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Controlling points/turnouts with Servos via the Arduino

I've managed to cook many Tomix Turnouts during my tinkering with the Arduino. The main issue has been applying too much current for too long. The actual switches that come with Tomix points are momentary and, internally, the circuit is only completed very quickly via a spring mechanism. The goal, of course, is to prevent the user from holding the power to the turnout magnet for too long. Unfortunately, I've managed to (via both coding and circuitry mistakes) apply too much power in the past and it takes very little time to melt the roadbase of Tomix Finetrack.

Due to all this, and the desire to use Peco Setrack, I'd decided that instead of Peco Turnout Motors (which also require large amounts of voltage) I'd use the smallest RC servos I could find. Off to eBay I went and the following arrived not so long ago.

9g Servo 9g Servo 9g Servo

I had no idea what servos to purchase: they seem to be rated in grams as to how much they can lift? I read in a few random locations across the web that 9g would be more than enough for switching points.

Hooking up Servos to your Arduino

This could not be easier. Arduino 1.0 already comes with the Servo Library in-built. Simply include this header and then implement the following code. The basic idea is to initialise the Servo on a specific pin (they only require one pin to be controlled) and then hook up the external power source. As per usual, it's not recommended to power too many off the USB 5v.

BareBonesWithServoAndEncoder

Mounting Servos to control Turnouts

Got some piano wire? A paperclip? Resistor legs? Any solid piece of wire/metal/rod will work to connect your turnout to a Servo. As you can see below, I've used a .55mm 'hobby wire' to connect everything up as I don't need flexibility and I want it to be robust.

Prototype Arduino + Servo controlling point Prototype Arduino + Servo controlling point

You could also be very tricky and build a full interlocking frame to control multiple points at once. I bought a few 'hinges' (no idea what the real word should be) to allow the rodding to turn corners but thought it easier in the end to just install another Servo :).

Rotary Encoders allow you to switch the Turnout yourself...

Rotary Encoders are 'endless' switches which usually come with 5 pins + GND. You can continually turn them, allowing for applications where you want an inifinitely adjustable value. The pins are as follows: one side has two pins which are for the 'pushbutton', as the actual stem can be pushed into the base and provides a momentary switch. The other three on the other side are for the rotor location. The inside pin is 'common' and needs to go to ground; the outside pins are 'data' and need to be hooked into digital inputs somewhere on your Arduino.

Rotary Encoder Rotary Encoder

You then simply download the rotary encoder library from PJRC, drop the main Encoder.cpp and 'utils' folder into your sketch folder and include the following source lines.

#define ENCODER_DO_NOT_USE_INTERRUPTS

#include "Encoder.h"
#include <Servo.h>

Encoder myEnc(7, 8);

Servo myservo;
long position = -999;
long srv = 0;
void setup() {                
  myservo.attach(5);
}

void loop() {
  long newPos = myEnc.read();
  if (newPos != position) {
    if (newPos < position) {
      srv += 1;
      if (srv > 180) srv = 180;
    } else {
      srv -= 1;
      if (srv < 0) srv = 0;
    }
    position = newPos;
    myservo.write(srv);    
  }
}

The code above will check if the rotary encoder has moved and, if it has, then check which direction and adjust the servo accordingly. Note that the servos will hum/jam if you try to turn them past any restrictions: i.e. once hooked to a turnout, the servo's movement will be limited and you should only move them as much as required... don't try and move them past the limits of the turnout. I'm quite sure that you will ruin either the servo or the turnout if you let it hum for too long past the movement of the switch.

What to do next?

Control your points based on timing? Or even based on track occupancy detection. Computerised turnout control will allow you to automate any movement over your layout. Of course, my current goal is to build a node for the OpenLCB project to control points via servos. This will need to store data, allow max/min settings per point, etc... but more as it gets built!

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About stevenh

Trains... trains... trains... + Electronics + Japan.
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  1. The ‘hinges’ you refer to are usually called cranks.


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