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Simple, sensitive metal detector
Geotech
Page 1
E
T
I: How T
o
Bui
l
d Gold

&

T
r
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De
te
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1981



Co
pyri
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Mode
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METAL DETECTORS depend on
detecting one of several effects that can
be observed when a metal object influ-
ences the magnetic field surrounding a
coil of wire carrying an alternating cur-
rent. The principal effects are: the
pattern of the magnetic field surround-
ing the coil will be altered and the
inductance of the coil will change.
The various types of metal detector
devised exploit these changes, electroni-
cally detecting the alteration induced in
the coil by the metallic object. Non-
metallic objects or material can also
affect the coil in similar ways.
There are three basic methods
employed to exploit the above effects.
Induction Balance (IB) metal detec-
tors employ two coils. One is driven by
a modulated oscillator. The other is con-
nected to a detector and amplifier. The
two coils are carefully positioned with
respect to one another such that the
receiver coil picks up very little of the
energy radiated by the transmitter coil
when no metal or mineral material is
nearby. When the coils are brought near
a metal object, the field pattern is dis-
torted, greatly increasing the transmitted
energy picked up by the receiver coil.
The modulated signal is detected and
can be indicated by amplifying the
recovered modulation to speaker level
as well as indicating it on a meter. For
obvious reasons, this type of metal
detector is often referred to as a trans-
mit-receive or TR detector, sometimes
as an IB/TR detector. Chief advantages
are good pinpointing ability and good
depth penetration, and they are not sen-
sitive to small ferrous objects.
Sensitivity suffers badly in mineralised
or ironstone ground. We described an
IB/TR metal detector back in our May
1977 issue (Project 549) and it is still a
popular project. The problem for the
home constructor lies in correct con-
struction and alignment of the coils.
Most IB detectors operate at a fre-
quency between 85 kHz and 150 kHz.
As they are badly affected by minera-
lised ground a technique was developed
using very low frequency to energise
the transmit coil. The VLF types oper-
ate at frequencies around 4 - 6 kHz, a
frequency range which penetrates all
types of soil quite well. However, they
need to run at a fairly high power to
achieve sufficient sensitivity with small
objects, hence battery drain is quite
high, and pinpointing ability is poor.
Pulse Induction detectors employ
coils in the search head that are set up in
much the same manner as the IB detec-
tor. However, the transmitter is pulsed
so that high energy bursts are transmit-
ted by the search coil. The receiver then
compares the phase of portion of the
received pulse with the transmit signal.
When a ferrous or magnetic object is
brought near the search coils the phase
of the received signal is advanced with
respect to the transmit signal. The oppo-
site occurs when a non-magnetic
conductor is brought near the search
coils. Thus, this type of detector can
effectively discriminate between fer-
rous and non-ferrous metals as well as
exclude ground effects simply by
setting the detection circuitry to exclude
signals of the unwanted phase charac-
teristics. Thus, a Ground Exclusion
Simple, sensitive metal detector
Phil Wait
The metal detecting hobby is enjoying quite a boom at the
moment and treasure hunters are not just after gold. Though the
price of the precious metal has fallen in recent months, at
around $600 an ounce its worth going after. Old coins and rel-
ics fetch high prices too, so theres lots to find out there...
FEATURES
Good sensitivity
Excellent stability
Good pinpointing ability
Loudspeaker output
Simple construction and set up
Tuning allows for ground
Low cost Geotech
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1981



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control is often featured with these
detectors. As the strength of the
received signal also varies, depending
on the target objects characteristics,
this effect may also be included in the
detection process.
Clearly, an IP detector presents many
problems to the home constructor.
The simplest technique detects the
change in inductance of a single search
coil. If this coil is part of the tuned cir-
cuit of an oscillator, then comparing the
frequency of the search oscillator with
a stable reference oscillator will indi-
cate the presence of a metal object. This
detector is called the Beat Frequency
Oscillator or BFO type. The two oscil-
lators are set such that there is a slight
difference in their frequencies and their
outputs mixed. The resultant will be a
beat frequency which is equal to the
difference between the two oscillator
frequencies. The main advantages of
this type are simple circuitry and set-
ting up along with good pinpointing
ability. In the past, most published
designs have suffered from a distinct
lack of sensitivity as well as poor tun-
ing stability. A cunning mixing
technique and a few other fillips can
overcome these problems.
Hence, our new metal detector is a
BFO type incorporating some modern
refinements. It has proved to have simi-
lar sensitivity to our IB detector, the
ETI-549, but is generally easier to build
and set up, there being no critical
adjustments.
Design features
Our new metal detector has three con-
trols: COARSE frequency adjust, FINE
frequency adjust and VOLUME on/off
The coarse frequency control is used to
initially set the frequency of the search
oscillator, compensating for the various
factors affecting any drift in this oscilla-
tor (mainly temperature and battery
voltage). The fine frequency control is
then used to set the note to a low pitch
when the detector is placed over the
ground, permitting compensation for the
effect of the ground on the frequency of
the search oscillator. The volume con-
trol adjusts the loudness of the output
from the speaker.
The two main design problems this
type of detector presents are the fre-
quency stability of the two oscillators
and the minute frequency change which
has to be detected.
The search oscillator we finally used
was settled on after some experimenta-
tion. Our first try employed an LC
oscillator built around a CMOS gate
chip. This proved to be not as stable as
we required and we found that trying to
obtain dc control of the frequency by
varying the supply rail voltage had
drawbacks. After some experimenta-
tion with oscillator configurations we
hit on a discrete component oscillator
which we found behaved much as we
were seeking.
The search coil in the circuit we used
is the inductor in a Colpitts oscillator.
However, this particular circuit may be
a little unfamiliar to many readers. To
increase the RF current in the coil, it is
placed in the collector circuit of Q1.
Feedback is between collector and emit-
ter and the base is effectively at RF
ground. The frequency determining
capacitance of the tuned circuit is
tapped to provide feedback, C2 and
C3 performing this function. Careful
attention has been paid to the basic fre-
quency stability of this oscillator. Good
quality styroseal capacitors have been
used for C2 and C3. These have a tem-
perature coefficient roughly opposite to
that of other temperature influences on
the frequency of the oscillator. In gen-
eral, the short-term stability of this
oscillator is quite good.
The particular circuit configuration of
the oscillator gave us a very useful
bonus dc control of the oscillator fre-
quency over a small range. Varying the
base bias on a transistor will vary the
collector-base capacitance. In this cir-
cuit, the c-b capacitance is part of the
overall stray capacitance that deter-
mines the exact frequency of
oscillation. As the base bias is increased
the c-b capacitance decreases, increas-
ing the oscillator frequency. In this way,
the oscillator frequency can be varied
over a range of about ten percent. We
have provided two controls, the FINE
control providing a variation of about
one-tenth that of the COARSE control.
The search oscillator is loosely cou-
pled via a 47p capacitor to a following
CMOS Schmitt trigger and two inve