BY LES KERR
Semaphore
Signal
For OO-Gauge Model Railways
This realistic-looking OO-gauge Semaphore has been modelled on a
real British semaphore. It has a red/white ‘flag’ that tilts down by 45°
and lights a green LED to signal an oncoming train to continue, or is
horizontal with a red light, indicating it should stop. It’s made from parts
that are relatively easy to obtain, although it requires some access to
machine tools and experience to build.
S
emaphore signalling was
one of the first signalling systems
used by railways. Semaphore signals were first patented in England in
the early 1840s. They were so successful that they were adopted throughout
the railway world. With the advent
of coloured lights, they were slowly
replaced, but a few remain in use. Adding them to a model railway makes it
look very realistic.
British signals come in two forms:
lower and upper quadrant. Lower quadrant signals pivot the arm downwards
for the off indication (trains can pass),
while upper quadrant signals pivot the
arm upwards for off. I decided to make a
lower quadrant signal as most of the old
signal photos I found showed this form.
Current British practice mandates
that semaphore signals, both upper and
lower quadrant types, are inclined at 45°
from horizontal to indicate ‘off’.
The British semaphore signal arm
consists of two parts: a timber or metal
arm (or ‘blade’) that pivots at different angles and a ‘spectacle’ holding
coloured lenses that move in front of
a lamp so the signal is visible at night.
To save having to make coloured
lenses, the lamp is replaced with a 3mm
red/green bicolour LED in the model.
When the arm is horizontal, the red
colour is switched on, and when it is
down, the green colour is on.
A miniature servo motor moves the
signal arm up and down (see Fig.1).
The servo collar is connected to the
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connecting rod (#10), which in turn is
connected to the lever (#3). When the
servo moves through 45°, the connecting lever does the same. As the connecting lever is joined to the signal blade by
the pin (#4), the signal blade follows the
movement of the servo collar.
In real life, the height of the signal
blade above ground was determined
by how far away it could be seen from
an approaching train. If you only have
a small layout, you can easily lower its
height to make it look to scale. This is
done simply by reducing the length of
the connecting rod (#10) and the mounting pole (#11).
We will present details of both the
mechanical and electronic assembly.
Just about any hobbyist should be able
to assemble the control board as it is
a simple single-sided design using all
through-hole parts. However, note that
making the parts for the mechanical
assembly will require some machining
experience and some machine tools.
Specifically, you will need a lathe;
just about any small one will do, as long
as it’s built to reasonable tolerances.
Most of the machining involves either
brass or aluminium, both of which are
relatively soft. You will also need a drill
press and a good selection of drill bits.
While you can probably get away
without it, to produce an exact copy
of the Semaphore presented here, you
will also need a basic mill with an end
mill tool, and the knowledge and ability
to use it – or a friend with these skills.
A video showing the Semaphore operating is here: https://bit.ly/pe-mar23-sem
Circuit description
The straightforward controlling circuit
is shown in Fig.2. The speed at which a
servo motor rotates is a function of the
servo itself. In the case of the semaphore
signal, we need it to rotate much slower
than its maximum speed to make it look
realistic. This is achieved by feeding a
series of pulses to the servo’s control
terminal, with a time delay between
each pulse.
When the up/down switch (S1) is
moved to the up position, digital input
RB0 of microcontroller IC1 (pin 6) goes
high, causing the microcontroller to produce a series of such pulses at its digital output RB1 (pin 7). The result is that
the servo motor moves slowly clockwise
by 45°. At the same time, digital output
RB2 (pin 8) is brought high and output
RB5 (pin 11) low, causing the red LED
to light.
The 100nF capacitor from pin 6 of
IC1 to +5V stops any contact bounce
produced by the switch. If the switch is
returned to the down position, RB0 is
pulled low by the 10kW resistor, resulting in another series of pulses from output RB1 that returns the servo motor to
its original position. At the same time,
output RB2 goes low and output RB5
high, resulting in the LED changing
colour back to green.
Servo motors are not as accurate as
stepper motors when moving through
Practical Electronics | March | 2023
Fig.1: this shows in detail what the Semaphore Signal looks like when it’s assembled and where all the pieces go. It’s
essential to refer to this diagram during each construction step to make sure the parts go together correctly.
a specific angle, being out by as much
as 10%. Similarly, any variation in the
position of the signal blade hole, the
LED plate or the connecting lever and
the servo collar can produce errors.
To solve this, two 1kW trimpots are
provided. The first varies the position
of the signal blade in the horizontal
position, and the second in the 45°
down position. The trim potentiometers vary the voltage on analogus inputs
RA0 and RB7 of IC1 (pins 17 and 13,
respectively). These feed into IC1’s
internal analogue-to-digital converter
(ADC) which converts the voltages
into numbers.
The microprocessor uses these values
to determine the pulse widths to produce in the two static positions.
Mechanical assembly
Many of the mechanical Semaphore
parts need to be made, and the details
of these are shown in Fig.3 (#1-9) and
Fig.4 (#10-14). They are made as follows.
#1 Cap and cap pin – I turned the cap
from a piece of 6mm aluminium rod by
mounting the rod in the three-jaw chuck
of a lathe, facing the end (ie, squaring it
off) and turning down the diameter to
5.2mm for 5mm. I then cut the 127.6°
taper. I reversed the job in the chuck and
parted it off to 3mm, then used a centre
Practical Electronics | March | 2023
drill followed by a 2mm drill to a depth
of 2mm, taking care not to break through
to the taper.
I made the cap pin from an 8mm
length of 2mm rod, glued in the hole
I drilled in the cap using Loctite GO2.
The shape of this item isn’t critical,
as it varied between different signal
manufacturers. Paint the cap assembly red.
#2 LED plate – I made this from a piece
of 1/32-inch (0.8mm) thick brass sheet.
The distance between the holes is the
critical dimension. Drill the holes, then
cut the plate to size. Finally, clean up
the edges.
#6 Railing – This was also made from
0.8mm (1/32-inch) diameter brass rod.
I turned a short length of scrap round
to 11.2mm diameter and used that as
a mandrel to form the curve. A small
amount of heat applied by a gas torch
makes bending easier.
The finished
Semaphore
will look
like this,
with wires
connected
to the PCB.
#3 Connecting lever – This was made
from a piece of 1/16-inch (1.6mm) thick,
1/4-inch (6.35mm) wide brass. Again,
the distance between the holes is critical. Drill the holes first, then cut and
file the lever to size. Paint the connecting lever blue-black.
#4 Pin – Cut a piece of 1/16-inch
(1.6mm) diameter steel rod to a length of
11mm. Clean up any burrs on the ends.
#5 Pillar (4 required) – Similarly, I made
these from 0.8mm diameter (1/32-inch)
brass rod cut to 12mm in length. Again,
clean up any burrs on the ends.
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Model Railway Semaphore Controller
Fig.2: the control circuit, which runs from a 5V supply, is quite simple. Microcontroller IC1 monitors switch S1 and,
depending on its position, sends pulses to the servo to control its angle while lighting either the red or green elements of
LED1. Trimpots VR1 and VR2 fine-tune the angles of the flag in the horizontal and down positions, respectively.
#7 Platform base – This is made from a
piece of 1/32-inch (0.8mm) brass sheet.
Drill all the holes, then cut the plate to
size. Next, using a fine saw and file, cut
out the square section so that it is a tight
fit around the 1/8 inch square mounting pole (see #11 below). Finally, clean
up the edges.
#8 Ladder support – This is made from
a length of 0.8mm (1/32-inch) diameter brass rod. Use a piece of 1/8-inch
(3.2mm) square brass as a mandrel to
form the shape. Again, a small amount
of heat applied by a gas torch makes
bending much easier.
#9 Support – Place a piece of 12mm
diameter aluminium rod in the threejaw chuck of a lathe and face the end.
Turn it down to 20mm to make it a slide
fit in a 3/8-inch (9.5mm) diameter hole.
Use a centre drill followed by a 4.3mm
(11/64-inch) drill to bore it out to a
depth of 20mm. Next, reduce the end
to 5.25mm diameter for 8mm and part
it off to length.
Finally, drill and tap the hole in the
side for the 2.5mm grub screw. Paint the
support blue-black and when dry, then
fit the 2.5mm grub screw.
#10 Connecting rod – I made this from
0.8mm (1/32-inch) diameter brass
rod. Bend one end of the rod through
90° but only bend the other through
about 20°. This is because the rod has
to pass through the 2.6mm hole in the
5.5mm-thick base. We will bend it to
90° later in the assembly process. Paint
the connecting rod blue-black.
#11 Mounting pole – The mounting
pole is made from a length of 1/8-inch
(3.2mm) square hollow brass tube. Drill
the 1/16-inch (1.6mm) diameter hole at
40
92mm from the pole end. You can make
the slot by drilling two 1mm holes 1mm
apart and using a file to remove the
remaining metal.
Make sure that the insides of the slot
and the insides of each end are free of
swarf and are smooth – when we insert
the LED wires, we don’t want to cut
their insulation.
#12 Base – My layout is made on a 2-inch
(51mm) thick sheet of polyurethane
foam. I buried the signal in the foam so
that it was flush with the top of the base.
This left a 0.5mm step down all around
the Semaphore that I later filled with
ornamental grass, so that the base was
more in keeping with the scale. Depending on your layout, you might decide to
leave out this step down.
The base is made from 6mm aluminium plate. Cut it to size, then drill and
tap the required holes. I made the step
using an end mill in a milling machine.
Paint the base blue-black and when dry,
fit the 2.5mm grub screw.
#13 Servo bracket – This is made from
1/16-inch (1.6mm) thick aluminium
sheet. Drill the two 3mm holes 29mm
apart, then cut it to size. Clean up the
edges with a file.
#14 Servo collar – Place a length of
12mm-diameter aluminium bar in the
lathe three-jaw chuck, face the end and
turn it down to a diameter of 9.8mm
for 10mm. Bore it out to a depth of at
least 5mm using a centre drill followed
by a 4.8mm diameter drill. Part off a
3mm section, transfer this to the drilling machine and drill the 2mm hole for
the grub screw. Thread the hole with a
2.5mm tap and fit the grub screw.
Finally, drill the 0.8mm diameter hole
exactly 4mm from the centre.
Mechanical assembly
With the parts now made, refer back to
Fig.1 to see how they all go together.
The LED plate (#2), platform (#7) and
ladder support (#8) are all soldered to
the mounting post. Clean, tin and flux
the mating surfaces between the LED
plate and the mounting post. Insert a
temporary pin in the 1/16-inch (1.6mm)
hole and use it to align the two pieces.
Using a small blowtorch, heat the assembly until you see solder coming out of
the joint. File off any excess solder.
Now clean, tin and flux the mating surfaces between the platform and
the mounting post. To align the plate
squarely, use a small timber cube as a
support and clamp it to the mounting
post. Using a small blowtorch, heat the
assembly until you see solder coming
out of the joint.
The next step is to solder the four
12mm pillars into the platform. Do this
one at a time using a soldering iron. To
keep them vertical in this operation,
drill a 0.8mm hole vertically into a piece
of scrap timber into which you insert the
pin. The railing can then be soldered
into place, making sure it is parallel to
the platform. File off any excess solder.
Next, clean, tin and flux the mating
surfaces between the ladder support and
the mounting post. To keep it level, make
a small timber cube for it to rest on and
clamp that to the mounting post. Using a
small blowtorch, heat the assembly until
you see solder coming out of the joint.
File off any excess solder. The whole
assembly can then be painted white.
Signal blade
The signal blade is a PCB, coded
09103222, measuring 31 x 20.5mm and
available from the PE PCB Service – see
Fig.5. Using a small pair of side cutters,
carefully remove the blade from the
Practical Electronics | March | 2023
Fig.3: this shows the smaller parts (#1-#9) that need to be made. Some can be made on a lathe, while others require a saw, files
and drilling. #6 and #8 are made by bending thin cylindrical bar stock on rectangular formers. Note: all dimensions are in mm.
Fig.4: the remaining parts to make, including the larger items (#10-#12)
plus a detailed view of the partially assembled Semaphore at right.
PCB. You can also snap it at the weak
points deliberately created by holes
drilled into the supports. Clean up the
blade edges with a file.
The PCB is coloured red/white, and
you can easily paint the spectacle area
(see Fig.6) by masking it and applying
spray paint, painting it with a brush, or
even using a black permanent marker.
However, if you aren’t happy with
the PCB colour, or you made the flag
Practical Electronics | March | 2023
some other way, you can download the
artwork (Fig.6) from the March 2023
page of the PE website (https://bit.ly/pedownloads), print it on a colour printer
and cut out the front/back shapes.
Use two-part five-minute epoxy to
glue the front shape onto the face of
the blade. Once dry, carefully clear
the paper from the holes. Glue the rear
label on and again remove the paper
from the holes.
Control module
The heart of the semaphore signal
is built on a single-sided PCB coded
09103221, which measures 51 x 37mm;
also available from the PE PCB Service.
Fig.7 is the PCB component overlay diagram. Start its assembly by fitting the
PCB pins, then the IC socket.
The reason for the IC socket is that
there is no provision for in-circuit programming. Therefore, the next step is to
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download the firmware from the March 2023 page of the PE
website and program the PIC16F88-I/P microcontroller using
an external programmer now, before fitting it.
Take care to orient the socket/IC correctly. Next, add the
vertically-mounted resistors; you can replace the 0W resistor with a wire link. Follow with the capacitors; check that
the electrolytic types are the correct way around, with the
longer leads to the + symbols. Next, add the 1kW trimmer
potentiometers and temporarily connect the servo motor
and LED1 (as per Fig.8).
Finally, connect the positive of the 5V power pack to +5V
and the negative to 0V. Check that all the connections are correct and that there are no dry joints or solder bridges.
At this stage, don’t plug in IC1 yet if you have used a socket.
Testing
Switch on the power supply and connect the negative lead
of a voltmeter to pin 5 of the IC socket and the positive lead
to pin 14. The meter should read +5V. If it reads –5V then the
IC socket or IC is the wrong way around.
Switch off the power and insert the IC (if you used a socket),
checking that it is correctly oriented. Switch the power on, and
Parts List – Semaphore Signal
1 single-sided PCB coded 09103221, 51 x 37mm (controller)
1 double-sided red PCB coded 09103222, 31 x 20.5mm
(blade) – both PCBs available form the PE PCB Service
1 PIC16F88-I/P microcontroller programmed with
0910322A.hex (IC1)
1 5V DC power supply
1 DF9GMS 9g micro servo [eg, Core electronics
SER0006]
1 18-pin DIL socket (optional; for IC1)
2 1kW mini top-adjust trimpots (VR1, VR2)
1 3mm red/green LED, three-lead type (LED1)
[element14 Cat 2148798]
1 miniature SPDT toggle switch (S1) [eg, Jaycar ST0300]
2 M3 x 16mm panhead machine screws (for mounting servo)
10 1mm PCB pins
1 10mm length of 1mm diameter heatshrink tubing
various lengths and colours of light-duty hookup wire
1 tube of Loctite GO2 adhesive
1 tube of Tarzan’s Grip or similar adhesive
Capacitors
1 100μF 16V electrolytic
2 10μF 16V electrolytic
2 100nF 50V multi-layer ceramic
Resistors (all 0.25W 1% metal film)
1 10kW
1 5.6kW
1 4.7kW
1 2.2kW
1 820W
2 680W
Mechanical parts
1 300mm+ lengths of 0.8mm (1/32-inch) dia brass rod
1 20mm+ length of 1.6mm (1/16-inch) diameter steel rod
1 20mm+ length of 2mm diameter aluminium rod
1 20mm+ length of 6mm diameter aluminium rod
1 40mm+ length of 12mm diameter aluminium rod
1 103mm length of 3.2mm (1/8-inch) square hollow
brass tube
1 20mm+ length of 1.6mm (1/16-inch) thick, 6.53mm
(1/4-inch) wide brass bar
1 20 x 20mm rectangle of 0.8mm (1/32-inch) thick
brass sheet
1 46 x 55mm rectangle of 6mm-thick aluminium sheet
1 35 x 7.5mm rectangle of 1/16-inch (1.6mm) thick
aluminium sheet
1 OO-scale ladder
3 2.5mm grub screws
42
the LED should glow green. Short S1’s two terminals together
and the LED should now glow red, while the servo motor
should rotate 45° clockwise (looking at the shaft). Remove the
short, and the servo should rotate 45° anti-clockwise while the
LED should change to green.
Add the short again and switch off the power. Leave the
servo in this position as it will make the final assembly process easier. Now is a good time to give the bottom of the PCB
a coat of clear varnish to protect it from corrosion.
Final assembly
Refer back to Fig.1 during final assembly to see how the Semaphore goes together.
1 Push the red/green LED into the LED plate. Before trimming the leads as short as possible, note which is the shortest as this connects to the red LED. The centre lead is the
common, and the other goes to the green LED.
2 The connecting wires must be very fine to fit through the
mounting pole. I found suitable wires in an old computer
mouse connecting cable. I selected red, yellow and black
and made them about 300mm long. Using a fine-tipped
soldering iron, connect the red wire to the red LED terminal, the yellow wire to the green LED terminal and the
black wire to the common (middle) terminal.
3 Cut a 5mm length of 1mm diameter heatshrink tubing and
slide it over the wires. Insert the wires one at a time into
the post until they protrude from the end. Be very careful
not to strip the insulation off in this process. Straighten
up the wires and shrink the tubing down over the exposed
portion of the wires using a heat gun.
4 Insert #4 (the 1/16in [1.6mm] diameter steel pin) into the
signal blade and lock it into place using Loctite GO2. When
dry, slide the assembly into the mounting pole (#11).
5 Push the support (#9) into the base (#12) with the grub
screw in the support on the right-hand side when looking
at the front of the signal. Tighten the grub screw in the base.
6 Push the three wires at the bottom of the post through the
hole in the support, then push the post into the support
and lock it temporarily in place using the grub screw in
the support.
7 Take the connecting rod (#10) and push the end with the
20° bend up through the base and platform to the signal
blade height. Use pliers to increase the 20° bend to 90°.
8 Insert the end of the connecting rod into the 0.8mm (1/32in)
hole in the connecting lever (#3) and push the pin attached
to the signal blade into the 1.6mm (1/16-inch) hole in its
other end. With the signal blade horizontal, adjust the position of the connecting lever so that it is parallel to the axis
of the signal blade. Lock the connecting lever temporarily
in place with a blob of glue.
9 Attach the servo bracket (#13) to the base using the two
16mm M3 screws. Align the servo motor as shown in Fig.1,
and attach the servo collar to the shaft with the grub screw
hole at the bottom.
10 Loosen the grub screw holding the mounting post in
place. Slide the servo motor assembly under the retaining
bracket. By adjusting the height of the post, you should be
able to align the connecting rod with the 0.8mm (1/32in)
hole in the servo collar. Push the end of the connecting
rod into the collar.
11 Move the servo until the connecting rod is vertical, then
lock it in place by tightening the screws. Adjust the column
height until the connecting lever at the top of the signal is
horizontal. Tighten the grub screw holding the mounting
post in place and the grub screw in the servo collar.
12 Check that the signal blade is parallel to the front of
the mounting base. If it is not, loosen the grub screw
in the base and rotate the post until it is. Tighten the
grub screw.
Practical Electronics | March | 2023
Fig.5: the semaphore flag is too small
for most PCB manufacturers to make
by itself, but they will make this larger
PCB which can be snapped or cut apart
(at the holes represented by black filled
circles) to give you something very close
to the correct flag shape. After cutting
or snapping it out, all you have to do is
file the top and bottom edges flat.
Fig.6: this
artwork can be
downloaded,
printed, cut out and
glued to the flag if it
isn’t already coloured
or you aren’t happy
with the colour or
surface finish.
Wiring
Wire up the signal as shown in the wiring diagram, Fig.8. Check this before
applying power, as reversing the supply polarity will destroy IC1.
Then, with the switch closed, apply
power. The LED should glow red, and
the signal blade should be horizontal.
Open the switch; the LED should light
green and the signal blade should move
down about 45°. Operate the signal
several times to make sure it changes
over smoothly and that nothing is binding. Check the tightness of the three grub
screws and the servo screws.
The two potentiometers on the PCB
allow you to fine-tune the position of
the two holes over the LED in the signal blade. The potentiometer closer to
the LED connections on the PCB (VR1)
adjusts the position of the signal blade
in the horizontal position and the other
(VR2) in the down (45°) position. Once
you are happy with the blade position,
use a drop of Loctite GO2 to glue the
connecting lever in place.
Fitting the cap and ladder
Attach the red cap and pin assembly
into the top of the mounting pole using
Loctite GO2.
Take an OO-scale ladder length and
paint it blue-black. When dry, lay the
ladder up against the platform support
and check that the top rung is level
with the platform. Cut it to size and
use Loctite GO2 to glue the ladder to
the platform and support. I deliberately
didn’t glue the ladder to the base, as that
would stop the post assembly from being
adjusted later.
Using it
The Semaphore could be combined with
a level crossing, such as my design (July
2022), or you could use it on its own,
such as before a switch or a station.
The simplest method is manual control. Position a toggle switch at a convenient location in the layout. With the
Semaphore in the stop (horizontal) position, manually stop the train in front of
it. Then, switch the Semaphore off at
an appropriate time, and the train can
move away.
There are also methods to automate
it. For example, if used near a level
crossing, you could arrange for the
Semaphore to usually be in the stop
(horizontal) position and then automatically switch to the down position
when the level crossing boom gates are
fully down. It could change back to the
stop position as soon as the boom gates
start to lift.
All you need to organise this is to have
a microswitch or reed switch arranged
so that it is open when the boom gates
are fully down and closed the rest of
the time.
If you can’t easily do that, the other
option is to use a delay circuit that’s triggered by the same signal that activates
the level crossing. Set the delay so that
it closes a set of relay contacts or activates an open-collector/drain transistor
after the boom gates have had a chance
to fully lower.
Use those contacts or that transistor to
trigger the Semaphore into its off position, and arrange it so that the contacts
open or transistor switches off as soon as
the Level Crossing trigger switches off.
You could also consider positioning a reed switch under the tracks and
placing a magnet in the train. This way,
when the train pulls to a stop in front
of the Semaphore, it triggers a delay circuit that disables the Semaphore signal
after a couple of seconds. It would need
to hold it off until the train has passed,
possibly sensed by a second reed switch.
I’ll leave the details of that arrangement
as an exercise for the reader.
Reproduced by arrangement with
SILICON CHIP magazine 2023.
www.siliconchip.com.au
Fig.8: once you’ve assembled the
Semaphore and the control PCB, here
is how to wire them up. Be very careful
to get this right, especially the 5V power
and servo wiring, or you could damage
IC1 or the servo when you apply power.
Fig.7: it shouldn’t take long to
assemble the PCB as it only has a
handful of parts on it. Make sure the
chip is programmed first if you’re
going to solder it directly to the board
and watch the orientations of the
electrolytic capacitors (the longer
leads are positive).
Practical Electronics | March | 2023
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