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.
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.
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.