MRServo Slow Motion Turnout Control

Ever wanted to automate the turnouts on your layout?  Maybe you have some that are hard to reach?  Or maybe you have a hidden staging yard?  Do you model a modern CTC-controlled subdivision and want to give your dispatcher realistic control over the turnouts?  Or maybe you have a multi-deck layout and other switch machines are too bulky to sit below the upper decks?  No matter what the situation, MRServo is a cost-effective solution!


What is MRServo?

MRServo-1 Control Board

MRServo is a low-cost, low-profile, slow motion model railroad switch machine using small and inexpensive RC servos to control the turnouts on your layout.  Three options are available to suit almost any situation.  The first, MRServo-1, is the simplest and least expensive.  It does its job of controlling the turnout, and does it well, without any bells or whistles.  MRServo-2 and MRServo-3 provide extra features in the form of accessory contacts.  With two sets of accessory contacts (DPDT form), the MRServo-2 is well suited for simple power routing and integration with signal systems and control panel indicators.  MRServo-3 provides a single set of accessory contacts in addition to a set of PowerFrog contacts which intelligently route power to solid frog turnouts.  PowerFrog eliminates the need for special current limiting circuitry or frog “juicers” in these situations.

Each kit comes complete with the control board, servo, hardware, throw wires, and a piece of heavy-duty mounting tape.  Instructions are also provided in each kit and an installation tutorial can be found here.


As with all Iowa Scaled Engineering products, MRServo is guaranteed for life against manufacturing defects.  And for you adventurous modelers, it is an open-source / open-hardware product.  Full schematics, design files, and source code are available on the product documentation pages.  We encourage you to build your own, improve upon it and, if you so choose, submit your ideas back to us so everyone can benefit.


PowerFrog Technology

MRServo-3 Control Board

Available exclusively on the MRServo-3, PowerFrog technology provides a reliable, short-proof way to route power to solid frog turnouts. (Examples: handlaid, Peco Electrofrog™, Shinohara™, and Walthers™ models.) These turnouts will create a short if the points make contact with the opposing rail before the contacts driving the frog switch polarity. Traditionally, the solutions have included additional gaps in the point rails to isolate the frog, various current limiting devices, or active frog “juicers”.  With PowerFrog, this is no longer necessary.  When MRServo starts to throw the points, it momentarily isolates the frog. Once the servo completes throwing the points, it sets the direction relay to the correct polarity and, after a small delay, re-energizes the frog.

Putting this intelligence in the switch machine has many benefits.  First of all, it works with any control system: DCC (any make), pre-DCC command control, traditional DC, 3-rail, etc.  Secondly, since the switch machine already knows the position of the turnout, you no longer have to create an intentional short-circuit to signal the frog “juicer” to change the polarity.  Third, the overall cost can be significantly less.  So, protect your wallet and your investment in turnouts and use MRServo as your switch machine of choice!


Installation of MRServo is straightforward, but please be sure to follow the directions.  After you have installed a few, the process goes very quickly.  A tutorial on our recommended installation procedure can be found here.

As mentioned above, MRServo is ideal for space-constrained applications such as multi-deck layouts.  Both the control board and the servo itself are small, resulting in a very low-profile installation.




  1. Looks like a good product. For doing a double crossover would need two of the crossover packs with one control module?

    1. We don’t recommend driving more than 2 servos from each control board due to the peak current that each servo can pull. Your best bet is to use 2 controllers, each with the crossover option, and wire the control inputs together.

  2. The MRServo turnout controllers just take a logic level input signal, so any DCC accessory decoder, C/MRI output, or similar device supported under JMRI can be used to drive the control input on the MRServo board. For interfacing to accessory decoders, see our post on that topic here.

  3. Hi. Very interested in MRServos, – not just for turnouts, but I also have two interlocking semaphores I want to actuate. Problem is, it is a mechanical interlocking (“Armstrong”) so they should change fairly fast, not slow motion. Can I adjust the speed of the servo or must that be preprogrammed at the factory? I don’t want them to “snap” but not move too slow, either. Picture pulling a lever in the tower probably one second or less. Thanks.


    1. The throw angle and speed are adjustable, but the controller must be reprogrammed. We can do this for you prior to shipping, or if you are familiar with programming microcontrollers, you can do this yourself. One of these days we will post a how-to on (re-)programming MRServo controllers. In the meantime, I will try to get a video of the default angle and speed posted in the next few days so you can see the defaults and then let us know how you would like it modified.

        1. Unfortunately, not yet. We can send some basic instructions if you are interested – please email us: You will need a PICkit2 or 3 and the necessary software installed to program the PIC microcontroller. The source code can be found on GitHub.

  4. working with N.J. International #2162 N scale grade crossing lights with crossing guard. I will need a Flasher for the lights,a dection unit and a MR-1 servo unit to activate the crossing. A Dallee #367, A Circuitron #FL-1 and a MR-1. Do I have it correct?

    1. Sorry it’s taken me a little bit to get back to you – Michael’s out this week and it’s just me minding the assembly, testing, support, and shipping departments.

      Did you mean a Circuitron DT-1, by chance? Then yes, that looks like about the right list. You’d connect the “OUT” terminal of the DT-1 to the CNTL terminal on the MRServo-1 and the negative input terminal on the 367. The GND terminal from the MRS-1 would connect to the “-” terminal on the DT-1, as well as the negative side of your 12VDC power supply. The +V on the MRS would connect to the common + input terminal on the 367, the + terminal on the DT-1, and the positive side of your 12VDC power supply.

      The only thing is that the MRServos are designed specifically for use with turnouts and have a fixed throw angle to make installation easier. If you’re using it to animate something like a crossing gate, you may have to do some tinkering with the length of the throw arm to get the right amount of throw. Also, I can’t say as I’ve ever seen a 2162’s mechanism, but you might need to add some sort of stops to keep it from throwing too far and damaging something. The included 0.025″ throw wire is pretty springy, however, and will flex nicely if you provide it some stops.

  5. Does your PowerFrog technology take care of a short circuit that would occur during a ‘wrong-way’ entry into a turnout?

    My understanding of the frog ‘juicers’ on the market detect a short circuit & then quickly reverse polarity, but don’t do anything regarding the points set in the wrong way, leading to a derail.

    Seems like the ideal situation would be to detect a short circuit, reverse polarity AND activate the turnout to change to proper setting. Does your PowerFrog control board do this? Seems like it would be possible since your electronics is all on the same board.

    Forgive me if I’m misunderstanding this, as I haven’t used DCC yet. Here’s a little background the help understand my question.
    I’m in the planning stage of a small switching layout which will be my first experience into new technology beyond old DC. It will be DCC or Ring Engineering RailPro. Leaning to RailPro.

    I want to use automated slow motion turnouts. I haven’t decided on ‘stall motor’ or servo but am leaning to servos. I like your open source concept. I also like the sensor switches like the Barrett Hill ‘touch’ sensors that can light the direction on a control panel diagram. Would those sensors be usable to activate your controllers? If not, would a momentary contact switch activate them?

    I am trying to think through everything that would be required. Lots of questions I know. I appreciate any help you could offer.

    1. Sorry for the delay in responding – with Christmas last week, I’m a bit behind on things.

      Tam Valley’s Frog Juicers work as you describe – they’re essentially little DCC auto-reversers. They detect a momentary short and immediately change polarity. However, you are correct that they will not change how the turnout is lined.

      Our Power Frog idea works a little differently, and is geared at solving a slightly different problem. It’s specifically designed for the older Shinohara, Walthers, Peco Electrofrog, and some handlaid turnouts where the point rails and frog are electrically one piece. If you feed the points/frog on one of these turnouts through the auxilliary contacts on a conventional switch machine, if the track points don’t change at the exact moment the switch machine’s auxilliary contacts change, it’s possible to create a dead short. While it would clear itself after a fraction of a second, on DCC that’s still enough to trip off the booster or breaker. PowerFrog provides a set of contacts that actually interrupt power to the frog & points entirely while the points are moving, and then reconnect it once the throw is complete. It solves much the same problem that the Juicers do, just in a different way.

      Unfortunately, neither of these does what you’re looking for – change the points over if the frog shorts out. The problem would be distinguishing the frog shorting because of a train vs. the frog shorting because the points are moving. It could be solved relatively easily, but currently neither of our products fit your needs.

      As far as Barrett Hill’s Touch Toggles, they certainly can work with our controllers. I have a bunch that I ordered to test out for my own layout. Expect a blog post about it sometime in the next couple of weeks, but the short version is that you have to get +5V to power them from either the servo output or from the programming pins, and otherwise ground and the control line just connect up on the input terminals. If you’re interested I can try to get my blog article done sooner.

  6. As a former R/C flyer, I have several micro servos laying around and was wondering if you would sell just the MRServo controller board?

  7. Guys,

    I was wondering is it possible to wire in indicator leds with the MServo-1 controller? And what is the limitation on cable length? I am using Cat3/4 cable.

    1. Yes. There is a 6-pin header (J3) where you can solder wires and get access to the signals that would normally drive the relays on the -2 and -3 versions. See the schematic for details. Length shouldn’t be an issue for any practical purposes.

      1. Michael,

        Thanks for the reply and it was great to meeting and talking with you at RPM. Looking at the schematic, I not sure if its possible to use bi-colored LED’s. Can you make a recommendation?

        1. A bi-color LED can’t be driven directly. You’d need some extra circuitry in order to get both colors to display.

          1. Michael,

            Nathan was helping me with this Bi-colored LED option at the RPM meet this past June. He provided details for me to wiring in a 3 legged LED to the MRServo-1 board but I have seemed to misplaced it. Can you or Nathan provide me those instructions again?

            Thanks a bunch,
            Jim Tatum

  8. I would like to use 4 MRServo-1 controllers with 2-SP90 servos per controller using wye servo cables, plus allow 90 degree servo motion/actuation – not about 30 degrees for turnouts! Is this possible? How long can the servo cables be? What is the cost for you to reprogram them?

    Thanks Dennis

    1. We can custom program 90 degree travel, just let us know in the comments when you place the order. I’m not sure of the maximum (and it depends on the situation), but the servo cables can be at least several feet long.

Leave a Reply

Your email address will not be published. Required fields are marked *