Point Controller
for Motor/Gearbox
using DPDT Switch

Kit is available from
Talking Electronics for $15.50
(incl servo) plus $6.50 postage. 
Click HERE  to buy kit.



Talking Electronics has produced a number of Point Motor Controllers, but this project uses a motor and gearbox to control the points.
In other words it is a controller-circuit AND a POINT MOTOR - all for $15.50
The kit contains a miniature motor / gearbox. This is a miniature SERVO that needs to be modified by removing the electronics and connecting two leads directly to the motor.
A DPDT push-switch is also included in the kit. This push-push switch can be substituted for a toggle switch for $1.50 extra.
In other words this project is a controller-circuit AND a POINT MOTOR - all for $15.50
All you have to do is connected the arm (commonly referred to as a "horn"), on the output shaft of the servo, to a manual set of points and you can control them remotely. Each time the DPDT switch is changed, the voltage to the circuit reverses and the opposite transistor conducts for a short period of time while the electrolytics on the base charge. This activates the motor to change the position of the points.
There is a small leakage through the transistors of about 20mA at all times and that's why the project cannot be connected to a battery. 

The "servo" is just a motor and gearbox (electronics removed)
It can be fitted in many different ways.
Laying it on its side will be the best option.

The block diagram shows the project uses a Double-Pole Double-Throw switch to reverses the voltage to a motor contained in a servo, to change the position of the rails at a "point." The project also has a timing feature to prevent the motor "jamming" and burning-out.

A Simplified block diagram of the project

The miniature motor and gearbox comes from a servo (commonly called a servo motor) and all you have to do is remove the PC board inside the case and connect two leads directly to the motor.
The gearbox delivers a very high torque to the output shaft and we need to be careful not to supply full voltage to the motor when it has reached its "limit-of-travel."
That's why the circuit reduces the current to the motor by the time it reaches the "end-stop."
The transistors are designed to limit the current by the action of the electrolytic charging via a resistor and slowly reducing the current into the base.
When the supply is reversed via the switch, the initial current is high and gradually tapers off when the arm reaches the limit of its travel. This means the motor has very little torque by the time it reaches the end and prevents "jamming."
The 4screw terminals on the PC board have been positioned so each connector aligns with the 4 terminals on a DPDT switch. Two criss-cross wires are fitted to the switch to connect the other two terminals. This makes it easy to wire the switch. The kit comes with a DPDT push-switch that you can change for a toggle switch.
The PC board also contains a red LED and green LED to let you know the position of the points.
This is a very low-cost project for activating a set of points.

PCB overlay and trackwork

Point Controller project connects to Motor/Gearbox  (modified Servo)
The DPDT toggle switch is $1.50 extra

The Push Switch is provided in the kit.

All the parts fit neatly on the PC board.
The transistors, diodes, electrolytics and LEDs are identified above and the diagram shows how they are fitted to the board.
There is one unusual thing on the board. The AHEAD and SIDING LEDs are placed in opposite directions. One LED has the cathode lead "down" and the other has the cathode lead "up," so one LED illuminates when the voltage is in one-direction and the other LED illuminates when the voltage is reversed.
The screw terminals allow the board to be easily connected to your power supply via leads into the 2-connector block. The motor connects to another 2-screw connector while the DPDT toggle switch connects to the 4-screw block. 

Only a motor and gearbox are required for this project. We have used a servo because they are cheap and compact.
Open up the servo and remove the PC board with electronics. Connect two leads directly to the motor and re-fit the case. Check the motor on 1.5v for activation in both directions.
Fit an arm to the output shaft and use the tiny screw provided to hold the arm in place.
It is now ready for connecting to the track with a push-rod.

The arm of the servo produces a certain amount of THROW.
This is the amount of lineal distance moved by each of the holes in the arm.  If you select a hole near the shaft, the "throw" will be very small. As you select a hole further from the shaft, the "throw" increases.
You have to match this distance with the amount of movement needed by the moveable rails, called "blades."  Once you select the required hole, the unused part of the arm can be removed. This will allow you to place the servo in a number of positions next to the track, so the arm can be connected with a push-rod. This can be a small length of spring-steel from a spring.
The servo has two mounting holes and you can make a small "L-bracket" to fit it to the base-board. Otherwise it can be glued in pace with clear silicon sealant. Make sure the action of the servo produces a small amount of pressure to keep the blades against the fixed rails when it reaches the end of travel in each direction. 

Point Controller
for Motor/Gearbox
$15.50 plus $6.50 postage. 
 HERE to buy kit

3 - 1k
2 - 2k2
2 - 4k7

2 - 100n monoblocks - 0.1spacing
4 - 100u 25v electrolytics

1 - BC337
1 - BC557
1 - 3mm red LEDs
2 - 3mm green LED

4 - 1N4004 diodes
1 - 7805 regulator

4  -  2-Connector terminal blocks

1  -  DPDT push-switch
4 - short leads for switch
1  - Miniature Servo (needs to be modified)
      with servo arm
1 - Point Controller for Motor/Gearbox
           PC Board

DPDT toggle switch (
in place of push-switch) $1.50 extra