construcción de detector de metales
DESCRIPTION
diagramas para poder hacer tu propio detector de metalesTRANSCRIPT
-
Geotech Page 1
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
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 betweentrash 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 thereceiver 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
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
passed to a speaker or headphones.Thats 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. Theattitude 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
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
block 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 whichsquares 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
-
Page 4
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
-
Geotech Page 5
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
As th
e g
ener
al p
rinci
ple
s of
op
erat
ion
hav
ebe
en di
scuss
ed
with
re
gard
to
th
e bl
ock
dia-
gra
m in
th
e te
xt, th
is de
scrip
tion is
co
nfin
ed
toth
e Ci
rcu
it al
one.
Com
me
nci
ng
with
th
e
tran
smitt
er,
Q3
isco
nfig
ure
d as
a
Colp
itts
Osc
illato
r, th
e tra
ns-
mit
coil
in
the
sea
rch
hea
d fo
rmin
g th
ein
duct
ance
w
hich
reso
nat
es w
ith th
e c
om
bin
a-tio
n C9
an
d C1
0. Bi
as is
ap
plie
d to
Q3
vi
aR
18, D
1 an
d R
19. Em
itte
r bi
as
is pr
ovi
ded
byR
17. Th
e tra
nsm
it si
gna
l to
the
phas
e-le
ad an
dph
ase-
lag
circ
uitr
y (gr
ound
and
disc
rimin
ate
contro
ls) i
s ta
pped
of
f the
co
llect
or o
f Q3
via
C8 to
th
e jun
ctio
n of
R
11 a
nd R
14. Th
e gr
oun
dco
ntro
l circ
uitr
y co
nn
ects
vi
a R
11 w
hile
th
e di
s-cr
imin
ate
circ
uitr
y co
nne
cts
via
R14
.Th
e s
ign
al fro
m th
e r
ecei
ve co
il is
a
mpl
ified
by Q1
an
d ap
plie
d to
ga
te 1
(see
bl
ock
dia
-gr
am
), on
e CM
OS
gate
in
IC
1 (IC
1b),
via
an
emitt
er-fo
llow
ing
buffe
r st
age,
Q2
.
No
te
that
Q1 is
o
pera
ted
as
a gr
oun
ded-
base
a
mpl
ifier.
The
phas
e of
th
e re
ceiv
ed si
gna
l thr
ou
gh Q1
and
Q2 is
n
ot al
tere
d. O
utp
ut f
rom
th
e em
itter
of Q2
is
ap
plie
d to
th
e dr
ain o
f IC1
b.Th
e
base
o
f IC
1b
is
driv
en
by a
squ
are
wav
e de
rived
fro
m
the
tran
smit
sign
al,
the
phas
e o
f w
hich
ca
n be
va
ried
by ei
ther
th
egr
oun
d or
di
scrim
inat
e co
ntro
ls.
In th
e V
LF m
ode
, th
e p
hase
of t
he tr
ans
mit
sign
al ta
pped
of
f fro
m Q3
ca
n b
e va
ried
usi
ngR
V1.
This
pro
vide
s a
pha
se-a
dvan
ced
sign
alth
at
can
be
va
ried
ove
r th
e r
ange
fro
m a
bout
+10
to
+
40.
A le
adi
ng
phas
e R
C n
etw
ork
isfo
rme
d by
R10
and
RV1
in
co
njun
ctio
n w
ithC7
. Th
is si
gnal
is
ap
plie
d to
th
e in
verti
ng in
put
of
an
op-
am
p,
lC4b
.
As
this
is
op
era
ted
atm
axi
mum
ga
in w
ith a
high
si
gna
l lev
el a
t th
ein
put,
it w
ilI s
quar
e up
th
e si
gna
l at i
ts ou
tput
(pin 7),
w
hich
dr
ives
th
e ga
te of
IC
1b.
In th
e di
scrim
ina
te m
ode,
sw
itch
SW1
con-
ne
cts
the
tran
smit
sign
al t
o
circ
uitr
y w
hich
pro
vide
s a
lagg
ing
pha
se si
gnal
w
hich
can
be
varie
d o
ver
a r
ange
set
by
RV2
(the
di
scrim
i-na
te
con
trol)
and
a se
t of
ra
nge
re
sist
ors:
R12
, R
13,
R15
and
R16
. Th
ese
for
m a
lag-
ging
pha
se
RC
net
wor
k in
co
njun
ctio
n w
ithC1
1.
The
sig
nal
is th
en
buffe
red
by
a no
n-
inve
rting
op-
amp,
lC
3b,
and
app
lied
to
the
inve
rting
inpu
t of l
C4b
via
SW
1c.
The
so
urc
e o
f IC1
b is
co
nn
ecte
d to
a
n in
te-
grat
or
stag
e fo
rme
d a
rou
nd
lC3d
. Th
e o
utpu
to
f thi
s st
age
is
co
nn
ecte
d di
rect
ly to
th
e se
nsi
-
tivity
co
ntro
l, R
V3.
The
w
iper
o
f th
ispo
tentio
me
ter
goe
s di
rect
ly to
th
e in
put o
f a d
ca
mpl
ifier,
lC
3a,
to
whi
ch
we
sh
all
retu
rnsh
ortl
y. Th
e w
ipe
r o
f RV3
is al
so co
nne
cted
toth
e sa
mpl
e a
nd h
old
ci
rcuit,
vi
a R
25,
whi
chin
volv
es
IC5,
IC
1a, th
e t
un
e co
ntro
l RV4
and
the
tun
e m
em
ory
push
butto
n, PB
1.Th
e s
ampl
e a
nd h
old
circ
uit
wo
rks
in th
e fo
l-lo
win
g w
ay. T
he jun
ctio
n o
f res
isto
rs R
25, R
26a
nd
R32
will
be a
t a d
c le
vel d
ete
rmin
ed
by th
edc
le
vel a
t the
wip
er of
R
V3 a
nd th
e dc
le
vel a
tth
e w
iper
o
f RV4
, the
tun
e co
ntro
l pot
entio
me
-
ter.
Th
e
dc
leve
l at
th
e
wip
er
of
RV3
w
illde
pen
d on
th
e si
gnal
le
vel a
nd
phas
e sw
itche
dth
rou
gh t
o th
e i
nte
grat
or by
IC
1b. W
hen
th
etu
ne
m
emor
y pu
shbu
tton
PB
1,
is
pres
sed,
IC1a
(al
so
a CM
OS
switc
h) w
ill a
pply
a
dcle
vel t
o th
e in
put
of th
e sa
mpl
e a
nd
hold
ci
r-cu
it pr
opor
tiona
l to
the
dc
leve
l at t
he ju
nct
ions
of
R25
, R
26 a
nd R
32. Th
is w
ill ch
arge
C1
3a
nd th
e o
utp
ut
of I
C5 w
ill se
ttle
at
this
va
lue.
This
dc
le
vel
is th
en
appl
ied
to th
e dr
ain
of
IC1b
, vi
a R
30.
Thu
s, th
e r
ecei
ved
sign
al a
nd
this
dc
leve
la
re m
ixed
a
t th
e in
put
to ga
te 1
(i.e.:
IC1b
),th
e co
mpo
site
si
gnal
be
ing
appl
ied
to
the
inte
gra
tor.
The
m
ete
r, M
1, is
dr
iven
by
a
dc
am
plifie
r,IC
3a. Th
e in
put t
o th
is o
p-am
p co
me
s fro
m th
ese
nsi
tivity
co
ntro
l an
d is
app
lied
to th
e n
on-
inve
rting
inpu
t (pin
3). T
his
stag
e ha
s a
gain
of
abo
ut 30
a
nd a
little
s
moo
thin
g (in
tegr
atio
n) of
the
sign
al is
ap
plie
d ar
oun
d th
e fe
edba
ck by
havi
ng
a c
apa
cito
r (C
14) c
onne
cted
in pa
ralle
lw
ith th
e fe
edba
ck re
sist
or,
R
36.
Apa
rt fro
m d
rivin
g th
e m
ete
r, th
e o
utpu
t of
IC3a
is
fe
d to
th
e so
urc
e o
f lC2
b w
hich
ga
tes
the
audi
o os
cilla
tor
thro
ugh
to
th
e a
udio
out
-
put s
tage
(i.e.
: ga
te 2).
Th
e dc
le
vel f
rom
pi
n 1
of l
C3a
goes
vi
a D
2 a
nd
R39
to
pi
n 1
1 of
IC
2b.
A po
sitiv
e b
ias
is a
pplie
d to
th
e ca
thod
e of
D
2fro
m th
e +
5 V
rail
via
R
38. O
nly
w
hen
th
e dc
leve
l a
t th
e o
utp
ut
of lC
3a go
es
high
er th
an0.
6V a
bove
the
bia
s ap
plie
d to
th
e c
atho
de o
fD
2, w
ill lC
2b be
bi
ase
d o
n.
The
audi
o os
cilla
tor
invo
lves
lC
4a, co
nfig
-
ure
d a
s a
n as
tabl
e m
ulti
vibr
ator
op
era
ting
at a
few
hu
ndr
ed
Hertz
.
The
ou
tput,
pin
1,
isa
pplie
d to
th
e g
ate
of
lC
2b. W
hen
lC2b
tu
rns
on,
th
e s
igna
l is
a
pplie
d to
th
e i
npu
t of
th
ea
udio
out
put s
tage
.
One
ga
te fro
m IC
2 is
bi
ased
in
to its
lin
ear
regi
on a
nd a
cts
as a
sou
rce
-fo
llow
er
buffe
r a
tth
e in
put
of
the
au
dio
ou
tpu
t st
age.
Th
e v
ol-
um
e co
ntro
l, R
V5, is
th
e so
urc
e re
sist
or fo
r th
isst
age
an
d th
e o
utpu
t is
take
n fr
om
the
wip
er
of
RV5
to
th
e ba
se o
f Q4
, ca
paci
tivel
y co
uple
dvi
a C
18.
The
out
put s
tage
is
a
sim
ple
com
plem
enta
rycl
ass-
B st
age
em
ploy
ing
a
low
po
wer
NPN
/PN
P tra
nsis
tor
pair.
Th
e co
llect
or of
Q4
dr
ive
sth
e ou
tput s
tage
, its
co
llect
or lo
ad
als
o pr
ovid
-
ing
bias
to th
e ou
tput p
air
(R48
). Bo
th dc
an
da
c fe
edba
ck is
ap
plie
d to
th
e ba
se of
Q4
by
R49
fro
m th
e ou
tput
. Au
dio
outp
ut c
an b
e fro
ma
n 8
ohm
sp
eake
r or
head
phon
es, vi
a a
dc is
o-la
ting
capa
cito
r,
C20.
H
eadp
hone
vo
lum
e
isre
duce
d by
a
470
ohm
re
sisto
r, R
50, in
se
ries
with
on
e le
ad to
th
e he
adph
one
sock
et, SK
2.Po
wer
su
pply
for
the
circ
uitr
y is
sp
lit in
to tw
opa
rts. Th
e au
dio
out
put
sta
ge i
s su
pplie
d by
two
9 V
batte
ries
con
ne
cted
in pa
ralle
l (B1
an
dB2
). Th
ese
are
co
nne
cted
via
a re
vers
e-p
ola
r-ity
pro
tect
ion
dio
de,
D3,
a
nd o
ne
po
le of
SW
2w
hich
is a
sw
itch
on R
V5.
The
rest
o
f the
ci
rcu
itry
requ
ires
a +
10V
an
da
+
5V r
ail,
with
re
spe
ct t
o th
e co
mm
on
rail
(0V). T
his
is d
eriv
ed fr
om
tw
o 9V
ba
tterie
s, B3
and
B4,
co
nnec
ted
in se
ries
and
app
lied
to a
regu
lato
r ci
rcuit
via
a
reve
rse
-po
larit
y pr
otec
-
tion
di
ode,
D
4, an
d th
e o
ther
po
le o
f SW
2. Th
ere
gula
tor
is
basi
cally
a
con
vent
iona
l se
ries-
pass
ci
rcuit,
Q9
bein
g th
e re
gula
tor
tran
sist
or.
The
zene
r di
ode
ZD1
prov
ides
a
stab
le re
f-e
ren
ce vo
ltage
fo
r a
diffe
rentia
l pai
r, Q7
a
nd
Q8, th
e la
tter
con
trollin
g th
e ba
se cu
rre
nt
toQ9
. R
esi
stor
R
54 a
llow
s a
smal
l am
oun
t o
fcu
rren
t to
pa
ss
to
Q7/Q
8 at
sw
itch-
on
toe
nsu
re
the
regu
lato
r s
tarts
co
rrec
tly.
The
base
of
Q7
is
bi
ase
d a
t hal
f the
u
pper
su
pply
rail
volta
ge b
y R
51 a
nd
R52.
Th
is vo
ltage
is
buffe
red
by a
n op
-am
p, IC
3c, co
nfig
ure
d as
a
volta
ge f
ollo
wer
, a
nd u
sed
to dr
ive
the
+5V
line.
D
eco
upl
ing
is pr
ovi
ded
by C1
9, C2
1 an
dC2
2. N
ote
that
b
atte
ry te
st
faci
litie
s a
re pr
o-vi
ded
by R
44/P
B2 fo
r th
e 1
8V su
pply
and
byR
45/P
B3 fo
r th
e 9
V
supp
ly.TU
NIN
GD
urin
g th
e tu
nin
g o
pera
tion, w
hen
the
inst
ru-
me
nt i
s be
ing
initi
ally
set u
p, th
e ci
rcuit
wor
ksin
th
e fo
llow
ing
wa
y: W
ith th
e tu
ne
m
em
ory
push
butto
n op
erat
ed,
IC
1a is
ga
ted
on an
d a
dc n
egat
ive
fee
dbac
k lo
op
is e
sta
blis
hed
from
the
ou
tpu
t of
th
e s
ensi
tivity
co
ntro
l, ba
ck to
the
inpu
t of g
ate
1,
th
e dr
ain
o
f IC1
b, vi
a th
esa
mpl
e a
nd h
old
ci
rcuitr
y. A
porti
on o
f th
evo
ltage
fro
m th
e w
ipe
r o
f RV3
is
a
dde
d to
th
evo
ltage
de
term
ine
d by
th
e vo
ltage
di
vide
rR
33,
RV4
and
R34
. Th
is is
app
lied
to th
eso
urc
e of
IC
1a. As
th
e tu
ne
me
mo
ry bu
tton is
pre
sse
d,
IC1a
is
co
ndu
ctin
g an
d ca
paci
tor
C13
will
char
ge to
th
e v
alu
e o
f the
co
mpo
site
volta
ge a
pplie
d to
th
e so
urc
e o
f IC
1a. Th
eop
-am
p IC
5 is
a
low
in
put c
urr
ent d
evic
e a
ndth
e o
utpu
t, pi
n 6
, will
settl
e at
a
val
ue e
qual
to
the
co
mpo
site
vo
ltage
a
pplie
d to
its
no
n-
inve
rting
inpu
t (pin
2).
Th
is dc
le
vel i
s a
pplie
dto
th
e s
ourc
e o
f IC1
b an
d th
e s
ign
al ou
tput
from
th
e r
ecei
ve co
il a
mpl
ifier
is m
ixe
d w
ith it.
This
w
ill br
ing
abo
ut a
redu
ctio
n in
th
e dc
leve
l of
th
e si
gna
l ap
plie
d to
th
e in
tegr
ato
rin
put,
and
thu
s a
red
uctio
n in
th
e dc
leve
l at
the
inpu
t of I
C1a
an
d, w
ithin
a
sec
ond
or tw
o,a
new
dc
co
ndi
tion
is e
stab
lishe
d. W
hen
the
dc le
vel a
rou
nd
the
loop
se
ttles
, th
e m
ete
r w
illre
ad
zero
(ce
ntre
) an
d th
e tu
ne
m
em
ory
switc
h is
re
lea
sed.
Th
e d
c le
vel a
t the
ou
tput
of IC
5 (an
d th
us
at t
he d
rain
of
IC
1b) is
m
ain
-ta
ine
d by
th
e ch
arge
o
n ca
paci
tor
C13.
In
pra
ctic
e, it
will
drift
ve
ry sl
owly
, as
C1
3 w
ill be
gra
dual
ly di
scha
rged
by
th
e in
put
curr
ent
ofIC
5 an
d th
e ca
paci
tors
ow
n le
aka
ge. Fo
r th
isre
aso
n, th
e tu
ne
me
mo
ry bu
tton is
lo
cate
d on
the
cr
ook
of
th
e ha
ndl
e w
here
it ca
n b
e op
er-
ate
d by
yo
ur
thum
b e
very
now
an
d th
en to
re
-
centre
th
e m
ete
r.
FEA
TURE
S
VLF
an
d T/
R o
pera
tion
Th
ree
ra
nge
s o
f di
scrim
ina
te (T
/R) o
perat
ion
Can
tun
e o
ut a
lum
iniu
m rin
g-pu
ll ta
bs
Gro
und
bala
nce
circ
uitr
y in
clu
ded
Tu
ne
m
em
ory
(a
uto
-tu
ne
) but
ton
H
igh
sen
sitiv
ity (w
ill de
tect
20
c pi
ece
at d
epth
s o
ver
250
mm
)
Pre
-wo
un
d an
d al
igne
d wa
terp
roof
se
arc
h he
ad
Stra
ight
forw
ard
co
nst
ruct
ion
, n
o a
lignm
en
t ne
cess
ary
Lo
w b
atte
ry dr
ain
Us
es
com
mo
n N
o. 21
6 tra
nsi
sto
r ra
dio
ba
tterie
s
Cost
s a
rou
nd
$200
in ki
t fo
rm
HO
W IT
W
OR
KS
ET
I 150
0
-
Geotech Page 6
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
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
-
Geotech Page 7
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
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 markedbad, 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-
-
Geotech Page 8
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
tional 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 dont 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 readersresources (anyone who has attemptedour earlier induction balance metaldetector knows what its 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 Alteks 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 abought 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
-
Geotech Page 9
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
adjust 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 thecrook 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 dont create a short circuit whereits 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 dont overheat the leadsas this can cause melting of the capaci-tors 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 youre satisfied that allis OK, you can turn your attention to the
hardware. Start with the case thathouses the electronics. If not pre-drilled,youll 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 its 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. Ifyoure 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, thats 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
-
Geotech Page 10
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
search head around the stem so that it isheld quite rigidly and plug it into theDIN socket on the case.
Youre ready to roll!... once youvetested 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
youre 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 andgood 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 abad 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
-
Geotech Page 11
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
-
Geotech Page 12
ETI:
How
To
B
uild
Gol
d &
Tr
easu
re D
etec
tors
, 19
81
Co
pyrig
ht
M
urra
y/M
oder
n M
agaz
ines
, re
pro
duce
fo
r per
son
al u
se o
nly
the 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.