Servo controlled level crossing gates
Entry posted by Barry Ten
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Ideally you'd design and install an operating level crossing at the early stages of layout construction, but things don't always work that way. When I built the boards and put in the track for the layout, I hadn't really been thinking about servo motors and so on, regarding such things as far beyond my ability to work with. Once I got further along with the model, and started gaining more experience, I began to think it would be nice to have operating gates, so while there wasn't yet a plan in mind, I was careful to keep the area under the crossing free of additional clutter.
About a year ago, a model aircraft enthusiast gave me a demo of the servo system on one his planes, and it looked intuitive enough that I started looking around to see what was available. The push I needed, though, was Peco releasing their new SmartSwitch product, which includes four servos and associated electronics all in one package. I bought the kit and started playing around with simple ideas to mount the servos under the boards. Due to access issues, it would have to be done simply, and also without creating too much dust and debris, to shower down on the N scale layout underneath. In the end the only modification to the existing boards/scenery was the drilling down of a vertical hole beneath each gate post, which was accomplished without too much bother.
After opening the box of parts, I cobbled together a prototype mounting bracket which could be screwed under the boards, with an operating rod pushing up through the board and through another two inches of loft insulation. Peco recommend wiring up the servos on a test bench to gain some familiarity with the control logic, and I thoroughly endorse this. Though, I have to say, the programming and operation is very straightforward, and for once the manual is actually informative. With no prior hands-on experience of servos, I found it dead easy to set start/end points and operation speed - basically, it does what it says on the tin and you don't have to be an electronics bod to get it working.
The servos come with a set of different nylon cranks for various actions, but all I wanted was a direct drive like a slow-moving axle. However, I had my doubts about whether it was wise to simply fix a long rod directly into the centerline of the servo. In the end, I used one of the cranks, then fashioned a kind of "reverse crank" from piano wire to return the motion to the centerline - all a bit cumbersome, perhaps, and maybe not necessary, but it works. The crank passed up through a brass and plastic tube to the top of the layout, and the post of the crossing gate is then drilled to accept the last 20mm or so of the piano wire. I knocked up the mounting brackets from thick plastikard, and the tubes and wire were from my usual stocks of modelling supplies. Everything else was already in the Peco package, which comes with plenty of screws and bits of wire.
My plan had been to make the gates an interference fit on the piano wire, so that they could be easily removed, and would "give" under the impact of a train, should I fail to open the gates. However, that proved unworkable - two were stiff enough that it was practical, but one ended up with too sloppy a fit, and I reluctantly decided to retain the gates with a small drop of superglue. I'll just have to take care with the gates, like on the real railway...
Here's a shot of the servos in place on their brackets:
There was a lot of adjustment needed to get the four gates opening and closing neatly, and to compensate for small errors in drilling the posts and the holes through the boards, but after some trial and error I was happy with the consistency of movement of the gates.
The servos have nine speed steps, of which the default is step 5. I tried step 1, the lowest, but found it both too slow and too jerky, so in the end I settled for a compromise of 4, which seems about right for layout use. The gates are controlled by individual switches, which can be thrown as one, to open/close them in one go, or they can be worked separately.
All in all, I am very impressed with the Peco product. Doubtless you could source the individual components more economically if you knew what you were looking for, but to have it all in one box, with good, clear instructions, makes a lot of difference. If you're like me, you'll soon start thinking of other applications for servos, such as signals, animation etc.
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