METAL DETECTION

BY THE PROF

PROSPECTING WITH A HOOVER

Metal objects “call out” to metal detectors as they sweep the
ground by interfering with the detector’s circuitry. Different
detectors measure different aspects of the effect and convert
them to audible or visual signals.

Although the “treasure” hunting craze of the seventies seems characteristics of the coil that a piece of metal in reasonably
to have waned considerably during the eighties, metal detection close proximity is likely to have. However, some detectors have
still remains a popular aspect of electronics. It is also one that, coils which detect changes in a radiated signal, rather than
perhaps after some over-kill in the past, has received very little directly detecting the metal. As far as I can ascertain, all normal
attention in the technical press over the last few years. Even if forms of metal detector use a search coil of some sort or other.
you are not interested in going out into the wilds in search of
In this article I will provide basic details of all the types of
Practical Electronics July 1988 - Copyright  Wimbourne Publishing

buried treasure, the technology of metal detection still

metal detector that I have been able to locate. Most of these
represents an interesting aspect of electronics. It is also
methods I have seen in operation, but with some of the less
potentially a very interesting line of pursuit for the electronics
common types I am not reporting from first hand experience.
experimenter.
An important factor to keep in mind is that there is usually
Most metal detector designs for the home constructor seem to more than one way to exploit each basic method of detection.
be of either the bfo (beat frequency oscillator) or ib (induction Consequently, the block diagrams provided here to illustrate the
balance) varieties, but there are actually many other types in way in which each type of detector operates are only intended
existence. Although the term “metal detector” brings to mind as examples. Not all metal detectors of each type will operate
images of treasure hunter style equipment, this type of precisely as described here, but they will be based on the same
equipment is actually used in a wide range of applications. fundamental principle.
These include things as diverse as pipe and cable location,
airport and other security applications, medicine, and electronic BFO
ignition systems. The bfo (beat frequency oscillator) method of detection is
one of the most simple. and has been much used at the low cost
In this article we will consider a number of different methods
end of the detector market. Most of the early home constructor
of metal detection, including both the common and some of the
units were also of this type, and I would guess that many
more obscure systems. In most cases practical circuits with
readers of PE have built one of these. The block diagram of
detailed descriptions are not provided, but a metal locator
Fig.l shows the general make-up for a unit of this type.
project will be provided in a future issue.
The search coil forms part of an L-C tuned circuit which acts
BEGINNINGS as the frequency selective circuit in an oscillator. The detected
Electronic metal detection has its origins further back in time metal could be regarded as being analogous to the adjustable
than most people would imagine. In fact the first electronic core in an if. transformer, and it causes a shift in the frequency
metal detector was designed over one hundred years ago (in of the oscillator. The oscillator could operate at an audio
1879 by Professor D.E. Hughes apparently). What is perhaps frequency so that the shift in frequency would be audible, but
even more surprising is that the original design was of the this would give totally inadequate sensitivity in practice. The
induction balance variety. I own a sophisticated metal detector problem is simply that the shift in frequency is not very great,
produced by a well known manufacturer of this type of and with direct audio operation not even a well trained ear
equipment, and this is of the induction balance type. In fact would be able to detect it.
many of today’s more complex detectors seem to be of this
type. This is not to say that modern detectors are simply refined
versions of the original, and that no new techniques have
evolved over the past hundred years or so. As we shall see,
there are numerous types of detector currently in use.
Many of today’s metal detectors are quite complex pieces of
equipment, but the principles on which they operate are
relatively straightforward. Much of the complexity of most
detectors is in the “bells and whistles” rather than in the main
detector part of the circuit. Virtually all methods of detection
rely on a pick up coil or coils, and the effect on the electrical

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 A high operating frequency and the heterodyne principle are used in practice though. A very simple method that I have
used to give improved sensitivity. The output from the search found to be quite effective is to have an oscillator which
oscillator and a second oscillator are fed to a detector and rf includes the search coil in its L-C tuned circuit, and to also
filter. The output from the filter is the “beat” note, which is have an accurate voltage monitoring circuit of some type
merely the difference between the frequencies of the two included in the unit. When metal is brought near to the coil it
oscillators. These are tuned so that the beat note is a low audio produces a change in the operating frequency, but probably of
frequency. If, for example, the search oscillator operates at greater importance in this case, it alters the 0 value of the coil.
199kHz and a metal object produces. a 0.01% reduction in The voltages in an oscillator are not exclusively under the
frequency in absolute terms this represents a shift of 10Hz. This control of the bias network, and in many cases the bias circuit
is not very much, but if the beat note was set at 50Hz, this exerts very little control. The change in 0 and operating
would give a 20% reduction in output frequency, with an actual frequency will give small voltage shifts with many oscillator
output frequency of 40Hz. This would be clearly detectable by designs, and these can be detected by the voltage monitoring
the human hearing mechanism. By contrast, direct operation at circuit.
50Hz and a 0.01% shift in frequency would given an inaudible
Vast numbers of metal locators must have been manufactured
shift of just 0.005Hz!
over the last ten to fifteen years, and one might reasonably
SENSITIVITY expect that there would be several special integrated circuits for
this purpose. This does not seem to be the case though, and the
For optimum sensitivity a high search oscillator frequency
commercial detectors I have seen have had circuits based on
Practical Electronics July 1988 - Copyright  Wimbourne Publishing

and low beat note are required. In practice a high operating

74ICs and the like. The only chip designed for this application
frequency is not possible due to legal requirements, but drift in
that I have been able to locate is the intriguingly named CS209
oscillators would probably preclude the use of operating
“stud detector” from Cherry Semiconductors. This device is
frequencies of many megahertz anyway. With this type of
primarily intended for detecting studs and nails in walls, but it
detector the output is normally in the form of an audio tone
is suitable for other metal location applications. Fig. 2 shows in
from headphones or a loudspeaker, but the change in output
somewhat simplified form the general arrangement used in the
frequency can be used to give some other form of indication
CS209.
with the aid of some additional circuits. Most bfo detectors only
seem to give the standard audio output though. Unfortunately,
some people are literally tone deaf, and the straight audio
output is not something that everyone finds usable.
I suppose the main attraction of this type of detector is that it
is cheap, rather than being highly sensitive or easy to use. In the
hands of an experienced operator quite good results can be
obtained, and this method of detection provides discrimination
between ferrous and non-ferrous metals. One category gives an
increase in the output tone while the other produces a reduction
(which type metal has which effect depends on how the unit is Once again, we have an oscillator with the search coil acting
set up). as the inductor in its L-C tuned circuit. The feedback level must
be carefully adjusted to the point where the oscillator only just
A drawback of bfo detectors is that they suffer from ground manages to sustain oscillation. The oscillator is sensitive to
effect problems. This is where placing the search coil close to changes in the 0 value of the coil, and the effect of a metal
the ground gives an indication from the unit even with no metal object close to the search coil is to produce a drop in the
present. In theory this does not matter provided the coil is kept amplitude of the output signal. The output is demodulated and
a constant distance above the ground, but in use this is virtually fed to a level detector circuit. The latter detects the drop in
impossible even with relatively flat terrain. Fortunately, the signal level and activates two output stages. The two outputs
ground effect problems can be practically eliminated with the are both current sinks which can handle currents of up to 150
aid of a Faraday shield. This is a metal sheath placed over the milliamps, but one is normally switched off while the other is
coil, but broken at some point so that it does not quite form a active under stand-by conditions.
complete ring. In my experience this dramatically improves
results with this type of detector, and for a home constructor Simple arrangements of this type are capable of quite good
unit something as basic as a sheath of aluminium foil will work results, but can also be disappointing in practice due to stability
wonders for the user-friendliness of the unit. Multi-layer search problems. The CS209 seems to offer good performance in this
coils are sometimes used with bfo detectors in an attempt to respect as it has a high quality built-in voltage regulator and a
improve sensitivity at larger depths. transient suppressor circuit. Another potential flaw in circuits of
this type is the strong innate hysteresis. In this case it occurs
DROP-OUT DETECTOR due to the reluctance of the oscillator to restart once oscillation
There are types of detector which are even more simple than has been allowed to drop-out completely. In the CS209 this
the bfo variety, and which offer a similar level of problem is minimised by a level control circuit which ensures
performance.They seem to be relatively unknown and little

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 generally much clearer than the varying tone produced by a bfo
detector.
The output from the amplifier is fed to a detector circuit that
gives a dc output level which is proportional to the output level
from the amplifier. If meter indication is required, the meter can
be driven from the detector stage via a buffer amplifier. For an
audio output the next stage is a dc amplifier which incorporates
a shift control that can be used to raise the quiescent output
voltage above its normal level of close to zero. In practice this
control sets the volume of the output tone under stand-by
conditions, and results are generally better with a quiet tone
rather than having zero output under quiescent conditions. This
control is sometimes (rather confusingly) referred to as a
that the oscillator can not cease oscillating even if a very large
“tuning” control incidentally.
piece of metal is placed close to the search coil.
The next stage is a “chopper”, which is merely an electronic
For those who would like to experiment with the CS209 the
switch that gates the output of the dc amplifier on and off. This
basic metal detector circuit of Fig. 3 is provided. The data sheet
generates a squarewave signal having a peak to peak amplitude
for this device recommends a value of about l00µH for L1, but
Practical Electronics July 1988 - Copyright  Wimbourne Publishing

equal to the output voltage from the dc amplifier. Accordingly,

its exact value does not seem to be critical. VR1 must be
the volume of this audio signal rises and falls in sympathy with
carefully adjusted for the highest resistance that does not result
the signal level received by the pickup coil. Most people find a
in Dl switching on. In fact it might have to be backed-off
change in volume easier to work with than the change in pitch
slightly from this setting, since Dl might otherwise tend to hold
provided by bfo type detectors, but by using a voltage
in the on state when the unit is activated. The automatic level
controlled oscillator (vco) a change in pitch could be obtained if
control circuit minimises the amount of hysteresis, but it does
preferred. The chopper could be driven from an audio
not totally eliminate it. Although in this basic circuit only led
oscillator, but it is often controlled by the main oscillator via a
Dl is switched on when metal is detected, obviously an audio
divider stage. Unlike bfo designs, ib circuits often work at quite
alarm or other form of indicator could be controlled by the
low frequencies (typically about 15 to 20kHz).
circuit. The normally on output, incidentally, is at pin 5 of IC1.
A three coil induction balance circuit uses the arrangement
IN THE BALANCE shown in Fig. 5, which is what some metal detector experts
The induction balance system requires two coils, or what in consider to be the only true ib setup. It is virtually identical to
practice may be three coils with some tappings. For the moment the slightly simplified type described previously, and it differs
we will only consider the basic twin untapped coil arrangement only in that two coils are driven from the oscillator. In practice
as depicted in the block diagram of Fig. 4. these two coils are carefully positioned and phased so that there
is zero output from the third coil, with the pick-up from coil 1
coil being cancelled out by the pick-up from coil 3. The
standard search head arrangement is to have coil 2 sandwiched
between coils 1 and 3, with coils 1 and 3 having the same
number of turns but being wound in opposite directions.

We have the familiar oscillator and search coil, but this time
with a second coil feeding into an amplifier. Although a strong
coupling from the coil driven by oscillator to the second coil
would be expected, careful positioning of the two coils provides
what is a very inefficient coupling with no significant output This type of detector operates in what is essentially the same
from the pickup coil. This sounds difficult, but is achieved manner as the two coil ib type, with metal in the vicinity of the
merely by partially overlapping the two coils. A piece of metalsearch coil upsetting the balance of the system, and producing a
close to the coils produces an imbalance and gives a strong stronger output from the amplifier. With both types it seems to
output from the amplifier stage. be normal to have the system adjusted so that under quiescent
conditions there is considerably less than perfect balancing.
There must be endless ways of converting this rise in signal
This enables the unit to discriminate between ferrous and non-
level into some form of indication to the operator of the
ferrous metals. The latter give a decrease in coupling whereas
equipment, but the general method shown here seems to be a
the former produce increased coupling.
popular one. It provides an audio output tone that rises in
volume when metal is detected. This form of indication is

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 Many of the more up-market ib designs have quite potentiometer so that the signal level can be adjusted for precise
sophisticated discrimination capabilities. This generally takes cancelling. As for the ib type, metal close to the search head
the form of a control that enables the unit to be nulled on a alters the strength of the signal from the pick-up coil, giving an
piece of metal, and it will then fail to respond (or at least be imbalance in the signal levels fed to the mixer and producing an
very insensitive to) any pieces of that particular metal in the output from the mixer. This signal is fed to a detector stage, and
search area. A typical use of the discrimination facility is to the resultant dc signal is amplified before being applied to a
render the detector insensitive to aluminium foil, which is meter. Of course, the dc signal can be fed to a chopper and
found in large quantities in many popular types of hunting audio amplifier if an audio indication is required as well. As the
ground. Note that discrimination means the ability of a detector t/r type of detector operates on what is really the same principle
to ignore certain metals: it does not mean the ability to ignore as the ib type, it has very much the same advantages and
everything but (say) gold or silver. In my experience the drawbacks.
discrimination facilities of metal detectors are less worthwhile
OFF-RESONANCE
than much of the advertising literature would have you believe.
There is often a penalty to pay in the form of reduced This seems to be a little used type of detector, and it utilises
sensitivity, and setting up a machine which has adjustable the setup shown in Fig.7. It is another type of detector that
discrimination and other advanced features can make setting up relies on a balancing process, but the operating principle is not
a video recorder to automatically record a programme look like the same as the ib and t/r types. In fact it is much more like the
child’s play. bfo type of detector in this respect.
Practical Electronics July 1988 - Copyright  Wimbourne Publishing

Induction balance metal detectors suffer from ground effect

problems. Some ib detectors now have a facility to “tune” out
the ground, but variations in soil moisture etc can make
frequent readjustment necessary. A very useful facility is an
automatic nulling circuit, which, over a period of a second or
two always adjusts the circuit for zero output. On the face of it
this cures the ground effect problem, but renders the detector
insensitive to everything else as well! However, in use the
detector is swept quite rapidly over the earth, and if the search
head passes over any metal the unit will produce a clear
indication before the nulling circuit has a chance to rebalance A signal generator feeds a signal to a buffer amplifier and
the circuit. Having used this method to roughly locate an object, detector circuit by way of an L-C tuned circuit that incorporates
the unit is switched to the ordinary mode so that it can be pin- the search coil. Some of the direct output of the search coil is
pointed. fed via a balance control to another buffer amplifier and
IB OR NOT IB detector circuit. The output signals from the detectors are fed to
a differential amplifier, and under quiescent conditions the
There seems to be some lack of agreement as to which balance control is adjusted for zero output from this amplifier.
detectors are ib types, and which are t/r (transmit/receive) units. As with a bfo locator, metal near to the search coil causes a
The general consensus seems to be that the types described shift in the resonant frequency of the tuned circuit. In this case
above are forms of ib detector, while the system depicted in the the tuned circuit is used as a filter, and the signal generator is
block diagram of Fig.6 is the t/r type. tuned just off-resonance where the filter has a rapid roll-off
This system is very similar to the ib type, and it has one coil rate. The shift in resonant frequency therefore gives a change in
driven from an oscillator and a second one feeding into an signal level that gives an imbalance at the differential amplifier,
amplifier. The amplified signal and some of the output from the and a consequent meter deflection.
oscillator are fed to a mixer. The signal from the amplifier is Compared to a bfo type detector this system has the
coupled to the mixer via an inverter, or perhaps some other advantage of providing meter indication, or an audio signal of
form of phase shifter circuit. The idea is to have the signals at varying volume if suitable output circuitry is included. In other
the mixer out of phase so that they cancel each other out. The respects it should perform as well (or as badly as) an equivalent
signal from the oscillator is fed to the mixer by way of a bfo design.

PULSE TYPE
Pulse induction detectors operate on a principle that is
completely different to ib and bfo style detectors. Fig. 8 shows
a somewhat simplified block diagram for a pulse induction
detector.
The principle of operation may be different, but a search coil
is still central to the operation of the system. A pulse generator
supplies brief pulses to the coil, and this generates a magnetic

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 up that drives an amplifier. Note that the coils are not arranged
field around the coil. On the trailing edge of each pulse the in such a way as to give zero output from the pick-up coil.
magnetic field collapses and generates a reverse voltage across Operation of this system relies on a reasonably strong output
the coil. The coil must be damped (electrically that is!) so that signal being obtained from the pick-up coil. The output signal
the reverse voltage spike decays quite rapidly. If there is a piece of the oscillator and the output of the amplifier are fed to
of metal in the vicinity of the coil it will either produce an eddy squaring circuits, as shown in the block diagram of Fig. 9.
current or become magnetised by the field of the coil,
depending on whether the metal is non-ferrous or ferrous. As The effect of metal close to the search head is to produce a
far as the effect on the reverse pulse from the coil is concerned, phase shift in the output from the pick-up coil. Linear or digital
in either case the pulse becomes stretched. circuits could be used to detect this phase change and convert it
Practical Electronics July 1988 - Copyright  Wimbourne Publishing

to some form of visual or audible output indication, and the

Turning the elongated pulse into a clear visual or audible digital approach is probably the more simple but effective
indication is usually done with the aid of a gate and an method. In fact an ordinary logic gate can operate as an
integrator. The gate is opened by pulses from a second pulse effective phase detector. Things would normally be arranged so
generator, but this circuit is triggered from the main pulse that the two signals are normally in phase, and move out of
generator. Remember that it is the reverse pulse generated phase when metal is detected. This gives zero output or very
across the coil that must be allowed to pass through the gate brief pulses from the phase detector under stand-by conditions,
circuit, and not the signal from the main pulse generator. The but longer pulses as metal is brought close to the search head. A
second pulse generator is therefore triggered on the trailing lowpass filter is all that is needed to convert the pulses into a dc
edge of the signal from the main pulse generator. In fact it is the output signal, or perhaps some form of digital pulse duration
part of the reverse pulse where it nears 0 volts that is really of measuring circuit could be used to provide a visual indication
interest, and the gate pulse can usefully be delayed slightly so on a digital display.
that the initial part of the waveform is cut out. The integrator
generates an output voltage that is a product of time and input A unit of this type could be designed to operate at virtually
voltage, and the stretched pulses give a higher output voltage. any frequency, but it is advantageous to use a very low
This signal is amplified and fed to a meter, and the output from frequency as this apparently avoids problems with the dreaded
the amplifier can be used to drive some form of audio indicator ground effect.
circuit if desired.
CONCLUSION
This system has definite advantages over the other types There are several different types of metal detection circuit
described so far, including its immunity to ground effect currently in use, with each type having its advantages and
problems, and what is generally much better stability than other disadvantages. Clearly the perfect metal detector has yet to be
types of detector. It is not without drawbacks though, such as invented. People who have never used a metal detector tend to
an inability to differentiate between ferrous and nonferrous have an exaggerated idea of the sort of performance that can be
metals, and what is often a relatively high level of power obtained. It takes a good unit to detect a 2p coin at a depth of
consumption (although some other types of detector tend to be 150 millimetres or so, and few units can detect even quite large
so packed with circuitry that they also consume large amounts pieces of metal at much more than two or three times this
of power). A point that has to be borne in mind with this type of figure. Super high sensitivity is not necessarily an asset
detector is that it gives absolute rather than relative detection. anyway, and it can simply provide confusing results with every
In other words, whereas it is possible to adjust other types of speck of metal being detected. When using my detector at full
detector to nullify the effects of any metal which forms part of sensitivity it often detects what turns out to be a small patch of
the detector itself, this is not possible with a pulse induction rust!
type. It is therefore important to have metal-free construction in
the search head and at least the lower part of the detector’s Ready made metal detectors of fair performance seem to be
“stem”. quite expensive, and this is one aspect of electronics where it is
certainly possible to build a unit that will cost considerably less
VLF PHASE ANGLE than buying a “real one”. There is plenty of scope for anyone
This is an interesting type of detector, but I can not say that I wishing to try their hand at designing this type of equipment.
have ever encountered a unit which utilises this technique. It is One last point is that an operating license is no longer needed in
reminiscent of a simple ib or t/r detector in that one coil is order to operate a metal locator in the UK, but it is only legal to
driven from an oscillator, while a second coil is used as a pick- use detectors that fall within certain (not especially stringent)
specifications.
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