Metal Detector - Twin Loop Treasure Seeker - Robert and David Crone.pdf

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TWIN LOOP

TREASURE SEEKER

Robert and David Crone

The ground effect comes from a large area and is almost

Pulse metal detectors are powerful and versatile machines but

in their basic form they suffer from ground effect and radio constant over a flat surface like a wet sandy beach after the tide

interference. However a very simple modification can almost has gone out. If we were to position a second search coil about

100mm from the original then it would pick up the same

entirely eliminate these two problems.

amount of ground effect. Now if we were to subtract the

The principle of the pulse metal detector is very easy to

outputs of the two coils the ground effect from each would

understand. A large pulse of current is transmitted through a

cancel out. However the system would still pick up coins

coil of wire and the resulting magnetic field induces eddy

because the distance between the coils is large compared with a

currents in nearby coins or metal objects. The eddy currents

coin. By similar reasoning, medium and long wave radio

continue to flow after the transmitted pulse has ended and they

broadcasts will cancel out as the field strength of these signals

in turn induce small voltages back into the coil. These voltages

does not change significantly in 100mm and each coil will

are amplified and detected in a receiver which operates an

receive the same amount of interference.

audio indication, usually a click generator.

So the second coil is a modification to the pulse detection

A problem with this is that the transmitted pulse induces

system. Figure 1 shows a block diagram of the unit. The central

eddy currents in mineralised ground causing a ground effect

feature is the search coil assembly which in practice consists of

signal. Secondly the coil acts as a good aerial for long and

two coils each of 200mm diameter and overlapping by 100mm.

medium wave radio broadcasts, producing interference. So

what can be done about these problems?

1989

Wimbourne Publishing

The Transmitter

Figure 2 shows the circuit

diagram of the transmitter. IC1

is wired as an oscillator

running at 100Hz. IC2 is

triggered 100 times per second

from

IC1

via

the

differentiating network of R3

and C3. Each time IC2 is

triggered its output goes high

for 165µs and drives the two

power transistors hard on into

saturation. The full battery

voltage is now applied across

the coils and the current in

each one builds up to about

one amp.

The Timing Circuit

Fig. 3 shows the circuit

diagram of the timing circuit.

IC3 is triggered from the

transmitter at the end of the

165µs current pulse. Its

output goes high for 36µs and

then IC4 is triggered via C8

and R11. IC4 runs for 50µs

and its output goes to the

receiver where it switches on

the detector for 50µs.

Page 1

The Receiver

Fig. 4 shows the circuit diagram of the receiver.

The outputs from the coils are fed to the inputs of

the difference amplifier lC5. Here the ground effect

and interference cancel out but the coin signals are

amplified and passed on to the next stage. The 709

is used in the IC5 position because its noise figure is

good enough for the job. Diodes D1 to D6 protect

the op-amp inputs and are configured so that IC5

does not go into an indeterminate state when the

diodes are on. Q3 is switched on for 50µs by the

timing circuit and allows the coin signals to pass on

to the detector and amplifier IC6. When constructed,

set pin 6 of IC5 to -1V by adjusting RV1 and set the

1989

Wimbourne Publishing

receiver output to

−0.3V

by the

front panel control RV2.

The Click Generator

Fig. 5 shows the circuit of

the click generator. With no

input at all, Q4 is off and the

circuit is inoperative. However

with

−0.3V

coming in from the

receiver, Q4 starts to conduct

very slightly and the circuit

starts to click slowly. The

clicks rapidly turn into a high

pitched whistle as the search

coil approaches a coin.

Page 2

Construction

The circuit is built on a

single

PCB

and

the

components

should

mounted according to the

component overlay in Fig. 6.

The

usual

precautions

should be taken with the

ICM7555s as these are

CMOS devices. You need to

keep yourself earthed when

handling these chips. Once

all the components have

been mounted on the PCB,

the board can be drilled in

the four corners. The board

is held firm in a plastic

control box by four nylon

cuts and bolts. Terminal

pins were used on the PCB

for external connections to

the

switches,

potentiometers, sockets and

battery connections.

Drill the required holes in

the plastic control box. You

will probably have to do a little additional filing for the volume, mount the coils, overlapping by 100mm as in Fig. 7 on a

suitable piece of 6mm plywood and fasten them down with

click control pots and the audio socket.

plastic cable clips and plastic screws. Connect the coils up to a

To make the search coils first obtain a piece of scrap 25mm few feet of 3-core cable terminated at the other end in 4mm

chipboard and hammer into it a 200mm diameter circle of nails, plugs. Alternatively you could use 2-core screened audio cable

wind 30 turns of no 26swg enamelled copper wire around the and use the screen for the common connection.

nails and secure the windings with string or cotton ties. Pull out

At this stage you would be advised to bench test the machine

a few nails, remove the coil and then wind a second coil. Then

to check that you have wound the coils correctly so that the

1989

Wimbourne Publishing

Page 3

current in each coil flows in

the same circular direction.

A method of testing the

phasing or current direction

in each coil, apart from

inspection, would be to

pass a small direct current

through each coil and then

detect the magnetic field

produced with a small

compass. The coils would

need to be placed in the

vertical plane with the

compass positioned at the

centre of each ring. If the

currents are in the same

direction, the compass will

indicate that this is so.

The Printed Circuit

Board

1989

Wimbourne Publishing

Fig.

shows

the

component overlay. Make

sure the components are

placed in the correct positions. Once the l65µs pulse has

finished, the reservoir capacitor C1 starts to charge up with a

large current. This causes a voltage drop in the wiring. If any

voltage drop gets on to the earth rail, it will be amplified and

interfere with the system operation. For this reason separate

wiring for the two battery supplies must be used and nothing

but the battery may be connected to the left of C1.

this supply as these devices require an oscillator, the output of

which might get into the receiver and cause interference. All

the batteries are mounted inside the lid of the plastic control

box and secured with strong rubber bands.

Then encapsulate the coils with Araldite and put the

assembly into a warming compartment so that the Araldite

melts and permeates into the windings before setting. Use

plastic angle material to attach the

assembly to a plastic or wooden stem.

No metal should be used in the

construction of the coil assembly. Any

metal nuts, screws, washers or solder

tags will upset the system.

An 80cm length of 20mm plastic

tubing may be used to make the handle

for the control box and can be bent into

the traditional ‘shepherd’s crook’ shape

by means of a bending spring and hot

water. A bicycle handlebar grip slipped

on to the top end makes an ideal handle

hold.

A 50cm straight length of 16mm

plastic tubing can be used for the stem.

One end was dipped in hot water and

flattened with pliers and then attached

to the coil assembly by means of a

plastic nut and bolt. The stem is then

slid up into the handle until the total

length suits the operator and then bolted

into position. Alternatively one could

use a wooden walking stick or adapt

whatever non metallic material one has

Page 4

The Coils

Fig. 7 gives the details of the coil

assembly. Mount the coils on a

plywood frame and cut away as much

wood as possible to reduce the weight.

A few feet of 3-core mains cable is

suitable for connecting the coil

assembly to the 4mm sockets on the

plastic control box. Everything must be

plastic or wood. Finally keep in mind

that the current in each coil is flowing

in the same direction ie they an driven

in phase.

Batteries

Eight 1.2V AA size rechargeable

cells provide the -10V supply. The

machine consumes mound 80mA of

current so the batteries will give about

five hours of continuous running. When

the batteries are discharged, the click

generator will go out of control. A 9V

PP3 or MN1604 battery provides the

positive supply for the op-amps. A

voltage converter is not used to obtain

to hand. The only metal materials

permitted are a few screws in the

control box and the two screws

securing the control box to the

handle. Finally, insert a rubber

washer between stem and coil

assembly. This gives a non slip

attachment to stop the search

head angle being moved by

rough grass.

Parts List

Resistors (all 1/4W 5%)

R1,2,18,22,24

R3,12

R5,8

R6,7

R9,11

R10

R13,14

R15

R16

R17

R19

R20

R21

R23

R25

R26,27

R28

RV1

RV2

RV3

47k

4k7

15k

680R

150R

68k

3k3

470R

470k

390k

100k

180k

220R

1k0

1M5

18k

2k2

180R

100k horiz preset

47k lin

4k7 lin

Testing

The initial testing should be

done in a metal free environment.

Most work benches and tables

contain large numbers of nails,

screws and brackets so the reader

is advised to suspend the coil

assembly from the ceiling on a

length of string to ensure that it is

well clear of metal. With the

click generator set to one click

per second the operator will

notice a significant increase in

the click rate if a two pence coin

is taken to a distance of 180mm

from the search coil.

Once small pieces of metal

have been located with the

general purpose search coil, the

final pinpointing can be carried

out with a snout probe shown in

Fig. 7b and in the above

photograph. This probe was

constructed in a similar manner to the general purpose coil

except that the coils do not overlap. Each coil is made from 48

turns of 30 swg enamelled copper wire making the loops 50mm

in diameter and 70mm between centres.

Capacitors

C2,15,17

C4,7,9

C5,10,14

C6,8

C11

C12

C13

C16

2200µ axial electrolytic

100n polyester 7mm

1n0 polyester 7mm

10n polyester 7mm

22µ 16v tant bead

220p 63v ceramic

3p3 63v ceramic

10p 63v ceramic

470n polyester 7mm

220n polyester 7mm

1989

Wimbourne Publishing

Semiconductors

IC1,3,4,9

IC2

IC5

IC6

IC7

IC8

Q1,2

D1-5

ICM7555IPA

NE555

µA709CP

TL081

78L05

79L05

TIP31A

2N3819

BC178

1N4148

How It Works

The operation is as follows. The two switches in the

transmitter close simultaneously for 165µs and allow a current

of one amp to flow through each coil. This operation is repeated

every 10ms (a frequency of 100Hz). The coin signals picked up

by the coils along with the interference and ground effect are

then routed to the op-amp A in the receiver (Fig. 1). Here the

interference and ground effect cancel out and the amplified coin

signals are passed on to the detector D. Detector D is switched

on by the timing circuit 36µs after the end of the current pulse

and for a duration of 50µs. The

µs

delay is to allow the coils to

settle down because the sudden loss of the current causes a very

large voltage spike to appear across each coil. The DC output of

the detector now goes to the click generator which starts to

click rapidly as the search coil approaches a coin.

Miscellaneous

BATT1

BATT2

PL1-3

PL4

SK1-3

SK4

SW1

8x1.2V AA batteries

1x9V PP3 battery

4mm plugs: 2 red 1 black

2.5mm mono jack plug

4mm sockets: 2 red 1 black

mono 2.5mm chassis jack socket

DPDT switch

Case. Enamelled copper wire, 28swg and

30swg. Plastic tubing, 16mm and 20mm.

6mm plywood. Plastic angle. Cable grips.

Glue (Araldite).

Page 5

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