I need help please. Who can help me figure this out?

I need help please. Who can help me figure this out?

I am trying to construct a servo/servo controller to operate my HO model train switches (they are called turnouts in model railroad talk). Turnouts are a ‘fork’ in the train track that allow a train to go right or left on the tracks. This is a ‘switch’, to speak in a mechanical sense.

The train travel switches from to the right or the left by a turnout just as if you were walking down a path and came upon a fork in the path that went left and the other went right.To direct the train travel either left or right, a set of mechanical ‘points’ on the turnout move to the right or left a small distance.

Model Trains 101 - The two ways model trains are controlled:

A. DC Mode: Where the voltage on the track is variable from zero to 16 volts DC and adjusted by an audio or linear potentiometer, and

B. DCC Mode: Where the track has a constant voltage applied to it and the locomotive is controlled by a ‘decoder’ with a specific digital ‘address’. The constant voltage supply is +/- 12 VDC and can be up to 16 VDC.

I want to operate the turnouts by one or the other of the following methods: By DC momentary contact switch OR by DCC (Digital Command Control) through which a digital signal used to control the locomotives through a digital address ‘decoder’. Other electrical features like lights and turnouts can be controlled by connecting a decoder to the feature and assigning it a specific ‘address’. The decoder allows electrical power to the device being operated OR keeps power from it when the device is not to be operated.

I have the following criteria for my issue:

  1. I want to power the servo and servo controller off the power running to through the track 12-16 VDC up to 15 amps. I do not want to use batteries or have to wire separate power sources.

In the DC mode there will be no power when the system is shut down or a locomotive is not running. I understand that in DC mode I will have a lower voltage at times when operating the turnouts or no power even when the system is ‘on’.

In the DCC mode there will be no power when the system is shut down (the decoder maintains its address however with no power on and resurrects that address when the system is turned on).

  1. The servo and servo controller has to fit inside a container that is no more than 1 1/4" wide by 2 1/2" long by 7/16" deep. I have picked the Pololu #1053 servo and #1350 servo controller. The Pololu #1350 is a 6 channel servo controller but I only need a one channel servo controller I believe and I don’t know if there is such a thing.

  2. The movement of the ‘points’ of the turnout that are the object of the intended electrical to mechanical movement is no more than 3/16".

  3. The amount of resistance that force needs to overcome to move these points is negligible if the servo can provide a small positive pressure as the points come to rest at the end of their travel either to the left or right. The points move to the left to send the train on the right fork of the track, then move to the right to send the train to the left fork of the tracks.

  4. Each turnout servo/servo controller ‘set’ will have to be self contained and one set per turnout.

  5. The movement of the points over the 3/16” distance should be slow such as 2 to 3 seconds per movement.

  6. I want to be able to easily change from DC to DCC without taking this servo/servo controller apart. I run my locomotives by DC now but I intend to run my locomotives by both systems in the near future.

It’s a tall order. Now, who can help me figure this out?

Jack La Duke

P.S. I’m 64, disabled from my military service and have been out of electronics since my Air Force days of 1970. I am just learning from the webmaster how to post messages. I’m trying to get back into electronics but it will take me some time to get up to the speed you younger people are at. In the interim I need some help please.


Your goal is impossible as stated, since in DC mode there will be at times no power on the tracks, and you are not willing to have a separate power source. Are you okay with the servos not being able to move when the power is off?

If so, we can talk about how to do it. Please try to take this in really small steps. Consider getting a Maestro, a 5V wall-wart, and some servos and just playing with it without the railroad first.

I think your first goal involving the railroad should be to get a regulated 5V power supply for your servos from the railroad voltage. You could test this by connecting, for example, a small motor to the supply, and see if you can get it to spin at a constant speed regardless of what the trains are doing.

We supply a variety of regulators; of these I think the #2110 Pololu Step-Down Voltage Regulator D15V35F5S3 fits your requirements the best; it should be powerful enough to supply at least a couple of servos.

Please note that you will need to add something like a diode bridge to ensure that the regulator always sees a positive voltage, and you should consider installing big electrolytic capacitors on the input and output to try to keep the power as stable as possible and avoid Destructive LC Voltage Spikes. (That regulator has a big input capacitor already, but if you are working with really long tracks and potentially switching the power on and off very quickly, it might be a good idea to add even more capacitance.)

This is a complicated problem, and I do not know enough about model trains to be sure of how to do it, so I cannot guarantee that I have the right solution for you. But I will be very interested to hear about how your project goes!



Thanks for your evaluation of this. I have not checked the forum for a few days as I received no email notification of a reply being posted.

I will review your post.

Thank you,

You’re a scholar and a gentleman.

Merry Christmas!