Post on 06-Mar-2018
Geotech Page 1
ET
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Gol
d &
Tre
asur
e D
etec
tors
, 198
1 —
Cop
yrig
ht
Mur
ray/
Mod
ern
Mag
azin
es, r
epro
duce
for
per
sona
l use
onl
y
“GOLD FEVER,” shrieked the newsheadlines following the finding of the27 kg Hand of Faith nugget at Wedder-burn in Victoria recently. It wasunearthed by a couple of amateur fos-sickers using a metal detector, justabout the most sophisticated tool everbrought to bear in the hunt for gold.
Designs for metal detectors genu-inely able to discriminate between‘trash’ and ‘treasure’ have generallybeen well kept trade secrets. Even thegeneral principles of operation havebeen veiled in mystery. However, weare indebted to Lee Allen of AltekInstruments of the UK for providing uswith the circuit design of this metaldetector project via our British edition.The design incorporates all the featuresand refinements of modern commer-cially-made instruments and featuresperformance equivalent to units costingtwo to three times as much.
Principles of operationThis detector employs the basically
well-known induction balance tech-nique to detect the presence of ametallic ‘target’ in the ground, but
includes a number of refinements whichrespond to certain characteristics of thetarget. The ‘search head’ contains twocoils: an outer coil which is connectedto a low frequency oscillator operatingsomewhere in the range 15 - 20 kHz,and an inner coil which is placed so thatit is only very loosely coupled to the
the coupling between the transmit andreceive coils will be increased. Thisgenerally produces an increase in thesignal from the receive coil. In simpleinduction balance detectors, such as theETI-549 (May 1977), this signalincrease is detected and used to gate anaudio oscillator on so that a tone is
A �discriminating� metal detectorThis metal detector operates just like the bought onesbut costs only one-third to one-half as much to build ityourself. It features three discriminate ranges plus VLFoperation and includes an auto-tune button.
design: Lee Allen, Altek Instruments, UK
article: Phil Wait
outer coil. The latter is connected to the‘receiver’ input of the instrument. Beingonly very loosely coupled, the signalinduced in the receiver (inner) coil fromthe transmit (outer) coil is very smallwhen a target is not in the vicinity of thesearch head.
When the search head approaches ametallic target, the target will have anumber of influences on the two coils.Firstly, the magnetic field pattern of thetransmit coil will be disturbed, and thus
Geotech Page 2
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Gol
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Tre
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e D
etec
tors
, 198
1 —
Cop
yrig
ht
Mur
ray/
Mod
ern
Mag
azin
es, r
epro
duce
for
per
sona
l use
onl
y
passed to a speaker or headphones.That’s all quite straightforward, but
there are other influences to be takeninto account. The ground in which a tar-get is buried can have quite a profoundeffect on the coils in the search head.Firstly, if the ground is basically non-conducting, then it will have a perme-ability considerably different to that ofair. This will affect the couplingbetween the two coils in the searchhead, increasing the coupling if thetransmit and receive coils are initiallyset up away from the influence of theground. You can compensate for thiseffect by physically varying the posi-tion of one coil in relation to the otherwhen the search head is near theground. However, different soils willhave different compositions and thushave different values of permeability —even within quite a small area. The bestway to compensate is by electronicmeans and well go into that shortly.
If the soil contains an appreciableamount of iron minerals (maghemite,hematite etc... often referred to as “ironstone soils”), or mineral salts of onetype or another, then it will be partlyconducting.
Such soils will have a permeabilityoften greater than basically nonconduct-ing soils, affecting the couplingbetween the coils in the search head in asimilar way to that just explained.Again, as the composition of the soilsvaries, so will the coupling. Another
effect is that of ‘eddy currents’ inducedin the conductive soil. The ac magneticfield of the transmit coil will induce acurrent in the ground beneath the searchhead and the eddy current has an effectopposing the permeability effect of thesoil — and the whole effect varies in acomplex and unpredictable way as yousweep the search head over the ground.
The only way to compensate for thesevarying, and generally unpredictableeffects, is to devise circuitry that ‘recog-nises’ the effect.
Permeability effects will vary thephase as well as the amplitude of thesignal coupled into the receive coil fromthe transmit coil while eddy currenteffects vary the amplitude. Knowingthat, one can devise appropriate cir-cuitry to take the effects into account.
However, we need to know how ametallic target affects the phase andamplitude of the receive signal. If thetarget is ferrous, it will have a muchgreater effect on the magnetic field ofthe transmit coil than will the surround-ing soil as its permeability is greater andit will ‘bend’ or concentrate the fieldlines to a much greater degree. If thetarget is non-ferrous it will have a per-meability effect opposite to that offerrous targets, deflecting the field lines,but eddy currents also have someinfluence.
The eddy current effect in a targetdepends on the electrical and physicalcharacteristics of the target. Metals
which are good conductors will havegreater induced eddy currents than met-als which have a higher resistivity. it isa fortunate accident of nature that goldand silver are good conductors (lowresistivity) while iron (especially if itsoxidised or rusty) is not so good aconductor.
If the target is ring-shaped then theeddy current effect is enhanced,whereas if its a broken ring or just apeculiarly-shaped mass, the eddy cur-rent effect is less pronounced. The‘attitude’ or orientation of the targetwill also affect the eddy current effects.If the main plane of the target object isaligned such that the field lines from thetransmit coil cut it at right angles, thenthe eddy currents induced will be at amaximum. If the main plane of the tar-get is aligned parallel to the transmitfield then the eddy currents induced willbe at a minimum. Obviously, the atti-tude of the target with respect to thetransmit coils field will vary as the headpasses over it and the eddy currenteffect will vary accordingly — it maynot be maximum beneath the centre ofthe search head.
The permeability and eddy currenteffects combine in the receive coil andthe signal varies in phase and ampli-tude in characteristic ways.
The instrumentThe best way to understand how this
instrument operates is to look at it in
Geotech Page 3
ET
I: H
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Gol
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Tre
asur
e D
etec
tors
, 198
1 —
Cop
yrig
ht
Mur
ray/
Mod
ern
Mag
azin
es, r
epro
duce
for
per
sona
l use
onl
yblock diagram form. The accompany-ing diagram shows the basic circuitblocks employed. The transmit oscilla-tor drives the transmit coil in the searchhead and supplies a signal to two phasecontrol circuit blocks. The signal fromthe receive head is first amplified andthen ac-coupled to the input of a gate(gate 1). This gate is controlled by theoutput from one or other of the phasecontrol circuits via a block which‘squares up’ and inverts the signal. Theoutput gate consists of an ac signalsuperimposed on a dc level. This passesto an integrator which obtains the aver-age dc level of the composite signal.This is then passed to both a dc ampli-fier which drives a centre-zero meter,and to a ‘sample and hold’ circuit. Theoutput of this block provides a dc levelto the input of gate 1 which is a mea-sure of the average dc level of thecomposite signal. The initial dc levelapplied to the input of gate 1 is actuallyestablished by the tune control. Thus, adc negative feedback path is provided.
In addition to meter indication, anaudio indication is provided. The out-put of the dc amplifier driving the metercontrols a gate which switches on or offthe output of an audio oscillator. This isapplied to an audio amplifier and an on-board loudspeaker or headphones.
Power for the audio amplifier is pro-vided by two 9 V batteries in parallel.The rest of the circuitry requires twosupply rails at +10 V and +5 V withrespect to the common rail (0 V). Thisis supplied by a regulator from an 18 Vsource consisting of two 9 V batteriesconnected in series.
Initially, the instrument is set up inthe ‘VLF’ mode. The search head isheld in the air and the tune controladjusted to bring the meter to centrezero. This is done with the tune mem-ory button depressed. This activates thesample and hold circuit, storing the dclevel set by the feedback loop in thecapacitor of the sample and hold block.Thus, a particular dc level at the outputof the integrator corresponding to metercentre zero is set up.
The search head is then lowered tothe ground. Naturally, this will upset thecoupling between the transmit andreceive coils and the output at gate 1will change. This will change the dc
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ET
I: H
ow T
o B
uild
Gol
d &
Tre
asur
e D
etec
tors
, 198
1 —
Cop
yrig
ht
Mur
ray/
Mod
ern
Mag
azin
es, r
epro
duce
for
per
sona
l use
onl
y
Geotech Page 5
ET
I: H
ow T
o B
uild
Gol
d &
Tre
asur
e D
etec
tors
, 198
1 —
Cop
yrig
ht
Mur
ray/
Mod
ern
Mag
azin
es, r
epro
duce
for
per
sona
l use
onl
y
As
the
gene
ral
prin
cipl
es o
f op
erat
ion
have
been
dis
cuss
ed w
ith r
egar
d to
the
blo
ck d
ia-
gram
in t
he t
ext,
this
des
crip
tion
is c
onfin
ed t
oth
e C
ircui
t alo
ne.
Com
men
cing
w
ith
the
tran
smitt
er,
Q3
isco
nfig
ured
as
a C
olpi
tts O
scill
ator
, th
e tr
ans-
mit
coil
in
the
sear
ch
head
fo
rmin
g th
ein
duct
ance
whi
ch r
eson
ates
with
the
com
bina
-tio
n C
9 an
d C
10.
Bia
s is
app
lied
to Q
3 vi
aR
18,
D1
and
R19
. E
mitt
er b
ias
is p
rovi
ded
byR
17. T
he tr
ansm
it si
gnal
to th
e ph
ase-
lead
and
phas
e-la
g ci
rcui
try
(gro
und
and
disc
rimin
ate
cont
rols
) is
tap
ped
off
the
colle
ctor
of
Q3
via
C8
to th
e ju
nctio
n of
R11
and
R14
. The
gro
und
cont
rol c
ircui
try
conn
ects
via
R11
whi
le th
e di
s-cr
imin
ate
circ
uitr
y co
nnec
ts v
ia R
14.
The
sig
nal
from
the
rec
eive
coi
l is
ampl
ified
by Q
1 an
d ap
plie
d to
gat
e 1
(see
blo
ck d
ia-
gram
), o
ne C
MO
S g
ate
in I
C1
(IC
1b),
via
an
emitt
er-f
ollo
win
g bu
ffer
stag
e,
Q2.
Not
e th
atQ
1 is
ope
rate
d as
a g
roun
ded-
base
am
plifi
er.
The
pha
se o
f th
e re
ceiv
ed s
igna
l th
roug
h Q
1an
d Q
2 is
not
alte
red.
Out
put
from
the
em
itter
of Q
2 is
app
lied
to th
e dr
ain
of IC
1b.
The
bas
e of
IC
1b
is
driv
en b
y a
squa
rew
ave
deriv
ed
from
th
e tr
ansm
it si
gnal
, th
eph
ase
of w
hich
can
be
varie
d by
eith
er t
hegr
ound
or
disc
rimin
ate
cont
rols
.In
the
VLF
mod
e, t
he p
hase
of
the
tran
smit
sign
al t
appe
d of
f fr
om Q
3 ca
n be
var
ied
usin
gR
V1.
Thi
s pr
ovid
es a
pha
se-a
dvan
ced
sign
alth
at c
an b
e va
ried
over
the
ran
ge f
rom
abo
ut+1
0° t
o +
40°.
A l
eadi
ng p
hase
RC
net
wor
k is
form
ed b
y R
10 a
nd R
V1
in c
onju
nctio
n w
ithC
7. T
his
sign
al is
app
lied
to t
he in
vert
ing
inpu
tof
an
op-
amp,
lC
4b.
As
this
is
oper
ated
at
max
imum
gai
n w
ith a
hig
h si
gnal
lev
el a
t th
ein
put,
it w
ilI ‘s
quar
e up
’ the
sig
nal a
t its
out
put
(pin
7),
whi
ch d
rives
the
gate
of I
C1b
.In
the
dis
crim
inat
e m
ode,
sw
itch
SW
1 co
n-ne
cts
the
tran
smit
sign
al
to
circ
uitr
y w
hich
prov
ides
a l
aggi
ng p
hase
sig
nal
whi
ch c
an b
eva
ried
over
a r
ange
set
by
RV
2 (t
he d
iscr
imi-
nate
con
trol
) an
d a
set
of
‘rang
e’ r
esis
tors
:R
12,
R13
, R
15 a
nd R
16.
The
se f
orm
a l
ag-
ging
ph
ase
RC
ne
twor
k in
co
njun
ctio
n w
ithC
11.
The
sig
nal
is t
hen
buffe
red
by a
non
-
inve
rtin
g op
-am
p,
lC3b
, an
d ap
plie
d to
th
ein
vert
ing
inpu
t of l
C4b
via
SW
1c.
The
sou
rce
of I
C1b
is c
onne
cted
to
an in
te-
grat
or s
tage
for
med
aro
und
lC3d
. T
he o
utpu
tof
thi
s st
age
is c
onne
cted
dire
ctly
to
the
sens
i-tiv
ity
cont
rol,
RV
3.
The
w
iper
of
th
ispo
tent
iom
eter
goe
s di
rect
ly to
the
inpu
t of
a d
cam
plifi
er,
lC3a
, to
w
hich
w
e sh
all
retu
rnsh
ortly
. T
he w
iper
of
RV
3 is
als
o co
nnec
ted
toth
e sa
mpl
e an
d ho
ld c
ircui
t, vi
a R
25,
whi
chin
volv
es I
C5,
IC
1a,
the
tune
con
trol
RV
4 an
dth
e tu
ne m
emor
y pu
shbu
tton,
PB
1.T
he s
ampl
e an
d ho
ld c
ircui
t wor
ks in
the
fol-
low
ing
way
. T
he ju
nctio
n of
res
isto
rs R
25,
R26
and
R32
will
be
at a
dc
leve
l det
erm
ined
by
the
dc le
vel a
t th
e w
iper
of R
V3
and
the
dc le
vel a
tth
e w
iper
of
RV
4, t
he t
une
cont
rol p
oten
tiom
e-te
r.
The
dc
le
vel
at
the
wip
er
of
RV
3 w
illde
pend
on
the
sign
al le
vel a
nd p
hase
sw
itche
dth
roug
h to
the
int
egra
tor
by I
C1b
. W
hen
the
tune
m
emor
y pu
shbu
tton
PB
1,
is
pres
sed,
IC1a
(al
so a
CM
OS
sw
itch)
will
app
ly a
dc
leve
l to
the
inp
ut o
f th
e sa
mpl
e an
d ho
ld c
ir-cu
it pr
opor
tiona
l to
the
dc le
vel a
t th
e ju
nctio
nsof
R25
, R
26 a
nd R
32.
Thi
s w
ill c
harg
e C
13an
d th
e ou
tput
of
IC5
will
set
tle a
t th
is v
alue
.T
his
dc l
evel
is
then
app
lied
to t
he d
rain
of
IC1b
, via
R30
.T
hus,
the
rec
eive
d si
gnal
and
thi
s dc
lev
elar
e ‘m
ixed
’ at
the
inp
ut t
o ga
te 1
(i.e
.: IC
1b),
the
com
posi
te
sign
al
bein
g ap
plie
d to
th
ein
tegr
ator
.T
he m
eter
, M
1, i
s dr
iven
by
a dc
am
plifi
er,
IC3a
. T
he in
put
to t
his
op-a
mp
com
es fr
om t
hese
nsiti
vity
con
trol
and
is
appl
ied
to t
he n
on-
inve
rtin
g in
put
(pin
3).
Thi
s st
age
has
a ga
in o
fab
out 3
0 an
d a
little
‘sm
ooth
ing’
(in
tegr
atio
n) o
fth
e si
gnal
is
appl
ied
arou
nd t
he f
eedb
ack
byha
ving
a c
apac
itor
(C14
) co
nnec
ted
in p
aral
lel
with
the
feed
back
res
isto
r, R
36.
Apa
rt f
rom
driv
ing
the
met
er,
the
outp
ut o
fIC
3a i
s fe
d to
the
sou
rce
of l
C2b
whi
ch g
ates
the
audi
o os
cilla
tor
thro
ugh
to t
he a
udio
out
-pu
t st
age
(i.e.
: ga
te 2
). T
he d
c le
vel f
rom
pin
1of
lC3a
goe
s vi
a D
2 an
d R
39 to
pin
11
of IC
2b.
A p
ositi
ve b
ias
is a
pplie
d to
the
cat
hode
of
D2
from
the
+5
V r
ail
via
R38
. O
nly
whe
n th
e dc
leve
l at
the
out
put
of l
C3a
goe
s hi
gher
tha
n0.
6V a
bove
the
bia
s ap
plie
d to
the
cat
hode
of
D2,
will
lC2b
be
bias
ed o
n.T
he a
udio
osc
illat
or i
nvol
ves
lC4a
, co
nfig
-ur
ed a
s an
ast
able
mul
tivib
rato
r op
erat
ing
at a
few
hu
ndre
d H
ertz
. T
he
outp
ut,
pin
1,
isap
plie
d to
the
gat
e of
lC
2b.
Whe
n lC
2b t
urns
on,
the
sign
al i
s ap
plie
d to
the
inp
ut o
f th
eau
dio
outp
ut s
tage
.O
ne g
ate
from
IC
2 is
bia
sed
into
its
lin
ear
regi
on a
nd a
cts
as a
sou
rce-
follo
wer
buf
fer
atth
e in
put
of t
he a
udio
out
put
stag
e. T
he v
ol-
ume
cont
rol,
RV
5, is
the
sour
ce r
esis
tor
for
this
stag
e an
d th
e ou
tput
is t
aken
from
the
wip
er o
fR
V5
to t
he b
ase
of Q
4, c
apac
itive
ly c
oupl
edvi
a C
18.
The
out
put
stag
e is
a s
impl
e co
mpl
emen
tary
clas
s-B
st
age
empl
oyin
g a
low
po
wer
NP
N/
PN
P t
rans
isto
r pa
ir. T
he c
olle
ctor
of
Q4
driv
esth
e ou
tput
sta
ge,
its c
olle
ctor
loa
d al
so p
rovi
d-in
g bi
as t
o th
e ou
tput
pai
r (R
48).
Bot
h dc
and
ac f
eedb
ack
is a
pplie
d to
the
bas
e of
Q4
byR
49 f
rom
the
out
put.
Aud
io o
utpu
t ca
n be
fro
man
8 o
hm s
peak
er o
r he
adph
ones
, via
a d
c is
o-la
ting
capa
cito
r,
C20
. H
eadp
hone
vo
lum
e is
redu
ced
by a
470
ohm
res
isto
r, R
50,
in s
erie
sw
ith o
ne le
ad to
the
head
phon
e so
cket
, SK
2.P
ower
sup
ply
for
the
circ
uitr
y is
spl
it in
to tw
opa
rts.
The
aud
io o
utpu
t st
age
is s
uppl
ied
bytw
o 9
V b
atte
ries
conn
ecte
d in
par
alle
l (B
1 an
dB
2).
The
se a
re c
onne
cted
via
a r
ever
se-p
olar
-ity
pro
tect
ion
diod
e, D
3, a
nd o
ne p
ole
of S
W2
whi
ch is
a s
witc
h on
RV
5.T
he r
est o
f the
circ
uitr
y re
quire
s a
+10V
and
a +
5V r
ail,
with
res
pect
to
the
com
mon
rai
l(0
V).
Thi
s is
der
ived
fro
m t
wo
9V b
atte
ries,
B3
and
B4,
con
nect
ed i
n se
ries
and
appl
ied
to a
regu
lato
r ci
rcui
t vi
a a
reve
rse-
pola
rity
prot
ec-
tion
diod
e, D
4, a
nd th
e ot
her
pole
of S
W2.
The
regu
lato
r is
ba
sica
lly
a co
nven
tiona
l se
ries-
pass
circ
uit,
Q9
bein
g th
e re
gula
tor
tran
sist
or.
The
zen
er d
iode
ZD
1 pr
ovid
es a
sta
ble
ref-
eren
ce v
olta
ge f
or a
diff
eren
tial
pair,
Q7
and
Q8,
the
lat
ter
cont
rolli
ng t
he b
ase
curr
ent
toQ
9. R
esis
tor
R54
allo
ws
a sm
all
amou
nt o
fcu
rren
t to
pa
ss
to
Q7/
Q8
at
switc
h-on
to
ensu
re
the
regu
lato
r ‘s
tart
s’
corr
ectly
. T
he
base
of
Q7
is b
iase
d at
hal
f th
e up
per
supp
lyra
il vo
ltage
by
R51
and
R52
. T
his
volta
ge i
sbu
ffere
d by
an
op-a
mp,
IC3c
, co
nfig
ured
as
avo
ltage
fol
low
er,
and
used
to
driv
e th
e +
5Vlin
e. D
ecou
plin
g is
pro
vide
d by
C19
, C21
and
C22
. N
ote
that
‘ba
ttery
tes
t’ fa
cilit
ies
are
pro-
vide
d by
R44
/PB
2 fo
r th
e 18
V s
uppl
y an
d by
R45
/PB
3 fo
r th
e 9
V s
uppl
y.T
UN
ING
Dur
ing
the
tuni
ng o
pera
tion,
whe
n th
e in
stru
-m
ent
is b
eing
initi
ally
set
up,
the
circ
uit
wor
ksin
the
fol
low
ing
way
: W
ith t
he t
une
mem
ory
push
butto
n op
erat
ed,
IC1a
is g
ated
on
and
adc
neg
ativ
e fe
edba
ck lo
op is
est
ablis
hed
from
the
outp
ut o
f th
e se
nsiti
vity
con
trol
, ba
ck t
oth
e in
put
of g
ate
1, t
he d
rain
of
IC1b
, vi
a th
esa
mpl
e an
d ho
ld c
ircui
try.
A p
ortio
n of
the
volta
ge fr
om th
e w
iper
of R
V3
is a
dded
to
the
volta
ge
dete
rmin
ed
by
the
volta
ge
divi
der
R33
, R
V4
and
R34
. T
his
is a
pplie
d to
the
sour
ce o
f IC
1a. A
s th
e tu
ne m
emor
y bu
tton
ispr
esse
d,
IC1a
is
co
nduc
ting
and
capa
cito
rC
13 w
ill c
harg
e to
the
val
ue o
f th
e co
mpo
site
volta
ge a
pplie
d to
the
sou
rce
of I
C1a
. T
heop
-am
p IC
5 is
a lo
w in
put
curr
ent
devi
ce a
ndth
e ou
tput
, pin
6, w
ill s
ettle
at a
val
ue e
qual
toth
e co
mpo
site
vo
ltage
ap
plie
d to
its
no
n-in
vert
ing
inpu
t (p
in 2
). T
his
dc le
vel i
s ap
plie
dto
the
sou
rce
of I
C1b
and
the
sig
nal
outp
utfr
om t
he r
ecei
ve c
oil a
mpl
ifier
is m
ixed
with
it.
Thi
s w
ill b
ring
abou
t a
redu
ctio
n in
the
dc
leve
l of
the
sig
nal
appl
ied
to t
he i
nteg
rato
rin
put,
and
thus
a r
educ
tion
in t
he d
c le
vel
atth
e in
put o
f IC
1a a
nd, w
ithin
a s
econ
d or
tw
o,a
new
dc
cond
ition
is
esta
blis
hed.
Whe
n th
edc
leve
l aro
und
the
loop
set
tles,
the
met
er w
illre
ad
zero
(c
entr
e)
and
the
tune
m
emor
ysw
itch
is r
elea
sed.
The
dc
leve
l at
the
outp
utof
IC
5 (a
nd th
us a
t the
dra
in o
f IC
1b)
is m
ain-
tain
ed b
y th
e ch
arge
on
capa
cito
r C
13.
Inpr
actic
e, it
will
drif
t ver
y sl
owly
, as
C13
will
be
grad
ually
dis
char
ged
by t
he i
nput
cur
rent
of
IC5
and
the
capa
cito
r’s o
wn
leak
age.
For
thi
sre
ason
, th
e tu
ne m
emor
y bu
tton
is lo
cate
d on
the
croo
k of
the
han
dle
whe
re it
can
be
oper
-at
ed b
y yo
ur th
umb
ever
y no
w a
nd th
en to
re-
cent
re th
e m
eter
.
FE
AT
UR
ES
•
VL
F a
nd T
/R o
pera
tion
•T
hree
ran
ges
of ‘d
iscr
imin
ate
’ (T
/R)
oper
atio
n •
Can
tun
e ou
t alu
min
ium
rin
g-p
ull t
abs
•G
roun
d b
alan
ce c
ircu
itry
incl
ude
d•
Tun
e m
emor
y (‘a
uto
-tun
e’)
butto
n
•H
igh
sen
sitiv
ity (
will
det
ect
20c
pie
ce a
t dep
ths
over
250
mm
)•
Pre
-wou
nd a
nd a
ligne
d w
ater
proo
f sea
rch
head
•S
trai
ghtfo
rwar
d co
nstr
uctio
n, n
o a
lignm
ent
nece
ssar
y•
Low
bat
tery
dra
in•
Use
s co
mm
on N
o. 2
16 tr
ansi
stor
rad
io b
atte
ries
•C
osts
aro
und
$200
in k
it fo
rm
HO
W IT
WO
RK
S —
ET
I 150
0
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level at the output of the integrator. Theground balance control is then adjustedto bring the meter back to centre zero.What the ground balance circuit does isto provide a signal which leads thephase of the transmit signal and thusleads the phase of the signal induced inthe receive coil without the presence ofground. The ground balance controlvaries the phase of this signal over arange of about four to one. Thus, whenyou vary the ground balance control,this varies the phase of the signal con-trolling gate 1, thus varying the averagelevel of the signal passed to theintegrator.
The process is then repeated until nochange occurs when the search head islowered to the ground. This establishesa ‘normal’ condition for the output ofthe integrator and the sample and holdcircuit maintains the appropriate dclevel at the input to gate 1 such that themeter remains at centre zero.
If the search head then approaches ametallic object, the amplitude of the sig-nal in the receive coil will vary as thecoupling between the coils and thephase of the signal will be altered by the
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target. This will change the averagelevel of the composite signal out of gate1 and thus the dc level at the output ofthe integrator will change. This will beamplified and the meter will show anindication. Also, gate 2 will be oper-ated and a tone will be heard in thespeaker.
However, this method of operationwill not indicate the difference betweenthe characteristics of different targets.
In the discriminate or TR mode, theground balance control is not used. Theinstrument is initially set up using thetune control to bring the meter to cen-tre-zero. The different TR ranges permitvarying degrees of control with the dis-criminate potentiometer. The phase-lagcircuit block generates a signal whichlags the phase of the transmit signal andthe discriminate control provides aphase-variable signal to drive gate 1.
When a ferrous target is approached,the combined permeability and eddycurrent effects tend to reduce the ampli-tude of the signal picked up by thereceive coil. This will cause a reductionin the dc level of the signal out of gate 1and a reduction in the dc level out of the
integrator. Thus, the meter will move tothe negative (left hand) side of thescale. This side of the scale is marked“bad”, obviously.
When a non-ferrous target isapproached, the combined eddy currentand permeability effect tends to increasethe amplitude of the signal picked up bythe receive coil. This will cause anincrease in the dc level of the signal outof gate 1 and an increase in the dc levelout of the integrator. The meter willthus move toward the positive (righthand — “good”) end of the scale.
The effects we are considering areactually quite small, hence the circuithas a considerable amount of do gain.
Gate 2 only operates when the outputfrom the do amp increases (goes posi-tive) and thus the audio output is onlyheard in the discriminate mode whenthe meter shows “good”.
It is unfortunate that ring-pull tabsfrom drink cans are aluminium and thusindicate along with other nonferrousmetals. But, the discrimination ability ofthe instrument can be adjusted toexclude the small effect these targetsgenerate — along with small trinkets,
the smaller gold nuggets, etc — butwho wants the tiddlers anyway!
If the dc level applied to the input ofgate 1 drifts — and it may do for a widevariety of reasons, operating the tunememory button will restore the balanceof the circuit and re-centre the meter.Quite a cunning arrangement.
Search headThe most important properties of the
search head are its size, the relationshipbetween the transmit and receive coils,and the shielding against capacitiveeffects between the coils and theground. Surprisingly, the actual induc-tance of the coils is not of primaryimportance.
The greater the coil diameter thegreater the penetration depth but theless sensitive the detector will be tosmall objects. Penetration using simple,circular coils is about equal to thesearch coil diameter for small objectssuch as coins, while sensitivity isroughly proportional to the cube of theobject diameter (expressed as a func-tion of the search coil diameter).Sensitivity is also inversely propor-
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ytional to the sixth power of the distancebetween the coil and the object.
All this means that if the object size ishalved the sensitivity is reduced to one-eighth. If the depth is doubled the sensi-tivity is reduced to one sixty-fourth. Seewhy metal detectors designed to pick upsmall objects use small coils and reallyonly skim the surface? If the search coilis doubled in diameter for greater pene-tration the sensitivity to small objectsfalls to one eighth, apart from the coilassembly becoming mechanically lessrigid. The law of diminishing returnsagain or ‘you don’t get something fornothing’.
Our new detector improves penetra-tion while retaining sensitivity by usinga co-planar arrangement of coils in thesearch head which gives a slightly mag-nified field pattern downwards, into theground.
We mentioned earlier that the twocoils are only loosely coupled. Thepositioning of the receiver coil in rela-tion to the transmitter coil is verycritical and is the major factor affectingthe performance of the instrument. Infact. misplacement by a millimetre or sowill markedly affect the performance.
As the search head is moved around,
the changing capacitance between thecoils and the ground could completelymask the minute changes in the field weare looking for. To avoid this affect thecoils are enclosed in a Faraday shield.
By now it should be obvious that con-struction of the search head is not a taskto be tackled on the kitchen table on arainy Sunday afternoon. In fact, con-struction and alignment of the searchhead would be beyond most readers’resources (anyone who has attemptedour earlier induction balance metaldetector knows what it’s like). With thisin mind we chose to use the commer-cially built, pre-aligned search headmade by Altek Instruments. This will beavailable in Australia through All Elec-tronic Components in Melbourne whohave agreed to make the unit availablewholesale to other suppliers, as well asretailing parts themselves, along withhardware — the plastic extendable han-dle and the case for housing theelectronics.
ConstructionThe mechanical components for this
project — the search head, handle andcase, are available through Altek’s Aus-tralian agent, All ElectronicComponents, as mentioned earlier. Werecommend you obtain these as yourfinished instrument will then be a pro-fessional looking piece of equipment,with the features and operation of a‘bought one’ two to three times theprice. However, you can suit yourselfand make your own handle if you sodesire and we have designed the pcboard such that it will also fit in a largejiffy box. You will have to use thesearch head recommended though, forthe reasons we have explainedpreviously.
All the electronics mounts on a sin-gle, double-sided pc board. The Altekcase has two clamps on the rearenabling it to be clipped on to the han-dle. The Altek handle has two sections,the lower section sliding inside theupper section enabling the operator to
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yadjust the length of the handle to suithis height. Connection between thesearch head and the electronics is via alength of shielded cable (supplied withthe head) and a five-pin DIN plug/socket arrangement. The tune memorypushbutton is mounted in the end of the‘crook’ of the handle (see photographs)where it can be easily operated by thethumb. It connects to the electronics viaa length of shielded cable passedthrough the handle.
Construction should commence withthe pc board. As it is a double-sidedboard (i.e: copper tracks on each side),first identify the ‘front’ and ‘rear’ side.These are marked, respectively, ETI1500f and ETI 1500r. The rear side hasthe more complicated pattern of tracks.The components are mounted on thefront of the board, where there is theless complicated set of tracks. Note thatsome of the resistors, IC pins and pcboard pins (used for connecting exter-nal wiring to the board) must besoldered to copper tracks on each sideof the board.
Commence with the resistors. Takecare with those that cross tracks thatyou don’t create a short circuit whereit’s not wanted. Next mount the capaci-tors. Take care with the orientation ofthe electrolytics. Note that capacitorsC2, C7 and C11 are styroseal types,used for their good temperature stabil-ity. Be careful when soldering them inplace that you don’t overheat the leadsas this can cause melting of the capaci-tor’s case, possibly damaging it. Thesample and hold capacitor, C13, mustbe a low leakage type, preferably poly-carbonate or mylar. We used a greencapsuccessfully, but whatever you manageto obtain, make sure its a good qualitytype from a well-known supplier.
Now mount the semiconductors. Takecare with the orientation of these as youcan destroy devices if they are incor-rectly inserted when power is applied.Finally, solder the pc pins in place andthe four battery clips. The latter all goalong one edge of the board.
Overall assembly of the pc board isclear from the overlay picture on page17.
Once you have everything in place onthe pc board and you’re satisfied that allis OK, you can turn your attention to the
hardware. Start with the case thathouses the electronics. If not pre-drilled,you’ll need to mark out and drill all theholes in the case lid. The panel artworkcan be used as a template. Note that wedressed up the case with a Scotchcalpanel. These should be availablethrough the usual suppliers. Centrepunch holes before drilling. The cutoutfor the meter can be made with a holesaw or by drilling a series of 4 mmdiameter holes just inside the markededge of the hole. When you completethe circle, the centre piece can besnapped out and the edge of the holecleaned up with a half-round file untilthe meter drops in neatly.
The speaker is mounted on the lefthand side of the case lid (as you wouldnormally view the unit in use). It is heldin place by large washers placed underthe nuts of three bolts spaced around theouter rim of the speaker. Alternatively,you can glue it in place. Be careful notto get any glue on the speaker cone oryou might end up with a rather ‘stran-gled’ sound!
The pc board mounts in the bottompiece of the case, along with the DINsocket (for the search head connector)and two battery test push buttons. Asmall hole in the bottom passes thecable to the tune memory button. Thecase bottom has four integral mouldedstandoffs to provide support for the pcboard which is held in place withscrews.
Once all the mechanical work on thecase is satisfactory, the Scotchcal panel
may be stuck on. Take care when posi-tioning it as it’s almost impossible tomove if you misalign it. Carefullysmooth out all the bubbles toward theedge of the transfer.
The controls, meter etc. may bemounted next. Then you can wire all theexternal components to the pc boardpins. We used lengths of ribbon cablewhere possible to simplify the wiring.The easiest way to accomplish this partof the assembly is to place the bottomof the box, with the pc board mountedin it, on your left and the lid, with themeter and controls etc. mounted, facedown on your right. Follow the wiringdiagram on page 16 and complete allthe interconnections. You should nowappreciate pc board pins!
The tune memory button mounts in ahole in the end of the ‘crook’ of thehandle, as we explained earlier, and theshielded cable connecting to it passesthrough the handle, emerging through asmall hole drilled in the handle nearwhere the cable can enter the hole pro-vided for it in the bottom of the box.This cable is best inserted before youmount the pushbutton. Remove thehandgrip. Push the cable through thehole in the handle near the case, until itappears through the end of the handle.Solder the end of the lead to the push-button and mount the pushbutton in thehole in the end of the hand grip (easiersaid than done!). Put the handgrip backand you can pass the business end of thecable into the case and terminate it. Ifyou’re lucky, kit suppliers may sell theunits with this part already assembled.
Holding the batteries in place is gen-erally left to your ingenuity. We useddouble-sided sticky tape (ah, that’s use-ful stuff...). The battery life is quitegood as the circuit has been designedfor low current drain. Reverse polarityprotection is provided on the pc boardto avoid problems should you inadvert-ently attempt to connect a battery backto front.
When all wiring is complete, push thecase onto the handle and drill a smallhole through one of the clamps and thestem of the handle. Insert a nail or a boltand this will prevent the case fromrotating on the handle. Mount the searchhead and adjust the length of the stem tosuit yourself. Wrap the cable from the
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ysearch head around the stem so that it isheld quite rigidly and plug it into theDIN socket on the case.
You’re ready to roll!... once you’vetested it.
If you wish to make the search headcompletely waterproof, seal the holethrough which the cable passes withSilastic rubber or some similar caulk-ing compound.
OperationWhen construction is complete and
you’re satisfied all is well, turn thedetector on and advance the vo1umecontrol. Set all other controls tomidrange and switch the mode selectorto VLF. Hold the search head up in theair and well away from metal objects,press the tune memory button and rotatethe tune control. The meter shouldswing either to side of the centre posi-tion. Set the pointer to centre scale andrelease the tune button. The metershould remain at this position but maydrift slightly, which it will tend to doimmediately after switch on. Pressingthe tune memory button at any timeshould return the meter to centre posi-tion, set by the tune control.
The next step is to determine that thepolarity of the receive coil is correct.After tuning the detector as described,bring a piece of iron near the searchhead. If the meter swings to the rightyour circuit is correct, if it swings to theleft you will have to reverse the twowires on the DIN socket that connect tothe receiver section on the pc board.The meter should now swing to theright.
Ground balanceWith the detector tuned, lower the
search head to the ground. The metermay swing off scale. If it swings to theright turn the ground control to the left,if it swings to the left turn the groundcontrol to the right. Raise the searchhead from the ground, press the tunebutton and the meter will return to cen-tre scale. Lower the search head againand repeat the procedure until there islittle difference in the meter readingwhen the search head is lowered. Set-ting the ground control is quite criticaland may take some time to achieve thefirst time around. The detector can now
be used in the VLF mode.
Sensitivity controlThe sensitivity control sets the gain of
the dc amplifiers in the detector and willgenerally give best results at midrange.If the control is set fully clockwise thetuning will tend to drift, requiring morefrequent operation of the tune memorybutton.
Discriminate controlsThe mode switch selects one of three
discriminate ranges: TR1, TR2 or TR3,while a vernier action is provided by thediscriminate control. The discrimina-tion ability of this circuit is extremelyeffective and it is possible to discrimi-nate between an aluminium ring pull taband a gold ring. Remember that dis-crimination depends on the resistivity ofthe target object.
When set to TR1, discriminate con-trol at mid-range, the meter shouldshow ‘bad’ for ferrous objects and‘good’ for non-ferrous objects alongwith a tone from the speaker. As thediscrimination controls are advanced,some nonferrous objects such as brasswill start to give a ‘bad’ reading, whilegold and silver will give a ‘good’ read-ing. As the controls are advancedfurther aluminium will start to give a‘bad’ reading, and so on. As you use thedetector you will become familiar withits operation.
The best way of setting the discrimi-nation controls is to carry around a fewsample objects of the type you want todiscriminate against just for this pur-pose. One thing to remember is that acorroded object will require a differentsetting of the controls to a non-cor-roded one so carry samples typical ofwhat you are likely to dig up.
By careful setting of the controls,unwanted objects can be tuned out, giv-ing no meter movement at all so thedetector can be used to reject particularobjects and at the same time discrimi-nate between others.
Well, its now up to you. Remember,the secret of success in metal detectingis more knowing where to look than thetype of detector you have. There aremany books available on the subjectwhich could help put you on the righttrack.
Notes for constructors(courtesy of G.N. Vayro, Broadmead-ows, Victoria).
• Take special care with the orientationof IC5 (CA3130) if an 8-pin TO-5(circular metal case) type is supplied.Refer to the pinout diagram below.
• Take care with the wiring of theheadphone socket as not all types havethe same, or similar, connections.Check this by examination or with amulti- meter before wiring.
• Take care when wiring the DINsocket that connects the search head.The search head wiring is colour-coded,as shown on the circuit diagram. Thered and black wires come from thereceive coil. This coil has a dcresistance of around 50 ohms. Thetransmit coil is connected via the cableshield and the white wire. It has a dcresistance of around 12 ohms. Theremay be a yellow wire in the cable.Ignore it as it is not connected. TheFaraday coil shields are internallyconnected to the cable shield.
• The wiring to the two pushbuttonsPB2 and PB3 should first be sorted outwith an ohmmeter before soldering it inplace.
• The pushbutton in the handle needs tohave good ‘feel’ and positive contact.One of the small C & K or Swann typesshould fill the bill.
• If you have or are using a metal frontpanel, it should be earthed to reducespurious capacitive effects. The body ofthe discriminate control should connectto 0 V (Pin i) and a star washer shouldbe inserted under its nut to provide agood contact to the panel. Otherwise, aplastic Scotchcal panel is recommended(one was used on the prototype).
• It is strongly recommended that aflux-removing solvent be used to cleanthe pc board following assembly.Whilst flux does not cause problemswhen ‘new’, many atmosphericallybourne chemicals can and do react with
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ythe flux in time. This causes a leakagepath to be established between thetracks and is especially troublesome inhigh impedance circuits, such as aroundIC5. A de-fluxed pc board will obviatelater (or early) problems with the auto-tune circuit; it also looks moreprofessional and aids identification ofdefective solder joints. The effort isworth it.
• If you have trouble with handcapacitance effects, plastic knobs or
collet knobs may be used to advantageon the controls, particularly the variablediscriminate control.
• The wiring to the pushbutton in thehandle should be done with shieldedcable, passed through a hole drilled inthe rear of the case to avoid fouling thetelescopic shaft in the retracted position.
• A battery clamp, fashioned from asmall strip of aluminium, is recommen-ded.
• The case should be mounted as closeto the curve in the handle as possible foroptimum weight distribution.
• A screw or bolt should be placedthrough the rear case mounting clip tostop the case rotating on the shaft. Therear clip is recommended to allow theshaft to be telescoped to minimumlength.