Operating points remotely with servo testers

There are two sets of storage sidings on the Peckforton Light Railway - one is situated at one end of the line in the lean-to porch and the other is in the garage at the other end. Until now, the two turnouts at the throat of each set of storage roads have been controlled manually which usually entails hopping inside to change the points and out again when transferring stock to and from the sidings. This isn't a major inconvenience as the sidings are only used at the start and end of an operating session. However, I've always felt that it would be a lot easier if I could operate the points remotely from outside the garage and lean-to.

I solved the problem of operating the lean-to storage siding points by using point rodding (see Operate  points with simple rodding)

However, this option wasn't feasible for the points in the garage as the entrance is much tighter. I then wondered if I could operate them with servo motors - maybe controlling them with cheap servo testers.

These handy little gadgets can be bought for as little as £2.00(ukp) from a well known online auction site. They are used to test servos - twiddling the knob will make any connected servo move.

My mate Greg Hunter in Australia mentioned how Dave Bodnar had modified a tester to operate turnouts on his model railway and so I consulted Dave's article and started experimenting - http://www.trainelectronics.com/Servo-simple-controller/index.htm

I did a few initial experiments (see PostScript below) and found the modification I needed to make to the circuitry wasn't as complicated as Dave's original suggestion. He added a resistor, presumably because the amount of throw he needed for his 16.5mm gauge points was less than I needed for my 45mm gauge points.

So, here's what I did

 

The servo control box

Firstly, I bought a couple of servo testers from eBay, after checking with the seller that they did not go through an unnecessary initial warm-up routine when turned-on (see Postscript below)

The cover was taken off by pulling off the knob and ungluing the overlapping edge of the cover.

The board was flipped over and a wire was soldered to the middle contact of the potentiometer (the square thing with a knob on top in the middle of the board )

NOTE: Alternatively, the wire could be soldered to the middle leg on the upside of the board

The other end of the wire was then soldered to one of the contacts on a single pole double throw (SPDT) toggle switch

 NOTE: Any single pole on-off switch can be used - slide switch, latching push button, rotary switch.

Another wire was soldered to the middle contact on the toggle switch. Its other end was soldered to the negative pin on the board

 NOTE: The wire can be connected to any negative contact on the board or even on to the battery or power supply connector.

 At this stage I checked everything was working as anticipated and then adjusted the knob on the servo tester until the arm on the servo turned through the right amount of arc to operate the point when the toggle switch was flicked. On LGB points this is approximately 7mm.

 

As I wanted to operate two points, I did the same again with another servo tester.

I decided to mount my testers and switches in a small hand-held equipment case which I happened to have to hand. 

The servo testers were attached to the base of the case with double sided sticky pads

As the case included a battery compartment for a 9v PP3 battery, I soldered the leads of a battery snap connector to the input pads of a 5v voltage regulator board and soldered to wires from the output pads to the power input pins of the servo testers.

NOTE: There are several designs of voltage regulator boards available on eBay. I tend to use these with variable voltage outputs to cover all eventualities. Just search for "Step down buck converter". They should cost no more than £2 - £5 (ukp) each
 

 Some even come with screw connectors if you are unhappy with soldering on to small pads

I then mounted the toggle switches in the lid of the case, and included a third toggle switch to turn on the power from the battery.

A couple of JR extension leads were attached to the output from the servo testers and the case was then screwed back together

NOTE: The JR servo extension leads also came from eBay and can be bought in a variety of lengths from a few centimetres to 2m - eg.

 Mounting the servos

Although the track in the garage is screwed to chipboard baseboards, I wanted the servos to be mounted on the baseboards for ease of installation and for maintenance.

A couple of pieces of 4mm thick plywood (70mm x 35mm) were shaped to take the servos, by creating a 40mm x 20mm cut-out. Four 4mm diameter fixing holes were drilled using the servo brackets as a template.

 NOTE: I opted to use larger servos as I happened to have a couple on hand. Mini servos such as the Tower Pro Micro could be used, especially if space is tight. Plenty available on eBay for around £3.00 - £5.00 each (even cheaper if you buy in bulk)!

A couple of 70mm long pieces of 12mm x 20mm softwood were also cut out on which to mount the brackets

These were screwed together and the servos bolted on with M4 bolts and nuts.

The longest servo arms (or horns) were then screwed to the hubs of the servos.

Linkages were made from piano wire. The zig-zag is to allow for some expansion and contraction of the linkages to help ensure the point blades are firmly pressed against the stock rails.

 One servo was mounted beside the innermost turnout ensuring it was in the ideal position for the linkage to be effective.

As the outermost turnout is located in the access hole to the garage, there was insufficient room for the servo to be mounted alongside it and so one of the horns which came with the servo was modified to make a bell crank.

This was screwed into a Rawlplug inserted into a hole drilled in the brickwork beside the tie-bar of the point.

A short linkage was placed between the bell crank and the tie bar and a slightly longer linkage was fashioned to link the bell crank to the the servo.

After some extensive testing, the cables were pinned to the roof of the entrance hole to enable the control box to sit outside the opening when it's needed.

When not in use, the control box sits on the track inside the opening.

 

Conclusion

I am very happy with the way the servos and control box perform. It certainly overcomes the need for me to keep trotting in and out of the garage to change the innermost point when running stock in and out the storage roads.

Although I opted to power the control box with a 9v PP3 battery, I could just as easily used a 5v plug-in power supply as there is mains power in the garage. It would have required a power input lead to run alongside the servo cables but would have obviated the need for a step-down voltage regulator.

Again there are plenty of 5v 1A mains power adapters on eBay

Cut off the DC plug and attach one of the servo plugs from JR servo extension lead.

In summary

 

 

 

Post Script - and a warning!

Not all servo testers work in the same way - as I discovered to my cost! Make sure you buy the right type. They look identical but operate slightly differently.

Some testers go through a warm-up routine when first switched on. They send the servo through a complete rotation left and right before settling down. This will wreck the linkages between the servo and the tie-bar on the point.

Check with your seller before buying that his (or her) testers don't include a servo warm-up procedure. I simply sent a message to the seller on eBay and got his assurance before buying a couple. If the seller doesn't reply you can assume that he either hasn't got a clue or he can't be bothered to reply - so isn't worth your custom.

Initial testing and modifying

Before producing the final version, I set up a simple test-rig. I adapted the circuitry shown in Dave Bodnar's article ......

 ..... ie attaching a couple of wires to the contacts below the potentiometer and then connecting these wires via a resistor to a toggle switch

 I rigged up the tester to a servo connected to a point and did some testing.

All worked well. The turnout could be switched over just using the toggle switch. The knob on the tester being used to finely adjust the amount of throw. All was well until I switched everything off, had a cup of coffee and returned. 

When I switched everything back on, the servo did a complete rotation in each direction before settling down - and, in the process, pinged the linkage between the servo arm and the tie-bar across the workshop. OUCH!!

More testing revealed that of the six servo testers I had available, five of them performed the (for me unnecessary) warm-up routine - thereby making them useless for this project. The only one which didn't go through the warm-up was one which I had purchased several years ago.

I therefore had to buy another servo tester - but checked with the seller first that it didn't perform this unnecessary warm-up procedure.