Servos for Points

Points /turnouts on my layout were initially operated manually using a wire-in-tube mechanism. There are currently 44 points on the layout and I started converting to servo control about half-way through.

The wire-in-tube (WIT) method I used worked very well and is shown to the right.
The mechanism is relatively easily fabricated and the cover is removable with one screw to allow easy adjustment of the point blades at the fascia.

It will handle a second wire to allow push/pull operation of 2 points in a crossover.
If there is any interest, contact me and I will add a section on the design and fabrication of the WIT method and the fascia mounted lever frames..

I changed to servo operation for the following reasons:

  • with this type of mechanical operation, control panels would not be able to operate the points (there are 4 mini panels on the layout).
  • the system selected allows precision adjustment of each point blade; variable speed of movement; position indication
  • servos can be installed from the top of a 50mm (+) foam layout
  • points can be controlled from a central position; a local panel; by a computer; as a route by changing many points at once and other options.
  • the system I use is based on the UK MERG model. Circuits vary from simple to complex.
  • Plus – I like working with electronics.
The photo shows the basic setup. The servo is located to the side of the point/ turnout and connected to it with a short “L” shaped wire. I use 1.2mm steel wire running through a short guide tube. This connects to the servo arm via a “Quick Connector”. The servo cable exits under the layout. SEE PHOTO
The mounting slot in the foam is formed by drilling 12mm holes through 50mm foam to fit a micro servo … SEE DRILLING HOLES IN FOAM
The guide tube I use is a short piece of bicycle bowden cable outer – used for gear selection from memory. I just bought a metre or so length from the local bike shop.
The servos I use are Tower Pro SG90 style or my preference Micro 9g Metal Gear Servo.
Shown above is a “Micro 9g Metal Gear Servo For Futaba Hitec HS-55 GWS walkera RC HELICOPTER GA” as described on eBay.
Here is a LINK to the eBay site I used but check other sources for a possible better deal. The current cost is $4.87 (16 July 2016) with free postage to Australia. There are cheaper plastic gear models but I have found these ones to work more smoothly and to be much quieter. As you can see, they come with a variety of servo arms (aka servo horns) and any will work in this application.
To connect the servo to the point/ turnout a “Servo Quick Connect” (shown above) is very good. I bought a set of 20 for $4.12 !! ($3.83 in June 2017)
That’s a little over 20c each. They were sold as “Durable 2mm Aircraft Stopper Servo Connectors Connector with Screws – Set of 20” and one eBay supplier I used was at this LINK . These things are TINY. The Allen Key grubscrew is 3mm.
This what you get for about 20c (Aust). A beautiful piece of micro engineering.
I use any of the servo arms supplied. Select the second hole from the pivot point of the servo arm and drill a 2mm hole in there to take the servo quick-connect. The point needs to move less than 3mm (HO) and initially I went for maximum torque by using the first hole and that is what is shown in the very top photo of this post. That hole is too close to the hub of the servo arm and some fiddly trimming was needed. I discovered that there is no need to use it as there is plenty of torque when using the second hole as shown above.
Assembly order is shown for the quick-connect. It needs to be free to rotate in the servo arm and the nut is best secured using a tiny bit of thread locker. The grub screw allows positioning adjustment but the travel and end points need to be set by some electronics.
This servo is being retrofitted to a turnout on EPS foam. As the control wire to operate the point was already in place, an additionally longer access slot was needed to allow the servo to slide in under the actuating (control) wire to the throwbar. The brass foam “drill” can be seen on the right.
Shown above – 2 MERG boards in use on my layout. The left hand one controls the servos (8 of them) and the right hand one handles the switches controlling the servos operating the points. This system was devised by the MERG group in the UK – see their Website MERG.  The operating system uses a CAN bus (2 wires) to distribute control events around the layout in a manner similar to that used in modern motor vehicles. You still need a distribution bus for DCC (plus, in my case, sub buses for the yards etc) and a bus for 12V DC to operate points and the power the CBUS boards.
This is a MERG design for TESTING SERVOS with the left hand one designed only to test servos – in this case I use it to test new servos and to set them to their midpoint.
The right hand one is, in effect, a stand alone method of controlling one servo & its point with a switch. The 3 blue components are variable resistors used to control the speed of the servo and distance it moves Left & Right. I also use it for testing the servos.

MERG sells kits for the above 2 projects and they cost just £1.55ea  +postage from the UK for MERG members. Almost all of the more sophisticated CBUS kits are based on professionally manufactured Printed Circuit Board (PCBs) and usually kits of the necessary parts are available or you can buy the parts locally.

There are other alternatives:

The device shown to the right is a ” MegaPoints Controller” by a UK company and could be very good for those people not confident in building PCBs themselves. I have not used it or seen it in operation but it comes ready to connect to 12 servos and has 12 corresponding switch inputs. Here is their Website and here is a YouTube Demo There are 2 videos in sequence. Cost is said to be £50 in the UK.

I also notice that DCC Concepts have an “above board” system that looks interesting for someone not interested in a DIY approach – Cobalt SS.

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Rick Fletcher

Born in the steam era and developed an interest in railways when given a clockwork Hornby "set". Surrounded by steam when travelling to school (by train of course) and holidays were always by steam train as we had no car. How lucky was I?

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