'Matchless' Metal Locator (Detector) - By Thomas Scarborough (Silicon Chip - June 2002) 7s - GeoTech(1).pdf

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Silicon Chip Publications Pty Ltd, reproduce for personal use only
by Thomas Scarborough
An induction balance (IB) metal loca-
tor has a good depth of penetration and
distinguishes well between ferrous and
non-ferrous metals. It is also capable, to
a large extent, of rejecting iron and also
tin foil This is a boon for anyone who is
searching for coins or noble metals.
My aim with this design was to cre-
ate a ‘minimalist’ device — one that
would work well but without all the
bells and whistles of the expensive,
commercial designs. I found that it was
possible, with just a handful of compo-
nents, to design a high-quality metal
locator.
For instance, on comparison with the
first-class EE-Magenta Buccaneer, this
design delivers 95% of the perfor-
mance in the category where it really
matters — a clear indication of the pres-
ence of metal.
‘Matchless’
Metal
Locator
Want to find a fortune? Buried treasure, perhaps? Lost
coins on the beach? Or perhaps you fancy earning some
pocket money finding other people’s valuables. Either way,
this project should really interest you. It’s an el-cheapo
induction balance (IB) metal locator that delivers surpris-
ingly good performance.
Simple, but it works
An IB metal locator is usually far
more complex than the design shown
here — the EE-Magenta Buccaneer, for
example, uses more than 70 compo-
nents. This one uses less than 20.
The reason for the simplicity is that I
have dispensed with analog circuitry,
and instead used a digital transmitter
and receiver.
As the search coils pass over metal,
only digital signals of a certain ampli-
tude break through to a peak detector
(IC1b). Since these are in the audio
range, they are immediately transferred
to the piezo sounder or headphones.
On testing the sensitivity of this
design in air, with optimal tuning and
using a 25mm-diameter brass coin, it
gave a clear signal at 150mm, and a
‘screaming’ signal at 110mm. It was
also able to detect a pin at 30mm.
Silicon Chip, June 2002 — Copyright
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Silicon Chip Publications Pty Ltd, reproduce for personal use only
Note that these figures
may not apply in tbe ground, where
depth of penetration will depend largely
on the mineralisation present.
In contrast, the locator is far more
reluctant to pick up tin foil. A tin foil
disk of the same size as the brass coin
was only detected at half the distance in
Silicon Chip, June 2002 — Copyright
air.
This
rejection of tin foil is due in
part to the metal locator’s low fre-
quency, which avoids what is called
skin effect.
Besides this, if the two coils are posi-
tioned as described, ferrous metals
(iron) are, to a very large extent,
rejected — to such an extent, in fact,
that a 25mm diameter brass coin weigh-
ing seven grams looks the same to the
metal locator as a lump of iron weigh-
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Silicon Chip, June 2002 — Copyright
ing 20 times as much. Large nonferrous
ohjects are detected at half a metre dis-
tance and more.
The locator’s power consumption is
conveniently low. It draws around
10mA, which means that it may be
powered off a small 9V battery. If an
alkaline battery is used, this will pro-
vide about 48 hours’ continuous use. In
my experience, the number of coins that
are found on a beach in an hour or two
should easily make up for the cost of
batteries!
Finally, while the stability of the loca-
tor is not the best, it’s by no means the
worst either. Re-tuning is necessary
from time to time, especially in the first
few minutes of use. One soon becomes
accustomed to giving the Fine Tune
knob an occasional tweak — perhaps
with every 40 or 50 sweeps of the
search head.
Circuit description
The search head of a typical IB metal
locator contains two coils: a transmitter
(Tx) coil and receiver (Rx) coil.
In this case, the Tx coil is driven by a
square wave oscillator, which sets up an
alternating magnetic field in the coil.
The Rx coil is then positioned in such a
way that it partly overlaps the Tx coil.
By adjusting the amount of overlap, a
point can be found where the voltages
in the Rx coil ‘null’ or cancel out, so
that little or no electrical output is pro-
duced. A metal object which enters the
field then causes an imbalance, result-
ing in a signal.
The transmitter (IC1a) is a standard
555 oscillator configuration, using one
half of the ICM7556IPD dual low
power CMOS version of this IC.
Do NOT use the veteran NE556N IC,
by the way.
IC1a oscillates at about 700Hz, deter-
mined by R/C components around pins
1, 2 and 6. The 680Ω resistor limits the
current passing through the Tx coil.
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The receiver section (IC1b) is pre-
ceded by a simple yet sensitive
preamplifier stage, based on Q1, which
amplifies the signal received from the
Rx coil. This is fed directly to IC1b,
which is used here as a high-perfor-
mance sine-square convertor. Its input
at pins 8 and 12 is biased by the divider
formed by the 10kΩ resistor and pots
VR1-VR3, so that only pulses of a cer-
tain amplitude break through to output
pin 9.
There is a point at which, with care-
ful adjustment, the signal is just
breaking through in the form of a crack-
ling sound. When the locator’s output is
adjusted to a fast crackle, the presence
of metal turns this into a ‘scream’. This
is heard from the piezo sounder or
through standard headphones. The 7556
IC allows up to 100mA of output cur-
rent, therefore no further amplification
is required.
Winding the coils
The one drawback to any IB metal
locator design is its need for two coils,
which must be very carefully and rig-
idly positioned in relation to one
another. Sometimes there’s no room
even for a fraction-of-a-millimetre error
in positioning these coils. While this
particular design makes things easier
than usual, the placement of the coils
will still require some patience. On the
other hand, the winding of the coils is
relatively easy. Each coil also includes a
electrostatic (Faraday) shield, which
helps to minimise ground effect.
The winding of the (identical) coils is
not critical and a little give and take is
permissible.
I used 30SWG (0.315mm) enamelled
copper wire, winding 70 turns on a cir-
cular former, 120mm in diameter.
I created the former with a sheet of
stiff cardboard with 12 pins stuck
through it at a suitable angle (the heads
facing slightly outwards). The coil was
wound clockwise around the pins, then
temporarily held together with stubs of
insulating tape passed under the coil
and pressed together over the top. The
coil may be jumble-wound (that is, you
don’t have to wind the turns on side-by-
side in neat layers).
Once this has been done, the pins are
removed, and a second coil is wound in
Silicon Chip, June 2002 — Copyright
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Silicon Chip, June 2002 — Copyright
the same way. In each case, mark the
beginning and end wires. Each coil is
then tightly bound by winding insulat-
ing tape around its entire circumference.
Now we add a Faraday shield to each
coil. This is accomplished with some
long, thin strips of aluminium foil. First
scrape the enamel off each coil’s end
wire. Solder a 100mm length of bare
wire to the winding wire, and twist this
around the coil, over the insulating tape.
This provides electrical contact for the
Faraday shield.
Beginning at the base of this lead, the
foil is wound around the circumference
of the coil, so that no insulating tape is
still visible under the foil but the foil
should not complete a full 360°. Leave
a small gap (say 10mm) so that the end
of the foil does not meet the start after
having gone most of the way around.
Do this with both coils. Each coil is
now again tightly bound with insulat-
ing tape around its entire circumference.
Attach each of the coils to its own
length of quality single-core screened
audio cable, with the Faraday shield in
each case being soldered to the screen.
Do not use stereo or twin-core micro-
phone wire to run both leads together;
this may cause interference between the
coils.
Gently bend the completed coils until
each one is reasonably flat and circular,
with each end wire facing away from
you, and to the right of the beginning
wire. Now bend them further until they
form lopsided ovals like capital Ds (see
Fig.2). The backs of the Ds overlap
each other slightly in the centre of the
search head. This is the critical part of
the operation, which we shall complete
after having constructed the circuit.
Last of all, wind strips of absorbent
cloth around each coil (I used strips of
thin dishwashing cloth such as Chux),
using a little all-purpose glue to keep
them in place. Later, when epoxy resin
is poured over the coils, this cloth
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