, except instead of a servo he’s using a bell crank and model aircraft linkage. The springs are pretty easy to make out of fine diameter piano wire. I used to make them myself years ago by just bending it around a pencil a few times.

    Basically the key part is making sure you’ve got a spring in the linkage to keep tension on the points while letting the servo travel as much as it wants. Despite their size, these little servos can exert a great deal of force – certainly enough to damage the points if there’s no spring to absorb some of the mismatch. But with one of these (for lack of a better term) “elbow springs”, it should work just fine.

    If anybody knows what the proper name of these springs actually is, let me know and I’ll update the post.

  6. Hi, with the MR3, that utilises the “Frog Power routing” does this mean with the “Fast Tracks” style of Turnout that we won’t have to cut ANY gaps in the rail?

    Thanks in advance,


    1. Here’s a turnout I built for my own layout a couple weeks back, and I’ve circled the areas that would become problems using a -3. I’ll explain each below the picture.

      An N scale Fast Tracks turnout with the problem areas highlighted.

      The red areas are places where conduction through the PCB ties themselves become an issue. You’ve got to gap the ties and remove copper foil at just the right spots so that when the points are at either end, there’s no conduction through the ties. You’ve also got to take the copper off the throwbar under the stock rails so that it isn’t shorting things out. If you’re really careful, it might be possible. However, removing the copper foil that close to the joints will weaken things, as the foil isn’t adhered to as much of the fiberglass tie around the joint area.

      The purple spots are tight clearance areas. At least in N, and I’ve seen it HO, metal wheels will sometimes intermittently contact both the point and stock rails. If the point rails are both powered with the frog, there’s always the chance of a short. Things with long rigid wheelbases (steamers, 6 and 8 axle diesels, etc.) tend to have more of a problem in this department.

      As far as the green circle, it’s not really a problem area on the turnout itself, but just a reminder that you’ll still need some sort of gaps on the inner rails coming in on the frog end, as I presume these will be powered from the correct polarity somewhere down the line. Personally I’ve gone to omitting the gaps at the frog and just put an insulated joint where I go back into flex track.

      Personally, I’d recommend a -2, and leave the point-end gaps where they are. It makes for a very electrically reliable turnout, even if the gaps are a bit unsightly. If you really feel up to the challenge of building a turnout with the very precise PCB work and clearance checking needed, however, it is theoretically possible to use a -3 and omit the rail gaps between the frog and the points.

  7. Our club has a great application for retrofitting you MRServo-3 servo product BUT our turnouts are all twin coil controlled by 2 push buttons, one for normal and one for the reversed position. Could/will you have an option for pushbutton control on your MRServo boards?

    Dale Gloer

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