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This Cable and Wiring
Tester has a row of
four LEDs to indicate
the condition of a pair
of wires: open circuit,
short, reversed and
good. A diode is hooked
across the far end of the
wire pair to assist the
test which is done
automatically as soon
as you press the button.
By LEON WILLIAMS
Here’s an easy to build and simple to use tester
that will prove indispensable to anyone
involved in the installation or maintenance of
cables or wiring systems. Small enough to carry
in a pocket, the tester employs four LEDs to
speedily indicate the health of a pair of wires.
Tracing faults in cables, especially
those in large buildings can be very
difficult if you are working on your
own. If you have a partner and some
form of communication, you can use a
multimeter set to measure resistance at
your end while you get your partner to
apply a short circuit and then remove
it. With the short removed the meter
should show an infinite resistance,
and with the short applied a low
resistance.
This is obviously difficult on your
own, as you would have to travel
between ends to place the short and
remove it in between taking readings
40 Silicon Chip
with the meter. Thankfully there’s an
easier way.
Diode testing
A technique that has been used for a
long time to test cable pairs is to place
a diode across the A and B wires of the
pair at the remote end. When a meter
is placed across the pair at the local
end, a low resistance will be obtained
with the meter leads connected one
way and a very high (ideally infinite)
re
sistance with the leads reversed.
This happens because the diode only
passes current in one direction; ie,
when the anode is more positive than
the cathode by about 0.7V.
A big advantage of the diode test is
that fault conditions such as a short or
open circuit can be diagnosed quickly.
If a pair has a short circuit somewhere
along its length, a low resistance will
be seen when the meter is connected
either way. Conversely an open circuit
will show an infinite resistance with
the meter connected either way.
The diode test will also show a
reversed pair; ie, where the A and B
wires get crossed along the way, as
the low resist
ance/high resistance
results will be opposite to those for a
good pair. This goes to prove that the
best ideas are sometimes the simplest.
Fig.1 illustrates the four common pair
combinations and the results obtained.
Easy to use
Carrying around an expensive
multimeter, continuously turning it
on and off and reversing the leads to
test pairs is tedious. With the Cable
and Wiring tester all you have to do is
connect the two test leads to the pair
under test and press the Test button.
The tester will automatically test the
pair and display the result on one of
four LEDs.
The orange LED (O) will flash to indicate an open circuit and the yellow
LED (S) will flash if the pair is short
circuit. A pair that is reversed will
cause the red LED (R) to flash, while
a pair that is in good condition will
cause the green LED (G) to flash. Of
course you will need to connect the
diode at the other end of the pair you
are testing.
Circuit description
The Cable and Wiring tester works
just like the manual diode testing
shown in Fig.1 but it does it automatically in two phases before it displays
the result. I will refer to these as phase
1 and phase 2. Fig.2 shows the circuit.
An oscillator is formed with IC2c,
one section of a 40106 hex Schmitt
trigger inverter, a 330kΩ resistor and
a 0.22µF capacitor. It produces a
square wave output with a frequency
of about 20Hz. IC2d, a 100kΩ resistor
and a 0.1µF capacitor form a delay
circuit. The output at pin 10 of IC2d
is a delayed and inverted replica of
the output from IC2c.
The reason for the delay circuit is
to separate the sample and display
pulses from the unstable periods when
the analog switches are swapping the
Fig.1: this series of the diagrams illustrates the method of testing a cable pair
with a multimeter and a diode connected to the far end. The Cable and Wiring
Tester runs through these tests automatically.
polarity of the line. The oscillator
controls the overall operation of the
tester and when its output is low, it
is in phase 1, and when its output is
high, it is in phase 2.
In phase 1, analog switch IC1a
connects wire A of the pair to pin 1
of IC2a, while IC1b connects wire B
to ground. If the pair is good (ie, not
reversed) and the diode is connected
Fig.2: the Cable and Wiring Tester works by alternately applying DC voltage to a cable pair in one direction
and then the other. The four possible conditions are indicated by the LEDs.
November 1997 41
Fig.3 (left): the component layout for
the PC board. Take care to ensure
that all polarised parts are correctly
orientated.
Fig.4 (below): this is the actual size
artwork for the PC board.
ent on the wires being tested, most
likely in the form of static charges,
each input is protected with a series
680Ω resistor and a 9.1V zener diode.
A .001µF capacitor is also connected
between the two inputs to shunt any
RF signals that might otherwise be
picked up by the wires under test.
The tester operates from a standard
9V battery which should last quite a
long time. Note that the Test switch
is also the power switch and is connected in the negative supply lead
instead of the positive supply lead
as is normal practice. This was done
simply because it made the PC board
layout easier.
Construction
with its cathode to wire A, no current
will flow through this circuit and
pin 1 of IC2a will be pulled high by
the 4.7kΩ resistor. If the pair is short
circuit or the diode is connected in
reverse, current will flow and pin 1 of
IC2a will be pulled to ground.
Assuming that all is well, pin 2 of
IC2a will be low. IC2f, a 33kΩ resistor
and a 0.1µF capacitor form a mono
stable which produces a narrow negative pulse when the output of IC2d
goes high, which is only within phase
1. The negative pulse from IC2f closes
analog switch IC1c and charges the
0.22µF “memory” capacitor connected
to pin 12 of IC1c to the voltage present
at pin 2 of IC2a.
When the pulse ends, the gate
opens but the charge on the capacitor
remains as the only discharge path is
via the very high input impedance of
inverter IC2b. The high output of IC2b
is applied directly to the B input of
the 4028 BCD-to-decimal decoder IC3.
During phase 2 the states of IC1a
& IC1b are reversed and wire A is
connected to ground while wire B
is connected to pin 1 of IC2a. With a
good pair, current will flow through
42 Silicon Chip
the circuit and pin 1 of IC2a will be
pulled to ground. The output (pin 2)
of IC2a is connected directly to the A
input of IC3. IC2e, a 0.1µF capacitor
and a 100kΩ resistor form a monostable which produces a positive-going
pulse when the output of IC2d goes
low, which is only during phase 2.
This pulse is applied to the C input
of IC3 and effectively becomes an
enable input, as with this input low
none of the LEDs can be selected. One
of the LEDs will be turned on when
the C input is high, depending on the
state of the A and B inputs. Note that
the D input is permanently connected to ground. With a good pair, both
A and B will be high. The LEDs are
only turned on for the period of the
pulse from IC2e which has the added
benefit that the current drain from the
batteries is less than if a LED was on
constantly.
In summary, the result of phase 1 is
stored in the memory capacitor until
the result of phase 2 is available, at
which point they are both applied to
the decoder and the respective LED
is turned on.
Since high voltages could be pres-
The Cable and Wiring Tester is
mounted in a small plastic case with a
row of four LEDs and a pushbutton on
top. At one end is a 3.5mm jack socket
to enable connection to a pair of wires.
Pressing the button flashes one of the
four LEDs depending in the test condition: Open (Orange); Short (Yellow);
Reversed (Red); and Good (green).
All the components apart from
the test socket are mounted on a single-sided PC board. Fig.3 shows the
wiring diagram. Begin construction
by soldering in the five tinned copper wire links, ensuring that they are
straight and lay flat on the board. Follow this with the resistors, the zener
diodes and the PC stakes.
Next, solder in the capacitors, remembering that the 22µF capacitor
is polarised and must be inserted the
right way. The integrated circuits can
be installed next, ensuring that they
are in the correct way. These are CMOS
types and can be destroyed by static
electricity, so earth yourself and take
care not to handle them too much.
The LEDs are installed with the
top of each LED 25mm above the PC
board. They should protrude slightly
from the lid of the case when it is fitted. Similarly, the pushbutton switch
is installed in a vertical position by
soldering its tags to two PC stakes.
Again, the switch should be at the
correct height with the case closed.
Install the PC board in the bottom
case half with four self-tapping screws.
If you find it won’t sit properly, you
can lightly file the edge of the board
or cut out the small plastic tabs inside
the edge of the case. Drill a hole in the
centre of the top endplate and mount
the 3.5mm test socket. Place the two
The four LEDs and the pushbutton switch are stood off the board so that they protrude through the lid of the case.
end plates in the slots on the bottom
half of the case. The bottom half has
four holes for the case mounting
screws while the top half has threaded
brass inserts.
Solder two wires from the socket to
the PC stakes on the PC board. Now
solder in the battery clip and trim the
length of the wires so that they sit
neatly with the battery positioned as
shown in the photographs. You may
find it necessary to cut off some of the
plastic tabs on the inside of the top half
to clear the battery clip when the two
halves are screwed together. Drill the
four holes for the LEDs and for the test
switch in the top half of the case. The
positions for these can be quite easily
found by firstly making measurements
with a ruler and then marking with a
pencil before drilling.
The test lead is made from a short
length of figure-8 cable. The type used
in the prototype was coloured red and
black. I soldered the red A wire to the
centre pin of the 3.5mm plug and the
Parts List
1 PC board, code 04411971, 51 x
88mm
1 plastic case, 120 x 60 x 30mm
1 3.5mm mono phono socket
1 3.5mm mono phono plug
2 small black alligator clips
2 small red alligator clips
1 normally open pushbutton switch
6 PC stakes
1 9 volt battery clip
4 No. 4 x 6mm self-tapper screws
1 5mm red LED (LED1)
1 5mm yellow LED (LED2)
1 5mm orange LED (LED3)
1 5mm green LED (LED4)
Semiconductors
1 4053 triple analog selector (IC1)
1 40106 or 74C14 hex Schmitt
trigger (IC2)
1 4028 BCD-to-decimal decoder
(IC3)
2 9.1V 1W zener diodes
(ZD1,ZD2)
1 1N4004 diode (remote test
diode)
Resistors (0.25W, 1%)
1 330kΩ
1 4.7kΩ
2 100kΩ
6 680Ω
1 33kΩ
Capacitors
1 22µF 16VW electrolytic
2 0.22µF MKT polyester
3 0.1µF MKT polyester
1 .01µF MKT polyester
1 .001µF MKT polyester
Miscellaneous
Tinned copper wire, hookup
wire, figure-8 cable, small piece
of scrap stripboard, heatshrink
tubing
Resistor Colour Codes
❏
No.
❏ 1
❏ 2
❏ 1
❏ 1
❏ 6
Value
330kΩ
100kΩ
33kΩ
4.7kΩ
680Ω
4-Band Code (1%)
orange orange yellow brown
brown black yellow brown
orange orange orange brown
yellow violet red brown
blue grey brown brown
5-Band Code (1%)
orange orange black orange brown
brown black black orange brown
orange orange black red brown
yellow violet black brown brown
blue grey black black brown
November 1997 43
together. Now clip the tester leads to
the diode leads, with the red A wire
clips connected together and the black
B wire clips connected together. Press
the Test button and verify that the “G”
LED flashes.
Now reverse the connection to the
diode leads, press the Test button and
check that the “R” LED flashes. Once
you are happy with the testing, screw
the case together with the four screws
supplied, checking that the drilled
holes line up with the LEDs and switch
without placing stress on them.
Using the tester
The basic operation of the tester
should be quite apparent. Simply connect the diode to the remote end with
the red clip connected to the A wire,
the tester to the local end with the red
clip connected to the A wire, press
the Test button and monitor the LEDs.
Multiple wire cables
Another view of the prototype Cable & Wiring Tester. Power comes from an
internal 9V battery.
black B wire to the ground pin. To finish the lead, solder a red alligator clip
to the red wire and a black alligator
clip to the black wire.
The diode assembly can be made
next. It simply comprises a diode
soldered to a length of figure-8 cable
as before. Its anode is soldered to the
black wire and the cathode to the
red wire. I used a scrap piece of strip
board to give the assembly some mechanical strength and then covered it
with heatshrink sleeving to prevent
accidental shorting. The red alligator
clip is soldered to the red wire and
the black alligator clip to the black
44 Silicon Chip
wire. Finally, fit a good 9V battery
into the case.
Testing
With the assembly complete, press
the test switch briefly and check that
the “O” LED flashes at about 20Hz. If
it does, you can proceed with the rest
of the testing. If it doesn’t work, have
a good look at the assembly again and
check it for construction errors.
Plug the test lead into the socket
and connect the two alligator clips
together. Press the Test button and
check that the “S” LED flashes to
indicate that the wires are shorted
So far this article has referred to just
testing a pair of wires, such as those
in a telephone cable or Local Area
Network (LAN) cabling. However,
the tester can be used to test cables
with multiple wires even if they are
not paired.
The simplest way is to select one
of the wires as a common A wire and
then progress through the other wires
as a second B wire. If you are working
on cable that has, for example three
pairs, you might construct a multiple
diode lead with three diodes and six
leads so that you could check all the
pairs at one time.
Some cabling systems use a special socket to terminate a multiple
pair cable. An example of this is an
RJ45 socket used in modern building
cabling where four pairs provide computer and telephone connections at
one socket. A plug could be adapted to
hold four diodes and plugged into the
remote socket while the tester could
be plugged into a mating socket at the
local end.
A switch would need to be incorporated in the tester leads to select the
pair to be tested. Finally the tester can
be used as a general continuity tester
to test diodes, speakers, audio/video
cables, etc. The tester will indicate a
short circuit with about 2kΩ or less
placed across the test leads but this
will vary from unit to unit and is dependent mainly on the characteristics
SC
of the ICs used.
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