Geotech Page 1

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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

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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

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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

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Geotech Page 5

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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

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p, th

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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 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-

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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

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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

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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

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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.