Point  Controller
for Servo


Cost: au$25.00 plus postage
Kits are available

see also:
Point Controller for Linear Actuator

This project is: Point Controller for a Servo
See: Point Controller for Linear Actuator

This is a project you will want to add to your layout.
It detects a train and produces a pulse to alter the points to alternately divert the train to the siding or allow it to travel ahead. 


This project controls a set of points via a SERVO.
A SERVO is a motor and gearbox with an output that rotates up to 270 degrees.
By adding an arm (or crank) to the output, we can get linear motion via a "push-rod" to change a set of points.
This project does not operate Peco solenoid points or Tortoise Controllers. It only controls a SERVO.
Don't get confused with a SERVO and STEPPER MOTOR.
A Stepper Motor produces accurate rotation by means of a directly applied signal to poles (coils) surrounding an armature and by pulsing the coils in the correct sequence, the armature rotates a few degrees on each impulse. It has no internal electronics and generally produces continual rotational movement. A Servo produces up to 270 degrees of rotation and then it generally reverses direction.

                         POINT CONTROLLER using a SERVO 

There are many ways to position the servo to allow the lever to control the points.
Placing the servo in its side will give better alignment.

Operates points using a SERVO.     
On-board LEDs indicate "ahead" or "siding"
Output to relay to power frog rails

These features are contained in our simple design and can be built in an evening.  A kit of components is available from Talking Electronics as well as a pre-programmed microcontroller.

Size-comparison of the TORTOISE™ Slow Motion Switch Machine and BluePoint with Servo:

Size comparison: Tortoise, BluePoint and Servo

Comparison of prices for converting from manual to automatic:

  - slow activating Point Motor  $20.00
Blue Point - slow activating Point Motor  $13.00
Servo  - free with Turtle Point Controller project.

Turtle Point Controller project - $25.00 (comes with ALL components, PCB and medium-size servo - not micro motor version).

Using a SERVO is the cheapest way to automate a set of points and produces fully automatic operation for  approx. $25.00

Turtle Point Controller project suits either a LINEAR ACTUATOR or SERVO. Both activate proportionally to the signal received via the signal line. Both have an IC that receives the signal and produces a pulse to the motor to turn it either clockwise or anticlockwise and the internal electronics compares the signal received from a pot or linear resistor to determine the angular or linear position of the arm.  This project does not work with TORTOISE  or Blue Point™ controllers.

R/C (Radio Control)  SERVO
A SERVO is the cheapest way to control a set of points.
This project operates the servo slowly to get the effect of a TORTOISE CONTROLLER with the advantage of placing the servo next to the track so you  don't have to cut a hole in the layout or any other awkward installation. It is also much cheaper than any other controller.

This is one of the servo's we tested. We call it MEDIUM SIZE. The LARGE servos are too big for this application.



"Cut-Away" view of an expensive servo with metal gears.
The all-plastic versions are much cheaper and have
a very similar compound gearbox to reduce the
output RPM and increase the torque.

A very SMALL SERVO uses a micro motor. A Micro Motor is designed "INSIDE-OUT."
The armature is wound on a former and the turns are GLUED TOGETHER with resin.
The former is then removed and the armature mounted over a strong cylindrical magnet that has a North and South pole, with the North on top and South on the bottom. There are 3 sets of windings, just like a 3-pole motor and the ends are terminated at 3 copper segments called a commentator. This commutator can surround the shaft or be located at then end of the winding. Two brushes touch the commutator segments to deliver current to the windings.
A coreless motor can produce twice the torque of a conventional motor because the armature does not have any iron. The iron absorbs magnetic flux and gets hot.
The armature is also further away from the centre of rotation and this produces more torque for the applied current. 
Micro Motors are also known as PAGER MOTORS, where they come with a weight connected to one side of the shaft (called an eccentric weight - not centric). When the shaft rotates, the motor vibrates (as in a mobile phone).
This type of motor can be produced with a case or body having a very small diameter and a short overall length and this allows a SMALL servo to be produced.

The commutator and brushes can clearly be seen in this photo.
A Micro Motor operates just like a 3-pole motor.
The winding is "skewed" (twisted) to produce
smooth rotational torque.

An "Exploded View" of a brushless motor

The servo in the kit is the MEDIUM SIZE  and comes with a set of HORNS. These are commonly called CRANKS or ARMS or LEVERS and connect to the output shaft with a splined connection (grooved) to prevent the arm slipping on the shaft. Some come with a screw to hold the arm in place.
Make sure you get a SERVO with a bag of horns.

Servo Arms or "Horns"
Single Horn or Arm
Star or Cross
Double Horn or Arm

The secret to making the servo operate slowly is a program in the micro. It creates a "MARK" (the length of time the control-line is HIGH) that tells the servo to advance (rotate) the output shaft a few degrees. The program then creates a Mark to advance the shaft a few more degrees.
This produces a slow, jerky movement, of the output. Each value is outputted a number of times and this creates the slow motion.   You can use an R/C servo or Linear Actuator.  

Connecting a servo to a set of points is very easy by using an arm and a push-rod (pull-rod).
The arm and rod turn the rotary motion of the servo into linear motion and the distance traveled by the rod is the greatest when the output of the servo rotates 180°.
 The distance is called the "THROW" and a number of holes on the arm (also called the "CRANK") selects the throw to suit the distance needed for the points.
The arm and rod is called the "LINKAGE" and this needs to be designed so that the rail sits with a small amount of pressure to keep it in place.

Using a "Star" or "Cross" to produce a linkage to operate the points with the servo near the track:

Using a "STAR"  in the linkage to connect to the rails
for  90° activation.

Max left and right movement produced by this
project is about  -85° + 85°

The project is designed to produce about 90 degrees of rotation for the servo. This produces enough "throw" to move the rails via a linkage. 

Here is a photo of the components in a servo from HOW AN R/C SERVO WORKS:

A Block Diagram of the electronics:

Arm positions for PULSE WIDTHS from 500uSecs to 2500uSecs

Most servos will rotate up to 180° and some will rotate to nearly 270°. Our project only needs about 45 degrees rotation to produce a THROW of about 10-12mm when a push-rod is connected to a hole near the axle. The track-points for HO gauge only need 8mm throw and this can be obtained very easily by connecting to a crank on the output of a servo.  

Servos come with different colours on the 3-pin connector:


The control line is called the "signal Line" and requires a waveform that is classified as a DIGITAL SIGNAL. This means it must rise to about 5v and down to about 0v to for the circuit inside the servo to respond.
The time when the signal is high is called the MARK and the low time is called the SPACE.
The width of the MARK determines the position of the output and it only takes a few cycles for the servo to respond and drive the motor to the angular location where the received signal matches the signal from the input potentiometer.
The signal on the control line is called PULSE CODED MODULATION and the HIGH will vary from 0.5mS to 2.5mS.
The LOW  time needs to b e about 20mS.
This means the coded signal arrives at about 50 cycles per second (50Hz).




Point Controller
for Servo
Parts List

Cost: au
$25.00 plus postage
Kits are available

1  -  180R   all 0.25watt
1  -  330R
1  -  220R
2  -  2k2
2  -  22k

2  -  100n monoblock capacitors
1  -  10u electrolytic
1  -  100u electrolytic
2  -  1,000u electrolytics @35v

1  -  1N4148 diode
1  -  LM78L05 voltage regulator
2  -  BD679 Darlington transistors
1  -  3mm red LED
1  -  3mm green LED
2  -  IR emitter LEDs
2  -  IR receiver transistors

1  -  PIC12F629 chip (with routine)
1  -  8 pin IC socket 
1  -  2m fine screened lead (for testing)
3  -  20cm hookup wire (for testing)
3  -  2-way terminal blocks
1  -  3-way terminal block
1  -  20cm very fine solder 
2  -  Surface Mount PC boards (for train 
         detector pair)
1  -  Small Servo
1  -  Point Controller for Servo PCB