Using servo motors to control turnouts

Servo motors are gaining the attention of model railroaders due to their low cost and small size. I will cover the Digitrax product this month and will cover the Walthers product next time.

No soldering is required! Even if you are good at soldering, the ease and speed of using a screw terminal or slip-on connector can be quite appealing, particularly when working upside-down under a layout.

The Digitrax DS78V is the servo motor-controlling stationary decoder from Digitrax (digitrax.com). It’s the same size as the company’s compact DS74 ❶ but can control twice as many switch machines (eight vs. four), twice as many routes (16 vs. eight), and optionally, trigger 16 routes. Like the DS74, you will need to use LocoNet for the routes feature to control turnouts on other accessory switch controllers. You can configure the routes using EZ-Routes or a DT602 throttle, just like the DS74 ❷.

Finally, you can power the DS78V from either your track bus or a dedicated 12V supply. See the January 2022 DCC Currents for more details. You can also use the DS78V to operate threeposition semaphore signals.

The DSXSV9 is Digitrax’s servo switch machine kit based on the popular 9G servo ❸. It consists of a servo motor, a mounting bracket, an arm on the servo, and a piano wire. Digitrax provides two pre-formed piano wires – 0.24" (0.6mm) diameter and 0.032" (0.8mm).

A little bit of assembly is necessary, requiring just three screws. You can put a switch machine together in just a couple of minutes.

A note of clarification is due. The motor mounts from the back of the mounting bracket, with the actuator arm sticking through the front. If you get this wrong, you’ll find that piano wire hangs up on the actuator arm. Fixing it is no big deal, and you’ll build the rest of your switch machines correctly.

For HO and N, Digitrax recommends using the hole farthest from the pivot on the servo arm. This will provide about 1⁄4" of travel 1" above the switch machine. You may want to experiment with the other holes closer to the servo’s pivot if you don’t need as much travel or holding force.

I found it interesting that the Digitrax instructions stated that the 0.024" piano wire would probably be adequate, especially since turnouts like the Peco Unifrog and the new Walthers turnouts have no hinges on the points and are a bit stiffer. I removed the spring from a Unifrog and tried the 0.024" piano wire with this switch machine. It didn’t hold the points against the stock rails. I found I needed the 0.032" piano wire to drive the points against the stock rails as desired.

I should also mention that the piano wires that come with this product are more than long enough to get through 1 1⁄4" of layout and 3⁄16" cork roadbed with plenty still protruding through the switch rod to snip off. Be sure to wear safety glasses when you do this, and use a cutter made for hardened steel wire. Piano wire will ruin your usual track cutters.

You should not try to move the servo arm manually. The DS78V has three buttons that you can easily use to center the arm as well as to verify that the servo properly moves the points to the closed and thrown positions.

Modelers love the challenge of saving money. If you

want to roll your own switch machine, you can buy 9G servos at a hobby shop that sells radio control products. The connector on the servo is an industry standard. Fans of eBay can find 9G servos there for even less ❹. There are also metal gears available for them on eBay. You’ll just have to fabricate or 3-D print your own mounting brackets to use these servos. Cables to extend the distance between the servo and the Digitrax controller can be bought on eBay, or you can use the Walthers cable kit, no. 942-113 (walthers.com).

Servos have no provisions for powering a turnout frog. If you want to power the frog, an all-electronic Frog Juicer by Tam Valley Depot (tamvalleydepot.com) will do it.

Like the DS74, you can use two DSXC4 breakout boards to make connections for wires coming from switches or sensors. Similarly, you can use the DSXCP1 to operate a turnout from your control panel. See the January 2022 DCC Currents for photos of these products.

Peco has expanded its Unifrog line. I first covered these in the March 2021 DCC Currents. The new offering is a code 70 no. 6 turnout. Peco (peco-uk.com) also hopes to have a no. 8 turnout Unifrog later this year.

The manufacturer didn’t just expand the line; it’s also made some refinements to the code 70 and code 83 Unifrog turnouts.

The manufacturer has extended the area of the frog that you can optionally power ❺. If you had a problem with metal wheels bridging both sides of the frog rails, causing a short, this refinement should take care of the problem ❻.

Peco has also relocated the jumpers on the back of the turnout that transferred power between the frog and closure rails ❼. They no longer risk the possibility of shorting to the wire for powering the frog. (To fix the original code 83 Unifrog, run this wire out to the side of the turnout, away from the frog, before dropping it through your benchwork. For insurance, I used Walthers Goo to hold it in place.)

Enjoy exploring the use of small servos and the new Unifrogs! For one convenient link to all the links in my columns, go to WiringForDCC. com/dcc_currents.htm.

MODELERS LOVE THE CHALLENGE OF SAVING MONEY. YOU CAN BUY 9G SERVOS AT A HOBBY SHOP THAT SELLS R/C PRODUCTS. – ALLAN

Here’s a money-saving tip: I recently discovered that the inexpensive 7-function digital meter available at Harbor Freight for $6.99 (item no. 63759) can accurately read DCC track voltages.

I was able to get the same reading when compared to an RRAmmeter.

❶ The Digitrax DS78V. All connections to Digitrax’s DS78V are solderless. The screw terminals are for connections to your track bus. You can power the DS78V from your track bus – a dedicated bus is preferred – or a wall transformer. The PS14 wall transformer can be plugged into the top, as can LocoNet. The servo motors plug onto the triplet connectors. Buttons can be connected to the two 10-pin connectors.

❸ Servo switch motor. When mounted vertically, the DSXCV9 requires a mere 2 inches of clearance beneath your benchwork. For some of you, this low amount of clearance required may be a game changer. When attaching the servo to the mounting bracket, insert the servo from the back of the bracket so that the pivot protrudes through the front.

❷ Programming routes. Routes within the DS78V are most easily programmed using the Digitrax DT602 throttle. See my January 2022 column for other features that the DS78V and DS74 share. Simulated throttle display used with permission of Digitrax

❺ Peco’s new code 70 Unifrog. Compare the frog improvement to the original Unifrog ❻. Note that the frog rails now contain small segments. These are electrically connected to the frog tip. Now wide metal wheels won’t short as they go over the frog rails.

❼ Relocated jumpers. You can see where the jumper wires that connected the closure rails to the frog rails were removed to prevent shorting to the frog wire. To route power to the frog rails, jumpers were added to connect them with the stock rails. I routed the frog wire up in this example. This isn’t necessary on the improved Unifrog.

❻ The original Unifrog. Compare to ❺ for the small but important change that should prevent wheels from shorting on the frog rails as they pass over the frog.

❹ Generic 9G servo. These devices, available at radiocontrol hobby shops and on eBay, are for those who want to make their own actuators. If you do this, don’t forget the arms for the pivot and connectors for the end of the servo cable.