Point  Controller
for Linear Actuator


Cost: au$25.00 plus postage
Kits are available

see also:
Point Controller for Servo

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

This project controls a set of points, using a miniature LINEAR ACTIVATOR.
The PC board is the same as for the circuit driving the SERVO as both the servo and Linear Actuator take the same signal to produce motion from one end of rotation to the other.
It is best to read the article on controlling a servo as the only difference is the ACTUATOR.

               POINT CONTROLLER for LINEAR ACTUATOR  (the relay to
                  power the frog rails is not included in the kit)

The Linear Actuator is very small and ideal for this application.

Operates points using a Linear Actuator.     
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.

There are a number of beautifully-designed LINEAR ACTUATORS on eBay and one design accepts commands identical to SERVOS, but they cost twice as much as a servo.

This Linear Actuator is just a micro motor and pot.
It does not have any proportional-control circuitry
and cannot be used in this project.

This tiny linear actuator can replace an R/C servo
as it has a chip on the underside that accepts PWM signals
and will operate the points slowly - the same as
a "Tortoise Controller."

This Linear Actuator is available from Waltzmart.com for $11.00 with free shipping. You get 2 Linear Actuators for $11.00

Underside of the Linear Actuator showing the electronics

Linear Actuator connected to point.
One supplier on the web sells the Linear Actuator
for less than $6.00, making it a very good choice.  


The secret to making the Linear Actuator 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.   

Connecting a Linear Actuator to a set of points is very easy by using a push-rod.


Linear Actuators 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).



for Linear Actuator
Parts List

Cost: au
$25.00 plus postage
Kits are available

1  -  180R
2  -  220R
2  -  22k

2  -  100n monoblock capacitors
1  -  10u electrolytic
1  -  100u electrolytic

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

1  -  PIC12F629 chip (with Lin 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  -  Miniature Linear Actuator
1  -  Point Controller for Linear Actuator PCB (PIC Chip has Lin