Model Railway
Level Crossing
BY LES KERR
This scale-model Railway Level Crossing has realistic moving barriers,
flashing lights and a bell sound recorded from a real level crossing. It can be
triggered automatically when a model train approaches. It’s controlled by a
couple of low-cost PIC microcontrollers and can be built for a modest sum,
assuming you have some basic model-making and electronic assembly skills.
D
uring the recent COVID-19
lockdown, I decided to build
a model railway layout in OO
gauge. As time went on, I added buildings, a tunnel, a bridge, a pond and
many other items, including a level
crossing. This level crossing can be
triggered manually, or automatically
when the train passes It includes arms
that automatically lower and raise,
flashing lights and a realistic bell sound
(video at: siliconchip.com.au/Videos/
Level+Crossing).
This article describes how you can
build your own level crossing just
like mine.
OO scale is 4mm:1ft which works
out to 1:76.2. I applied this scaling
to images of signs taken from fullsized crossings. For other items, like
the red flashing lights, servos, barrier,
and posts, I used slightly bigger parts
than the scaled-down real-life items.
The bell sounds were recorded from
an actual crossing.
The Level Crossing project involves
building two boxes with posts that sit
on either side of the railway tracks
where a road meets them. When the
train approaches, they drop their arms
to block vehicles from crossing the
tracks while simultaneously flashing
their lights and sounding alarm bells.
24
Once the train has passed, the lights
and bells turn off, and the arms lift
up again.
Initially, the arms/gates opened and
closed at a speed determined by the
servo motor manufacturer. This speed
was excessive compared with the reallife version, so I developed a circuit to
move the arms in small steps, with a
delay between each.
The easiest way to do this was to
use an inexpensive microcontroller
programmed to produce the correct
number of steps, with a delay between
each, covering the angle that the arm
needs to move through.
There are four red LEDs on each
post: two facing each way, and they
flash alternately in pairs (with the
LEDs connected back-to-back illuminated together).
Due to the alternate flashing, normally you would need three wires to
connect them up – one to each LED and
one common to both. But the hollow
post is so small that it is only possible
to fit one wire up the centre; using the
brass post itself as a conductor gives
just two wires.
The way around this is to put
the pairs of LEDs to be illuminated
together in series, then connect those
pairs in inverse parallel. This way, if
a current is applied across the set of
four LEDs in one direction, two are
illuminated, and if the current flow
direction is reversed, the other two
are illuminated.
The only problem with this is that
to illuminate all the LEDs you need a
‘full bridge’ type of driving arrangement that can drive one end of the
LEDs high while it drives the other
low, or vice versa. Luckily, this is easily achieved with a pair of microcontroller digital output pins.
Circuit description
Refer now to Fig.1, the Level Crossing circuit diagram. It is based mainly
around two PIC12F617 8-pin, 8-bit
microcontrollers.
When the start switch (S1) is closed,
digital input GP2 on IC1 (pin 5) is
taken high. The resistor and capacitor
help to debounce the switch contacts.
In response, IC1 brings its GP4 digital output high (pin 3), switching on
MOSFET Q1, which applies 5V to the
recording/playback chip (IC3) with
the bell sound recorded on it. IC3 is
wired in the continuous mode by connecting pin 2 to pin 13, which results
in the bell crossing sound being produced constantly from the connected
8Ω speaker.
Practical Electronics | July | 2022
Level Crossing Controller for Model Trains
Fig.1: circuit diagram for the Level Crossing Controller. This project uses two PIC12F617 ICs, which saves on extra
components – for example, a 555 timer and some transistors that would be needed to flash the LEDs.
The sound continues until Q1’s
gate is brought low by microcontroller IC1, switching it and the playback
module off.
I was going to use a 555 timer to
flash the LEDs, but the two-wire
requirement meant that I would have
to add extra transistors. An inexpensive microprocessor fits the needs perfectly, hence IC2. It probably would
have been possible to build this function into IC1, but that would make the
timing tricky as IC1 also has to generate
servo pulses with accurate timing. A
separate chip makes that easy.
At the same time as GP4 goes high,
IC1 also brings its digital output GP1
high, which indicates to IC2 to start
flashing the LEDs alternately. IC2’s
digital pins GP4 and GP5 are configured as outputs. Initially, GP4 is taken
low and GP5 high, resulting in two of
the LEDs on pole one and two on pole
two glowing red.
Half a second later, GP4 goes high
and GP5 low, causing the LEDs that
were lit to extinguish and the other
LEDs to light. This sequence is repeated
until the start switch opens and IC2’s
pin 6 input (GP1) goes low again.
Shortly after the lights and bells
are triggered, IC1’s GP0 digital output
produces a series of pulses that go to
the servos, causing them to move the
Practical Electronics | July | 2022
arms slowly down until the servo arm
is horizontal. It remains down until a
couple of seconds after the start switch
opens (at which point the flashing
lights and bells cease), resulting in
the arms moving up slowly to their
full upright position.
Switch options
The original design uses a toggle
switch for S1, with the Level Crossing operated manually. The operator simply switches it on when the
train approaches the crossing and
switches it off after the train has
passed through.
However, some constructors may
hanker after automatic operation.
This can be achieved by gluing a
strong magnet somewhere on the
train floor, then positioning two reed
switches at strategic points underneath the track. They must be positioned so that the magnet passes over
one before the train reaches the level
crossing, and the other after it has finished passing through.
Ideally, the magnet should be underneath the train so that it passes as
close to the tracks as possible without
actually hitting them. However, with
a strong enough magnet, you might
get away with fitting it inside one of
the carriages.
Be careful not to place the magnets
right next to the reed switches, as this
could demagnetise the switches, making them useless.
An alternative version of the firmware for IC1 (ending in B) changes the
function of pin 5 on IC1 to toggle the
Level Crossing on and off each time
that pin transitions from a low to a
high level. Therefore, wiring both reed
switches across the S1 terminals will
provide the required behaviour.
If you have more than one set of
tracks going through the level crossing
(eg, trains going in both directions),
you could wire more than two reed
switches in parallel. However, note
that odd things will happen if you have
trains passing through the crossing in
both directions at once.
If you want to support that case
properly, you will need to develop a
small external circuit that handles the
logic to trigger this circuit, and you’ll
probably want to stick with the A firmware in that case. The logic could consist of two S/R flip-flops with their outputs wired through an OR gate, going
into pin 5 of IC1.
Note that the B firmware could also
be used with a momentary pushbutton
type switch wired across S1, to allow
the operator to manually toggle it on
and off if desired.
25
Fig.2: a 1:1 scale diagram
of the mechanical
construction details for
the unit. Note that the
servomotors have their
mounting arms modified
so that they can be
mounted sideways.
Fig.3: the label artwork for the
various parts of the Railway Level
Crossing. This is shown at actual size
and can be downloaded from the July
2022 page of the PE website.
Construction
There are two main parts to the construction: the electronic assembly,
which is pretty straightforward, and
the fabrication of the boxes, poles,
arms and other pieces that make up
the level crossing, which generally
will take longer. As it is most of the
work, we’ll start with the mechanical assembly.
The mechanical parts drawing
(Fig.2) shows the dimensions and
quantity of the parts to build the crossing. I will go through each piece and
describe how I made them.
Mounting post
This was made from a length of hollow
square brass 3/32-inch (about 2.4mm)
extrusion. Mine was made by K&S metals, which most model shops stock.
You have to drill a 1.5mm hole 48mm
from the bottom as the exit hole for the
LED power wire. Using a small round
file, clean up the hole and the ends so
that all burrs are removed that might
cut the insulation on the wire.
Backing plates
There are six of these, all made from
0.5mm brass sheet, also from K&S
metal. You will need two of each of
26
the rail crossing backing plates, track
backing plates and stop backing plates.
Using a small metal saw, cut out the
required size and then use a file to
round the edges and remove any burrs.
Barrier
You will need two; I made them from
1/16-inch (1.6mm) blank PCB scraps.
You can draw up the shape on the PCB
or trace the shape from the label. Drill
the 7mm hole and cut the barrier from
the PCB using a saw and file.
LED holder
I turned these up on a lathe by bolting eight square pieces of 0.5mm-thick
brass together on a mandrel, each with
a 3mm hole in the centre.
Alternatively, buy some brass washers with a 3mm centre hole (the LED
diameter) and an outside diameter of
about 6mm (not critical). If the inner
hole is slightly larger than 3mm, you can
hold the LED in place using glue. The
washers should be painted matte black.
Post mount
This is an optional part that adds a
bit more realism. Because the base of
my model railway was made of polyurethane, I had to insert a metal plate
under the rails to which the crossing
parts were mounted. I drilled a 6mm
hole in the plate and held the post
mount in place with Loctite. It’s a simple turning job to make the part out of
aluminium round.
Servo arm
The miniature servo is supplied with
a servo arm that has to be cut to size.
The mechanical drawing shows the
arm before and after modification –
you need two, one for each barrier.
The barrier is glued to this part of the
assembly, as described later.
LED assembly
Make two LED assemblies, as shown
in Fig.2. Use pliers to bend the leads
so that you put limited stress on the
LED connections. Cut the leads to size
and solder them together. The anodes
of the LEDs are marked with ‘A’ on the
drawing. At this stage, don’t solder it
to the post.
Servos
So that the servomotors can be
mounted on their sides, it is necessary
to remove the mounting arms. Use a
hacksaw to cut them to the size shown
on the drawing.
Practical Electronics | July | 2022
An example of what the finished barrier and railway crossing sign looks like.
Sign labels
Fig.3 shows the three sign labels and
the covering for the barrier. To make
these, download the 1:1 scale label
drawing as a PDF from the July 2022
page of the PE website and use a colour
printer with 80gsm paper.
Print the drawing and measure the
100mm line. Let’s say it measures
99mm. This gives a calibration factor
of 100/99 = 1.01 or 101%. So if you
print the file again at 101% scale, the
100mm line should measure 100mm.
Mechanical parts assembly
The first step is to push the black LED
holders over the LEDs. Next, with the
mounting hole at the rear of the post,
clean a 2mm strip on the front with a
centre 50.25mm from the bottom and
tin that strip with solder.
Place the LED assembly over the
post, as shown in the drawing. Using
a soldering iron, attach it to the post.
Select about 100mm of thin wire
with high-temperature insulation
and slide it into the hollow post at
the bottom until it exits out at the
1.5mm hole, 48mm up. Strip off
about 2mm of insulation and solder
it to the LED assembly as shown in
the upper left photo.
The three backing plates are then
glued to the post as shown, using
Loctite GO 2. Leave it for 24 hours
for the glue to set. Using heatshrink tubing and masking tape,
cover the LEDs and then spray
the assembly with aluminiumcoloured paint. When dry, remove the
heatshrink tubing and masking tape
Practical Electronics | July | 2022
and attach the three labels to their
respective backing plates.
The final task is to connect the second power lead to the post on the two
post assemblies. This is done after they
are assembled on the crossing, as any
solder on the post would stop it from
going into its mounting hole.
Again, clean and tin a 2mm section at the bottom end of the post
and attach a wire to it. I will leave
the design of the road across the track
up to you, as the sizes will depend on
your particular railroad layout. Mine
consisted of timber wedges painted
matte black.
Electronic assembly
The heart of the level crossing circuit is built on a single-sided PCB
Fig.4: the overlay diagram for the
Level Crossing. Note the resistors are
mounted vertically.
coded 09108211. It is available from
the PE PCB Service and measures 48
x 43mm. The PCB overlay diagram
(see Fig.4) can be used as a guide
during construction. Start by fitting
the PCB pins, then the IC sockets.
We used IC sockets for the microprocessors and the recording ICs in
case we ever wanted to reprogram
or change the sound. Do take care to
orient them correctly.
Now add the resistors, which are
mounted vertically, followed by the
capacitors. Check that the 100µF
capacitor is the right way round. Next,
add the 2N7000 MOSFET Q1, oriented
as shown.
The wiring diagram (Fig.5) shows
how to connect the two post assemblies, the loudspeaker, the trigger
Here is an example of the
completed project fitted onto
a model railway track.
27
Fig.5: the wiring diagram for the
project. For triggering the device, we
recommend using a reed switch for
S1 which is hidden under the tracks,
so that it can be triggered by a magnet
mounted on the locomotive.
switch and the two servomotors.
Rather than using a pushbutton switch
as shown, we expect most constructors
will use a reed switch hidden under
a section of the track, with a magnet
on the model locomotive to trigger it
before the loco reaches the crossing.
Finally, connect the positive of the
5V power pack to the +5V point on the
board 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 the PIC
controllers (IC1 and IC2). There is
no provision for programming either
of the microcontrollers in-circuit, so
you will either need to program them
yourself using an external programmer (eg, PICkit 3 or 4) before plugging them in.
You can download the HEX files
from the July 2022 page of the PE website; the file ending in ‘A’ or ‘B’ is for
IC1 (depending on the type of switch
used) and the ‘C’ file is for IC2.
Recording the bell sound
The download package on our website
also includes a WAV audio file of the
bell sounds, which you need to transfer to IC3. This is supplied as part of
a module that is capable of recording by itself (see the photo opposite).
The simplest way to transfer the bell
sounds from a computer to the chip
is to place the module’s microphone
close to your computer speakers.
First, though, the module needs a
power source. Connect a 5V supply to
its power input terminal block. With
Fig.6: this box was designed to hide the servomotors when displayed on the
track. You can print this diagram on a suitable material, fold it and then paint it
if you want.
28
the green terminal block on the left,
make sure that the two slide switches
marked FT and repeat are switched
to the left-hand side. It’s also a good
idea to temporarily connect the 8Ω
speaker to this module so that you
will be able to hear and check what
you have recorded.
Hold the module so that its electret
microphone is about 100mm from
the computer loudspeaker. Play the
downloaded WAV file at the maximum reasonable volume, and after
it starts, hold down the REC button until LED D1 goes out (after the
maximum recording time of about
10 seconds).
Slide the repeat switch to the right
and momentarily press the PLAYE
button. This should verify that you
now have a continuous recording
of the level crossing bell sound on
the chip.
Testing the electronic assembly
Plug the 5V power pack into the mains
and, using a voltmeter, check that
you have 5V between pins 1 and 8
on IC1’s socket. Switch off the power
supply, remove the ISD1820P IC from
the recording and playback module
and insert it into the Level Crossing
PCB, oriented as shown in Fig.4. Do
the same for the PIC microprocessors,
making sure that you don’t get them
mixed up.
Switch the power on, close the start
switch and you should see the red LEDs
flashing alternately and hear the level
crossing bell sound from the speaker.
Half a second later, the servomotors
Practical Electronics | July | 2022
A more complicated approach to recording the bell sounds
I designed the circuit shown in Fig.7 to
provide a more elegant way of recording
the bell sounds from a computer onto
the ISD1820P chip. However, while it is
a better solution, the effort and expense
of building this circuit are not worthwhile
for a one-off recording. The speaker/
microphone method described in the
text provides decent results with minimal effort. Regardless, I am presenting the circuit here for those interested.
Audio from the PC’s output jack is
adjusted in level using VR1, then AC-coupled to two op amps, IC2a and IC2b.
These convert the single-ended computer audio into a balanced signal, ideal
for feeding to the ISD1820P’s balanced
microphone inputs at pins 4 and 5.
The components at the top of the
circuit detect when the audio playback
begins on the computer and automatically triggers recording on the ISD1820P
(IC4), so that you don’t have to try to
press both buttons simultaneously to
get the best results.
1a a pli e t e a io i al y
and feeds a diode charge pump (D1 /
1μF capacitor
i capacitor ic ly
charges as soon as a signal comes from
the computer. The other half of the dual
op amp, IC1b, is connected as a comparator, pulling the GP2 digital input of
The ISD1820P is often sold as a module similar to this. This version is sold
by Jaycar (www.jaycar.com.au/p/XC4605). But there are a wide variety of
alternatives available online that will also work. Note that they might have
different arrangements for feeding in power, jumpers instead of switches and
other minor variations. (Do ensure the IC is socket mounted – NOT soldered!)
pi
lo a oo a t e c ar e o
t at 1μF capacitor e cee a o t V
This also lights LED1.
e
it
icroco troller
detects that its pin 5 has gone low, it
generates a pulse from its GP1 digital
output (pin 6) to trigger recording mode
on IC4. This has an appropriate length to
recor t e ole ell o
e e ce
o
i acti a a p l e tretc er
Fig.7: a circuit I designed to record sound to the ISD1820P module directly from a computer’s audio output jack.
Practical Electronics | July | 2022
29
should move slowly clockwise about
70°. On opening the switch, the servomotors should slowly move back, the
flashing lights should extinguish, and
the bell sound should stop.
Final fitting
Glue the barrier covering labels to
each side of each barrier and trim any
excess overhang. Mount the servomotors side-on, as shown in the photo.
Apply power and close the start
switch. The servomotors will move
down to the barrier closed position.
Slide the barrier over the modified
servo arm bush as shown in the
photo, and then attach the servo arm
to the servomotor.
Glue the barrier onto the servo
arm so that it is horizontal and let it
dry. Do the same for the other servomotor. Open and close the switch to
check that the barriers operate, as in
the video.
To hide the servomotors, I made
boxes out of folded card and painted
them silver. Fig.6 is the cutting diagram for this box, and it is also available as a PDF download.
Print the 1:1 scale drawing on 80gsm
paper – or any suitable material of your
choosing – cut out the outline, fold
it up into a box and use super glue
to hold it together. In this operation,
remember to be very careful not to get
Parts List – Level Crossing Controller
1 control PCB assembly (see below)
1 5V DC supply (eg, USB charger with USB cable)
1 SPST toggle switch (S1) OR
1 momentary pushbutton switch (S1) OR
2 reed switches plus a magnet (S1; see text)
8 3mm high-intensity red LEDs with diffused lenses (LED1-LED8)
2 1.6kg.cm 9g 180° micro servos [eg, Core Electronics SER0006]
1 8Ω speaker [eg, Jaycar AS3006]
1 ISD1820P-based audio recording/playback module [eg, Jaycar XC4605, see text]
1 set of printed labels (see Fig.3)
various lengths and colours of light-duty hookup wire
various mechanical parts (see Fig.2)
Control PCB parts
1 single-sided PCB coded 09108211, 48 x 43mm
2 8-pin DIL IC sockets (for IC1 and IC2)
1 14-pin DIL IC socket (for IC3)
1 PIC12F617-I/P 8-bit microcontroller programmed with 0910821A.HEX
(for toggle switch) OR 0910821B.HEX (for momentary or reed switches) (IC1)
1 PIC12F617-I/P 8-bit microcontroller programmed with 0910821C.HEX (IC2)
1 ISD1820P audio recording/playback IC with bell sound recorded (IC3)
(from module listed above)
1 2N7000 small-signal N-channel MOSFET (Q1)
1 100μF 16V electrolytic capacitor
2 100nF 63V MKT or 50V ceramic capacitors
16 1mm PCB pins
Reproduced by arrangement with
Resistors (all 1/4W 1% axial metal film)
SILICON CHIP magazine 2022.
1 1MΩ
1 100kΩ
1 10kΩ
www.siliconchip.com.au
2 4.7kΩ
2 330Ω
1 220Ω
super glue on your fingers – unsticking them can be painful! Use tweezers
to hold the surfaces together when the
glue is setting.
Last, Paint the box silver, cut out
the hole for the servomotor and then
fit the box. (Repeat this build for the
other servomotor.)
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