|This projects converts a
manual point into a remotely-controlled point using a servo. The
switch in the kit controls the action and the servo in the kit does the
This project allows you to manually operate a point via a
Our project uses a SERVO, for
the activation of the point and
these have an output shaft that rotates nearly 300 degrees. But our
project limits the rotation to about 90 degrees and this is enough to
move the rails at the point via a rod called a "linkage."
An "arm" is fitted to the output of the servo to get linear motion
via a short
push-rod (from the arm to the rails).
The kit comes with the servo, switch and linkage to create a remotely
controlled point at less cost than any other way.
You need to connect the servo to the point with a stiff length of wire
called a LINKAGE (also called a PUSH-ROD).
This allows the servo to produce LINEAR MOTION and the project reduces
this linear motion to a very short distance to suit the "THROW" of the
point. This is done by reducing the rotational movement of the servo via
the circuit and using a hole close to the axel to produce just the right amount
This is the distance the point must be moved to change the rail(s) from
one position to the other.
Connection between the point and the servo is done with a short length
of copper or steel wire included in the kit and this can be called a PUSH ROD, PULL ROD or
It needs to be straight. It can be bent at the ends so it can be
connected to the servo and lever on the track but the actual rod should
be straight to deliver the "thrust."
You need to work out where to position the servo and then it
can be housed in a signal-man's hut as shown in the photos.
The servo is laid on its side to take up the least room and held in
position with double-sided tape.
When the servo has proven to be working correctly, it is glued to the
baseboard with acetone cement around the edge of the servo.
The circuit is quite unconventional and it's the first time you
will see such an usual design.
That's because we design unusual things.
The circuit delivers two different "timing values" to the 555 to
produce clockwise and anticlockwise rotation. These values produce about
90° rotation and this is all you need to get the "throw" required.
The LEDs illuminate
to show the direction of the point.
The project also includes a 2 metre extension lead so the servo can be
located 2 metres from your control panel.
The project will work from a wide range of voltages, including AC or DC
and as the voltage increases over 10v, the only effect will be the BC338
will get slightly warm if the point is constantly changed. The
transistor will pass 800mA so it is easily capable of delivering the 250mA for
The single diode on the power supply produces half-wave rectification
and although this does not produce very smooth DC, none of the circuit
is very sensitive to noise. The first 100u provides a small amount of
smoothing and the BC338 improves this enormously by only using the
voltage from 0v to 6v2 and the ripple will be above this voltage. This is
done by using a 6v2 zener on the base. The
output is improved further by the second 100u to give the smooth voltage
The 555 produces two different mark-space ratio arrangements at
basically the same frequency of 20mS cycles. (50Hz)
The first diagram below is an animation and it shows a signal that is HIGH for
1mS, every 20mS, and it will rotate the arm fully clockwise. When the HIGH is
about 2mS in width, (every 20mS), the arm will rotate to the fully anti-clockwise
(The diagram is not in proportion - the base-line is actually 20mS long)
The animation below shows approximately the same thing:
By selecting a HIGH between the two extremes, the arm will rotate about
The 47k and 100k produce the timing (the HIGH) for the 555 to move the arm on the
servo about 90°.
Either the 47k or the 100k is taken to the 5v6 rail via a PNP transistor
and is turned ON to feed
the 22n on the 555.
When the switch is changed to the other position, the other
transistor operates the servo via the 555 in the other direction.
The kit is cheaper than buying all the parts separately from local
stockists and the LEDs are
high bright because the current is only about 1 milliamp. The 90° header for
the servo allows the 3-pin plug
to be close to the board. There are a few components in the kit that are
not readily available separately and that's why a kit is the best
All the components fit on the board and the
LEDs show the position of the point.
The toggle switch can be fitted to the board or mounted on your control
The kit comes with a 2 metre extension lead (components) so the board can be
positioned near the operator and the servo plugged into the 3-pin
connector on the extension lead.
Close-up of the components
on the PC board
The output of the servo has the arm cut short so only one hole is
available for the thick copper wire.
Bend the wire around the arm and use a hot soldering iron to melt the
wire into the arm.
Keep the wire long so it provides a force to keep the moveable rail
pressed against the outer track.
BEFORE YOU START !!|
Before you start thinking about
buying the kit for this project, look at the range of POINT CONTROLLER
projects from Talking Electronics and make sure this is the one you
The "point" needs to be a manually operated point. This projects does all
It operates a single point and converts it to a
remotely-operated point. It is the cheapest conversion you can get and
uses a servo to do the work.
You can use AC or DC from about 10v to 14v (16v DC) and you can hide the
servo in a hut or shed. The LEDs on the PC board show the condition of
the track and the kit comes with a 2 metre extension lead. You just need
a little bit of expertise to fit the "push-rod" (linkage) between the arm
(on the servo) and the lever on the track that changes the position of the
point. The photos will help you do this.
Controller using 555 IC
plus $6.50 postage
(Pre-assembled kits $5.00 extra)
1 - 470R all
1 - 2k2
2 - 4k7
4 - 10k
1 - 47k
1 - 120k
1 - 2M2
1 - 22n ceramic
2 - 100u electrolytic
1 - 6v2 zener
1 - 1N4148 diode
1 - 1N4004 diode
1 - 3mm green LED (high bright)
1 - 3mm white LED (high bright)
1 - BC338 transistor
2 - BC557 transistors
1 - 555 IC
1 - 8 pin IC socket
2m - 3-core flat ribbon for
servo extn lead
1 - 2-screw terminal block
1 - 3-pin 90° header for
1 - 3-pin male for servo
1 - 3-pin female socket for
20cm very fine solder
1 - Servo with arms
1 - Double-sided tape to
temp hold servo
(glue needed to hold the servo in place)
1 - 10cm length thick copper wire for
1 - toggle switch SPDT
1 - Point Controller
555 IC PCB
|Talking Electronics has many
kits for Model Railways and each Kit has a different name so you
know which kit to order.