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If you do a lot of multi-conductor cable wiring or cable troubleshooting,
you’ll find this project especially handy. It consists of two very compact
units: a signal injector which produces a very distinctive ‘warbling
whistle’, and a sensitive signal tracer which can help you easily identify
the cable conductor(s) carrying the warbling test signal. The signal tracer
unit can easily be adapted for other kinds of signal tracing as well.
‘WHISTLE & POINT’
CABLE TRACER
By Jim Rowe
I
t should be easy but it’s often a
pain. Most multi-conductor cables are colour coded, so it should
be a snack to connect each one to the
appropriate pins of an RJ45 wall socket
or whatever.
But some of the colours are often a
bit hard to distinguish — especially in
poor lighting, when you’re crouch-ed
down behind a desk or in some other
awkward location.
It is surprisingly easy to mistake
the blue-and-white for the green-andwww.siliconchip.com.au
white, or the white-and-orange for
the white-and-brown. And then you
find that somebody’s PC doesn’t seem
to want to talk to the network server,
because you’ve swapped some of the
pair returns. . .
Or you might be running a length
of six-pair telephone cable and need
to make sure that you get the pairs
properly matched at each end. It can
be trickier than you’d expect.
What you need in your toolbox is
a compact little signal injector giz-
mo to squirt an easy-to-identify test
signal along the conductors from one
end, plus an equally compact little
‘sniffer’ or signal tracer gizmo so you
can make sure which conductor is
carrying the test signal at the other
end.
These two handy little gizmos are
exactly what you get when you build
this project, which we’ve dubbed the
‘Whistle & Point’ Cable Tracer. (Get it?
One device produces the ‘whistle’ to
draw attention to the wire you want,
October 2002 53
and the other then ‘points’ you to
it...)
We can’t take the credit for designing the gizmos themselves, because
they were dreamed up by the team at
Oatley Electronics — who are selling
kits for the two PC board assemblies
inside ’em.
However when they showed them
to us, we were so impressed that we
decided to work out how to house
them in low-cost cases. This turned
them into rugged little devices capable
of being carried around in the usual
toolbox and used reliably ‘on the job’.
We also dreamed up that weird
name for the project too, so it would
get your attention. (So blame us for
that, not Oatley!) You’ll be able to buy
both PCB kits from Oatley for only $24
plus post and packing (typically $7.00
within Australia).
We’ve calculated that you’ll only
have to spend an extra $16 or so at
most, to fit both boards into the more
expensive of the boxes we’ve used
with on-off slider switches and batteries.
So the total cost for the complete
project as a cable tracer set should
still be no more than $32, buying
everything from scratch. Not bad for
such a handy pair of tools, wouldn’t
you agree?
By the way if you want to turn the
signal tracer unit into a more general-purpose unit, this mainly involves
using a larger speaker and building it
into a larger box.
And Oatley Electronics can even
help you out there, too: as you’ll
find in the ‘Wheredyageddit?’ box,
for only $2.00 more they can supply
husky little 50mm speakers complete
with a larger plastic case which can
be used to house the complete tracer.
The box even contains an optional
power amp IC which can be used to
get more ‘grunt’.
How they work
Let’s look first at the signal injector
unit. The circuit for this is shown in
the upper part of the schematic diagram and, as you can see, it’s based on
a couple of very low cost 555 timer ICs,
plus a C8050 NPN transistor.
The first 555 (IC1) is connected as
S1
ON/OFF
8
3
9V
BATTERY
10F
5
B
6
K
8
C
3
2
10F
E
7
1
100nF
Q1
C8050
10k
4
IC1
555
a simple relaxation oscillator, with
its frequency of oscillation set by the
10kΩ feedback resistor from pin 3 to
pins 2 and 6, and the 10µF capacitor
from pins 2 and 6 to ground.
With these values IC1 oscillates
quite slowly at around 6Hz, producing a square wave at pin 3 and a
‘rounded sawtooth’ waveform at pins
2 and 6. It’s the rounded sawtooth
that we make use of, but since pins
2 and 6 are operating at a fairly high
impedance, we use transistor Q1 as
an impedance matching emitter follower.
This allows us to extract the sawtooth without loading down the
oscillator and disturbing its operation.
As you can see, the low impedance version of the sawtooth which
appears at the emitter of Q1 is then
coupled to pin 5 of the second 555,
IC2. This is the ‘control voltage’
input of the 555, so as a result the
sawtooth from IC1 is able to modulate the operation of IC2.
IC2 is again connected as simple
relaxation oscillator, just like IC1.
10F
4
IC2
555
5
6
2
A
47k
CLIP A
(+)
100
7
K
1
2.2k
D1
1N4148
SIGNAL
INJECTOR
UNIT
D2
1N4148
10nF
CLIP B
(q)
A
1N4148
K
A
BC549
B
ZD1
+
C8050
q
E
2N5484
B
C
C
S
E
G
D
S2
ON/OFF
680
+
PROBE
TIP
150pF
39k
Q2
2N5484
D
G
S
1.5nF
3.9k
Q3
BC549
B
IC3
LM386
1.5nF
2
SIGNAL
TRACER
UNIT
SC
2002
1M
4.7k
10k
1k
100F
q
6
100F
5
C
9V
BATTERY
3
4
E
10pF
ZD1
5.6V
100F
47k
4.7
PIEZO
SPEAKER
15nF
AWHISTLE & POINT˚ CABLE TRACER
54 Silicon Chip
www.siliconchip.com.au
The injector board (top) and
the tracer board following
assembly. Note the absence
of on/off switches as shown
in the drawings below: these
were added when they were
put into cases.
clips should be accidentally connected
to supply rails with voltages above or
below the 9V battery rails.
So that’s the injector unit. A simple, low cost circuit which generates
a strong and very easy-to-recognise
audio test signal, from a standard 9V
battery.
Now let’s look at the matching signal
tracer unit.
As you might expect this is basically
just a fairly high gain audio amplifier,
although it does have a few special aspects because of its being customised
for this application.
For example because we’re really
only interested in tracing the injector
unit’s warbling whistle signal, the
amplifier’s frequency response is tailored to mainly respond to frequencies
between 1100 and 1700Hz. This also
TONE GENERATOR
2.2k
IC1
555
+
10F 1
Figs. 1a (the injector board – top)
and 1b (the tracer board – bottom)
along with the wiring required.
100F
10F
1
D2
4148
D1 +
CLIP A
(+)
CLIP B
(–)
–
10nF
S2
3.9k
1
4.7
IC3
LM386
1.5nF
ZD1
1k
220
47k
Q3
39k
+
10k
1.5nF
Q2
680
5.6V
BC549
2N5484
1M
4.7k
150pF
10pF
PROBE
100F
+
+
100F
+
GND
IC2
555
10k
100nF
10F
47k
C8050
Q1
100
+9V
4148
S1
9V BATTERY
allows us to use a very small piezo-electric speaker mounted directly on the
board, as we’re not interested in reproducing frequencies below 1100Hz.
At the heart of the amplifier is
IC3, an LM386 audio output device.
This provides the drive for the piezo
speaker, as you can see, with the 4.7Ω
resistor and 15nF connected across the
output as a ‘Zobel network’ to ensure
stability.
As the LM386 has a fixed voltage
gain of 20 in this configuration, transistor Q3 is used ahead of it to provide
additional gain and make the tracer
suitably sensitive. Q3 is a BC549, used
in a standard common emitter stage.
The emitter is fully bypassed to give
a high voltage gain, while the use of
1.5nF coupling capacitors deliberately
limits the low frequency response.
+
However in this case the main timing
components are the 47kΩ feedback
resistor and the 10nF capacitor from
pins 2 and 6 to ground.
This gives a basic oscillation frequency of around 1400Hz but because
of the modulation from IC1 the actual
frequency of IC2 varies up and down
between about 1150Hz and 1700Hz.
This time we use the square wave
output from IC2, available at pin 3. This
provides a waveform of almost 9V peak
to peak, which becomes the injector’s
‘warbling whistle’ output signal.
It’s fed to the active output clip (clip
A) via the series 100Ω resistor — to
protect IC2 from damage due to accidental short circuits. ‘Catcher’ diodes
D1 and D2 are also connected so that
pins 3, 2 and 6 of IC2 are protected
against overvoltage damage if the test
15nF
+9V
PIEZO
SPEAKER
9V BATTERY
GND
TONE DETECTOR
www.siliconchip.com.au
October 2002 55
Q3 and IC3 together would probably serve quite well alone as a cable
tracer, providing plenty of gain plus
a reasonably low input impedance
(about 5kΩ).
However, a JFET source follower
stage has been added at the input, to
give the tracer a much higher input
impedance (nearer 1MΩ). This will
make the unit also very suitable for
signal tracing in low-frequency electronic circuits, where its high input
impedance won’t cause unnecessary
loading.
(When using it for signal tracing in
such circuits, you could use either
the injector unit to provide a suitable
signal for tracing, or a standard audio
generator set to produce a tone of about
1200-1400Hz.)
The input stage uses a 2N5484
N-channel JFET, with the signal from
the tracer’s probe tip coupled to its
gate via a 150pF capacitor. The 1MΩ
resistor provides the gate’s bias return,
while the 10pF capacitor shunts away
any RF that may also be picked up by
the probe tip.
Both Q2 and Q3 are powered from
a regulated 5.6V supply rail, which
is derived from the 9V battery via a
simple regulator circuit using the 680Ω
resistor and zener diode ZD1. The
LM386 chip runs directly from the 9V
Two different views of the
“opened out” tracer case showing
how everything is “shoe-horned”
in. It’s a tight fit but it will all go
in! Note the probe, the tinplate
shields, the insulating tape and
also the polystyrene packing
around the battery. All these are
explained in the text.
battery rail but both supply rails have
100µF reservoir capacitors to ensure
low frequency stability.
Construction
Apart from the 9V batteries and
on-off switches, virtually all of the
circuitry for both the injector and the
tracer units is fitted on two very small
PC boards.
The board for the injector unit
measures only 41 x 25mm and has the
Oatley code K181A, while the board
for the tracer measures 72 x 25mm and
is coded K181.
Despite the small size of both
boards, fitting the components should
be very straightforward as there’s not
all that many of them in either case.
The location and orientation of each
part is also shown clearly in the board
overlay and wiring diagram, so if you
follow this carefully you shouldn’t
have any problems.
As usual it will be easier if you fit
the low profile components (resistors
and diodes) first, then follow with
the smaller and larger capacitors, and
finally the transistors, ICs and the
piezo speaker.
Just make sure you fit each polarised component in the correct way
around, to prevent problems later.
When it comes to housing each board
in a protective case, you have a range
of choices.
The injector unit in particular can
go in virtually any small case, as long
as there’s room for the board assembly
itself, on-off switch S1 and the 9V battery and its snap lead. The two output
leads are simply taken out through a
grommetted hole and fitted with small
shrouded alligator clips.
To illustrate at least one of the
packaging options for the injector, we
Here’s how the injector board, battery and
switch all fit inside a piece of 32mm PVC electrical
conduit. Again, it’s a pretty tight fit inside the pipe!
56 Silicon Chip
www.siliconchip.com.au
housed the prototype unit in a 120mm
length of 32mm outside diameter PVC
conduit fitted with push-on plastic
end caps. This length was plenty to
fit both the board and battery end-toend, with the on-off switch mounted
in one end cap and the output leads
emerging through a grommetted hole
in the other end cap.
This makes a practical and quite
rugged little package, which can also
be easily opened when you need to
replace the battery. There are fewer
options for packaging the tracer unit,
because we found that its PC board
really needs to have a certain amount
of shielding.
This means that a very small metal
case would be quite OK, although
there aren’t too many suitably sized
and proportioned metal cases available — especially at a reasonable price.
You might have to make one up
yourself, or modify an existing metal
utility box. Of course you can always
use a low cost plastic case which lends
itself to fitting some shielding inside.
This can be quite practical, as we’ve
tried to show with our housing of the
prototype tracer unit shown in the
photos. The case we’ve used is a modular ABS unit measuring 90x50x32mm,
and sold by Dick Smith Electronics
(Cat. No. H-2832).
As you can see from the photos this
has just enough room to fit the PC
board assembly and battery on-edge
and side by side, with the on-off switch
S2 mounted in the rear panel (offset to
the side so it allows space for the PC
board) and the probe tip mounted in
the centre of the front panel.
As for the probe tip itself, we gave
this a bit of thought and ended up buy-
Parts List – Whistle & Point Cable Tracer
INJECTOR UNIT
1 PC board, code K181A, 25 x
41mm
1 Plastic case 90 x 50 x 32mm,
or
120mm length of 32mm PVC
conduit plus end caps (see text)
1 Miniature slider switch, SPST
(S1)
1 9V battery, 216 type
1 Snap lead for 9V battery
2 Small alligator clips, red and
black
1 Rubber grommet, 10mm hole
diameter
Semiconductors
2 LM555 timers (IC1,IC2)
1 C8050 NPN transistor (Q1)
2 1N4148 silicon diode (D1,D2)
Capacitors
3 10µF PCB electrolytic
1 100nF (0.1µF) metallised
polyester
1 10nF (.01µF) metallised
polyester
Resistors (0.25W 1%)
1 47kΩ
1 10kΩ
1 2.2kΩ
1 100Ω
ing one of the low-cost ‘solderless’ test
probes sold by DSE (Cat. No. P-1755).
It proved to be quite easy to remove
the metal probe tip from the plastic
body — they simply pull apart.
Then we used a small jeweller’s
hacksaw to cut off all but about 3mm
of the larger-diameter rear section of
the metal tip, leaving the remaining
section as a short ‘bolt head’ to go
TRACER UNIT
1 PC board, code K181, 72 x 25mm
1 Plastic case 90 x 50 x 32mm,
or
150mm length of 32mm PVC
conduit plus end caps (see text)
1 Miniature slider switch, SPST (S2)
1 9V battery, 216 type
1 Snap lead for 9V battery
1 Test probe (see text)
1 Piezo speaker, 19mm diameter
Semiconductors
1 LM386 amplifier (IC3)
1 BC549 NPN transistor (Q1)
1 2N5484 N-channel JFET (Q2)
1 5.6V 400mW zener diode (ZD1)
Capacitors
3 100µF 10VW PCB electrolytic
1 15nF (.015µF) metallised polyester
2 1.5nF (.0015µF) metallised
polyester
1 150pF ceramic
1 10pF ceramic
Resistors (0.25W 1%)
1 1MΩ
1 47kΩ
1 39kΩ
1 10kΩ
1 4.7kΩ
1 3.9kΩ
1 1kΩ
1 680Ω
1 220Ω
1 4.7Ω
behind the plastic case front panel.
The details of this should be clear
from the small diagram. We drilled
the front panel so that the threaded
section of the tip could be passed
through it, and the round knurled ‘nut’
(used originally to fasten the test probe
lead’s conductor) could then be used to
fasten the tip to the front panel.
The tracer’s own input wire was cut
Resistor Colour Codes
No.
1
1
1
5
2
2
2
2
2
2
1
1
www.siliconchip.com.au
Value
1MΩ
47kΩ
39kΩ
10kΩ
4.7kΩ
3.9kΩ
2.2kΩ
1kΩ
680Ω
220Ω
100Ω
4.7Ω
4-Band Code (1%)
brown black green brown
yellow violet orange brown
orange white orange brown
brown black orange brown
yellow violet red brown
orange white red brown
red red red brown
brown black red brown
blue grey brown brown
red red brown brown
brown black brown brown
yellow violet gold brown
5-Band Code (1%)
brown black black yellow brown
yellow violet black red brown
orange whiteblack red brown
brown black black red brown
yellow violet black brown brown
orange white black brown brown
red red black brown brown
brown black black brown brown
blue grey black black brown
red red black black brown
brown black black black brown
yellow violet black silver brown
October 2002 57
5
SILICON
CHIP
40
12.5
(ALL DIMENSIONS
IN MILLIMETRES)
20
‘WHISTLE & POINT’
CABLE TRACER
10
Dia
INJECTOR UNIT
30
35
(BEND UP AT ABOUT 70°)
Panel labels for the injector unit (above)
and tracer unit (below). As there are no
on-panel controls, placement is nto critical.
12.5
(BEND UP AT ABOUT 70°)
22
SHIELD PLATE
BEHIND FRONT PANEL
SHIELD PLATES FOR TOP &
BOTTOM OF CASE (2 OFF)
SILICON
CHIP
MATERIAL: 0.2mm TINPLATE (SEE TEXT)
We found that a shield was necessary inside the plastic case
to prevent the high-gain amplifier picking up too much RF
energy. Ours came from an empty pineapple tin – if you
don’t like pineapple eat something else.
very short and soldered directly to the
rear of the probe tip, behind the panel.
It all worked out quite neatly.
To provide the shielding, we cut
three small shield plates out of a strip
of 0.2mm tinplate salvaged from a
small tin which until recently contained pineapple pieces(!).
The shield plates were shaped as
shown in the small diagram, with
the barrel-shaped piece designed to
go behind the front panel (its 10mm
hole clearing the rear of the probe tip)
and the two more rectangular pieces
designed to go into the front sections
of the top and bottom halves of the
case itself.
These latter pieces have a 12.5mm
deep section on each side bent upwards at about 70°, so their centre
sections lie flat inside each half of
the case yet their shielding extends
around the sides.
When the shield plates had been cut
out and any sharp burrs removed, we
then fastened them to the rear of the
(TRACER BOX FRONT PANEL)
ORIGINAL WIRE
CLAMPING NUT NOW
ATTACHES TIP TO PANEL
front panel and inside the case halves.
We used small pieces of gaffer tape for
this, but you might prefer to use a few
dobs of epoxy cement (like 5-minute
Araldite).
Then we soldered some short
lengths of insulated hookup wire to
connect all three shields together, with
another short length so they could be
connected to the PC board’s earth line
when it was placed in position. (They
have to be connected to the board
earth, to provide correct shielding.)
Before mounting the PC board in
the case, we applied a small strip of
gaffer tape down the centre of each
case-half shield plate, to make sure
that the plates couldn’t cause any short
circuits at the edges of the board.
By the way, it’s OK to use Gaffer
Tape as insulation for low voltage
devices like this but only properly
rated electrical tape should be used
on higher voltages.
As well as cutting holes in the front
and rear panels to take the probe tip
‘WHISTLE & POINT’
CABLE TRACER
TRACER UNIT
and on-off switch, we also drilled
some small (2mm diameter) holes in
one side of each case half near the rear
end, to allow you to hear the sound
from the piezo speaker when the case
is screwed together. You can hopefully
see these holes in the photo.
When the board assembly was fitted
in the bottom half of the case, the front
panel with probe tip mounted on it
was slotted in too and the board input
wire carefully bent around to go into
the hole in the rear of the probe tip.
Then the two were soldered together
quickly, so as not to overheat either.
The rear panel with switch S2 fitted
was slotted in at the other end, and
the wires from the PC board and battery clip lead soldered to the contacts
of S2.
After this the wire from the shield
plates was carefully soldered to the
earthy copper at the front end of the PC
board. Then the battery was added and
squeezed in alongside the PC board (on
the copper side), with a small piece of
PULL AWAY PLASTIC SLEEVE
CUT OFF ALL EXCEPT 3mm OF TIP REAR
SECTION TO ACT AS 'HEAD' BEHIND PANEL
Here’s how we turned a multimeter probe into our tracer probe while at right is
a close-up photo showing how it mounts to the tracer case.
58 Silicon Chip
www.siliconchip.com.au
Capacitor Codes
Value Alt. Value IEC Code EIA Code
100nF 0.1uF
100n
104
15nF .015uF 15n
153
10nF .01uF
10n
103
1.5nF .0015uF 1n5
152
150pF
–
150p
150
10pF
–
10p
10
plastic material between the two to
prevent shorts.
Some small pieces of expanded
polystyrene were added as ‘packing
pieces’ at either end of the battery, to
prevent it moving forward or backward
and causing trouble.
Finally the top half of the case was
fitted, taking care to dress the battery
leads so they weren’t squashed between the case halves.
Using them
Checking cables using the two devices is quite straightforward. All you
need to do is connect injector output
clip A to one end of the wire you’re
trying to trace, and connect clip B to
either another wire, or a number of
other wires, or some earthy metalwork.
Then you turn both units on, and
start probing the far end of all of the
wires with the tracer unit. When you
contact the right wire with the probe
tip, you’ll hear the injector’s ‘warbling
whistle’ quite clearly.
Note that you don’t really need an
earth return wire for the tracer, because the tracer amplifier is very sensitive and there’s enough capacitance
between the shield plates and your
hand to provide a high impedance
return path.
Of course if you want to use the tracer to check signal paths in equipment
PC boards, then you will have to fit it
with an earth-return input lead.
This could consist of a 500mm
length of insulated hookup wire, with
one end soldered to the earthy copper
at the front of the tracer board, and the
wire brought out of the case through a
1.5mm hole drilled in the side.
The far end of the wire would be
fitted with a shrouded alligator clip
like those on the injector output leads.
You’ll only need to add this lead
to your tracer if you do want to use it
for general signal tracing work on PC
Wheredyageddit?
This project and the PC boards are
copyright Oatley Electronics.
Oatley have available a kit (K181) with
both PC boards and on-board compon-ents plus the 9V battery snap
leads and the piezo speaker for $24.00
plus $7.00 for packing and postage if
applicable. This does not include the
on-off slider switches, plastic cases,
batteries or probe tip as described in the
text. However Oatley can supply a larger
‘surplus’ plastic case with a PC board
containing a husky 50mm speaker and
an optional audio power amplifier — all
very suitable for ‘beefing up’ the signal
tracer unit — for an additional $2.00.
Oatley Electronics can be contacted by:
phone (02) 9584 3563; fax (02) 9584
3561; mail to PO Box 89, Oatley NSW
2223; email (sales<at>oatleyelectronics .com); or via their website (www.
oat-leyelectronics.com).
boards, though. For cable tracing, it’s
not needed.
SC
Book Review . . . by Leo Simpson
Firsts in High Fidelity. The Products and History of H. J. Leak
& Co, by Stephen Spicer. 1st edition published 2000 by Audio
Amateur Press, USA. Soft covers, 195 x 260mm, 272 pages.
ISBN 1-882580-31-1
Anyone who is over 45 probably is aware of the legendary English hifi company, H. J. Leak & Co Ltd, although unless you were
reasonably well-heeled in the years preceding 1970, it is unlikely
that you would have ever owned their products. I certainly knew of
their products in those years but they were priced way above my
means. So were the products of other notable English companies
such as Quad, A. R. Sugden and Wharfedale. It was more a matter
of admiring them from afar.
So it is with considerable interest that I received this sample
copy about the products of the Leak company. They made a range
of amplifiers, tuners and loudspeakers and all were notable in
some respect or other apart from high performance, for the day.
Of particular interest was the Leak sandwich loudspeaker, based
on a 13-inch woofer with a “sandwich” cone consisting of a core
of expanded polystyrene foam sandwiched between aluminium
skins. This very rigid cone was shown in adverts (in Radio TV &
Hobbies) supporting the full weight of the founder, Harold Leak.
One of the reasons why Leak amplifiers were so highly regarded
was the quality of construction. Not only was the under-chassis
layout beautifully symmetrical but the transformers were extremely
well made, with diecast covers. They were very good performers
too, with plenty of negative feedback (a supposed anathema to
today’s valve amplifier enthusiasts) and harmonic distortion of
less than 0.1%.
www.siliconchip.com.au
For me though, there was
a certain English eccentricity
about some Leak products and
none more so than the “troughline” FM tuner. I had always
assumed that this was another
English oddity but it turns out
that the “trough line” was quite
innovative in its day and used
a tubular transmission line in
place of the conventional coil
inductor used in the local
oscillator. This method of
construction gave very good
frequency stability. Nowadays, AFC (automatic
frequency control) largely achieves the same result.
Another interesting chapter in the book is devoted to the
Australian manufacture of Leak loudspeakers. This was started
by Syd McClory, a well-known personality on the local Sydney
hifi scene at the time.
All told, these book is a wonderful source of information on
Leak products, whether you just want to engage in nostalgia or
whether you are involved in restoring or building a Leak amplifier.
To that end, there are quite a few circuit diagrams for amplifiers
and tuners.
The book is priced at $59.95 plus $8 postage and packing. It
is available from Evatco, PO Box 487, Drysdale, Vic 3222. Phone
(03) 5257 2297. email evatco<at>mira.net
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