eti549 150, Wykrywacze Metalu
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ETI Project 549
INDUCTION BALANCE
METAL DETECTOR
A really sensitive design operating on a different principle from that of other published circuits. This Induction Balance
circuit will really sniff out those buried coins and other items of interest at great depths depending on the size of the object.
“
ANOTHER
METAL LOCATOR,” some of you will say.
Yes and no. Several designs have been published in the hobby
electronics magazines; some good, some downright lousy but
they have invariably been Beat Frequency Oscillator (BFO)
types. There’s nothing wrong with this principle - they are at
least easy to build and simple to set up. The design described
here works on a very different principle, that of induction
balance (IB). This is also known as the TR principle (Transmit-
Receive).
This part accounts for three-quarters of the effort. Great care,
neatness and patience is necessary and a sensitive ‘scope,
though not absolutely essential, is very useful. It has to be
stated categorically that sloppy construction of the coil will (not
may) invalidate the entire operation.
IB VERSUS BFO
The usual circuit for a metal locator is shown in Fig. 2a. A
search coil, usually 6in or so in diameter is connected in the
circuit to oscillate at between 100-150kHz. A second internal
oscillator operating on the same frequency is included and a
tiny part of each signal is taken to a mixer and a beat note is
produced. When the search coil is brought near metal, the
inductance of the coil is changed slightly, altering the frequency
and thus the tone of the note. A note is produced continually
and metal is identified by a frequency change in the audio note.
All metal locators have to work within a certain frequency
band to comply with regulations and a licence is necessary to
operate them. This costs £1.20 for five years and is available
from the Ministry of Posts and Telecommunications, Waterloo
Bridge House, Waterloo Road, London S.E.1.
First a word of warning. The electronic circuitry of this
project is straightforward and should present no difficulty even
to the beginner. However, successful operation depends almost
entirely upon the construction of the search head and its coils.
The IB principal uses two coils arranged in such a way that
there is virtually no inductive pick-up between the two. A
modulated signal is fed into one. When metal is brought near,
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the electromagnetic field is disturbed
and the receiver coil picks up an
appreciably higher signal.
However, it is impractical for there to
be no pickup - the two coils are after all
laid on top of each other. Also our ears
are poor at identifying changes in audio
level. The circuit is therefore arranged
so that the signal is gated and is set up
so that only the minutest part of the
signal is heard when no metal is
present. When the coil is near metal,
only a minute change in level becomes
an enormous change in volume.
BFO detectors are not as sensitive as
IB types and have to be fitted with a
Faraday screen (beware of those which
aren’t - they’re practically useless) to
reduce capacitive effects on the coil.
They are however, slightly better than
IB types when it comes to indentifying
exactly where the metal is buried - they
can pin-point more easily.
Our detector is extremely sensitive -
in fact a bit too sensitive for some
applications! For this reason we’ve
included a high-low sensitivity switch.
You may ask why low sensitivity is
useful. As a crude example, take a coin
lying on a wooden floor: on maximum
sensitivity the detector will pick up the
nails, etc., and give the same readings as for the coin, making it
difficult to find.
We have specified Q3 and Q4 types as BC109C (highest gain
group) for although lower gain transistors worked for us they
left little reserve of level on RV1 and really low gain types may
not work at all.
Treasure hunting is an art and the dual sensitivity may only
be appreciated after trials.
RV1 is the critical control and should be a high quality type -
it will be found that it has to be set very carefully for proper
operation.
Table 1 gives the distances at which various objects can be
detected. These are static readings and only give an indication
of range. If you are unimpressed with this performance you
should bear two things in mind: first compare this with any
other claims (ours are excellent and honest) and secondly bear
in mind how difficult it is to dig a hole over 1 ft of ground
every time you get a reading. Try it - it’s hard work!
The choice of an LM380 may seem surprising as only a small
part of its power can be utilised with battery operation. It is
however inexpensive and widely available unlike the
alternatives (note it does not require d.c. blocking at the input).
Output is connected for an 8 ohm speaker and to headphones.
Stereo types are the most common and the wiring of the jack
socket is such that the two sections are connected in series
presenting a l6 ohm load (this reduces current consumption
from the battery).
COMPONENT CHOICE
The injunction Q1 is not the normal 2N2646; we found
several examples of these erratic in their level - we are talking
about tiniest fractions of one per cent which would normally
not matter, but it does in this circuit. Even some examples of
the TIS43 did not work well - see the note in How it Works.
Secondly Q2 is deliberately a plastic type. Metal canned
transistors usually have the collector connected to the case and
due to the nature of the circuit we noted a very small change in
signal level due to capacitive effects when metal can types were
used.
CONSTRUCTION: CONTROL BOX
The majority of the components are mounted on the PCB
shown in Fig. 3. Component overlay and the additional wiring
is shown in Fig. 4.
Exceptional care should be taken to mount all components
firmly to the board. The trimmer capacitor CV1 is mounted at
right-angles to the board, its tags being bent over and soldered
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firmly to the copper pads. This enables it to be
trimmed with the box closed. A plastic trimming
tool should be used if possible. Poor connections
or dubious solder joints may be acceptable in
some circuits - not in this one. Take care to
mount the transistors, diodes and electrolytic
capacitors the right way around.
The PCB is fitted into the control box by
means of long screws and pillars. The control
box has to be drilled to take the speaker, the pots,
switches, headphone jack and the cable from the
search head.
THE HANDLE ASSEMBLY
The handle is made totally from standard parts.
The general construction can be seen in Fig. 5.
This is made from Marley 22mm cold water
plumbing available from many plumbing shops.
The hand grip is that for a bicycle - also easily
available and a perfect fit onto the plastic pipe. A
right-angled elbow and two sleeve connectors
are specified. The elbow should be glued firmly
and one end of each of the connectors should be
glued also.
HOW IT WORKS - ETI 549
Q1, Q2 and associated components form the transmitter section of the circuit. Q1 is a
unijunction which operates as a relaxation oscillator, the audio note produced being determined
by R1 and C1. The specified components give a tone of roughly 800Hz. R1 can lie in the range
33k to 1OOk if a different audio frequency is desired.
Q2 is connected as a Colpitt’s oscillator working at a nominal 130kHz; this signal is heavily
modulated by C3 feeding to the base of Q2. In fact the oscillator produces bursts of r.f. at 800Hz.
L1 in the search head is the transmitter coil.
L2 is arranged in the search head in such a way that the minimum possible signal from L1 is
induced into it (but see notes on setting up). On all the prototypes we made we reduced this to
about 20mV peak-to-peak in L2. L2 is tuned by C6 and C7 and peaked by CV1 and feeds to the
base of Q3, a high gain amplifier. This signal (which is still modulated r.f.) is detected by D1, D2
providing the bias for D1. The r.f. is eliminated by C10 and connects to the level control RV1.
The signal is further amplified by Q4 which has no d.c. bias connected to the base. In no-signal
conditions this will be turned off totally and will only conduct when the peaks of the 800Hz
exceed about O.6V across R11. Only the signal above this level is amplified.
On low sensitivity these peaks are connected to the volume control RV2 (any stray r.f. or very
sharp peaks being smoothed by C15) and fed to the IC amplifier and so to the speaker.
The high sensitivity stage Q6 is connected at all times and introduces another gating stage
serving the same purpose as the earlier stage of Q5. This emphasises the change in level in L2
even more dramatically. Note that RV1 has to be set differently for high and low sensitivity
settings of SW1.
Whichever setting is chosen for SW1, RV1 is set so that a signal can just be heard. In practice
it will be found that between no-signal and moderate-signal there is a setting for RV1 where a
‘crackle’ can be heard. Odd peaks of the 800Hz find their way through but they do not come
through as a tone. This is the correct setting for RV1.
The stage Q6 also feeds the meter circuit. Due to the nature of the pulses this need only be very
simple.
Since we are detecting really minute changes in level it is important that the supply voltage in
the early stages of the receiver are stabilised, for this reason ZD1 is included to hold the supply
steady independent of battery voltage (which will fall on high output due to the current drawn by
IC1).
It is also important that the supply voltage to Q1 and Q2 does not feed any signal through to
the receiver. If trouble is experienced (we didn’t get any) a separate 9V battery could be used to
supply this stage.
lC1 is being well underused so a heatsink is unnecessary.
Battery consumption is fairly high on signal conditions - between 60mA and 80mA on various
prototypes but this will only be for very short periods and is thus acceptable. A more modest
20mA or so is normal at the ‘crackling’ setting.
Stereo headphones are used and are connected in series to present 16 ohms to IC1 reducing
current consumption.
Selection of Q1 and Q2
We found that Q1 and to a lesser extent Q2 required careful selection. Q1 should be chosen for
the minimum possible ‘crackle’ - so that the transition from no-signal to hearing the 800Hz is as
definite as possible. Some transistors for Q1 and Q2 can produce higher odds peaks than others.
The reason for the connector near the base is
to facilitate easy removal of the head and the
control box for testing and initial setting up.
The control box is held to the handle by means
of two pipe clips - again available from
plumber’s merchants.
The connection to the search head is by means
of a 4-1/2in length of tubing which has to be
modified. Put 1-1/2in of this tube into boiling
water for about half a minute to soften the
plastic, take it out and quickly clamp it into a
vice to flatten half the length, at the same time
bending the flat to about 45 degrees. This will
now lie across the top of the search head and is
glued into position and held by a single 2BA
nylon nut and bolt through the top of the search
head.
THE COIL
Remember this is the key to the whole
operation. The casing of the coil is not so critical
but the layout is.
It is best first to make the 8mm plywood circle
to the dimensions shown in Fig. 5. A circle of
thinner plywood or hardboard is then firmly
clued onto this - it’s fairly easy to cut this after
glueing. Use good quality ply and a modern
wood glue to make this.
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PARTS LIST - ETI 549
Resistors
R1
Semiconductors
Q1
47k
1/4W, 5%
TIS43 Unijunction
R2
270R
1/4W, 5%
Q2
2N2926 - see text
R3
150k
1/4W, 5%
Q3, 4
BC109C
R4
39k
1/4W, 5%
Q5, 6
BC108
R5, 14
1k
1/4W, 5%
IC1
LM380 14 pin DIL
R6, 15
180R
1/4W, 5%
D1, 2, 3, 4
OA91
R7, 9
1M8
1/4W, 5%
ZD1
6.2 volt 400mW zener diode
R8, 10, 11, 12 ,13
4k7
1/4W, 5%
Miscellaneous
SW1 SW2, 2 pole, 2 way slide switches
Stereo jack socket
Miniature (2-1/4 in etc) 8 ohm loudspeaker
L1, L2 - See text and drawings
Vero box (65-2520J)
PCB Board, ETI 549
4 core, individually screened cable, 1.5 metres
Battery clip (PP6)
Battery (PP6)
Wood and laminate for search head
2 control knobs, 2BA nylon nut bolt
M1 signal level meter, 150
Potentiometers
RV1
4k7
log rotary
RV2
4k7
log rotary
Capacitors
C1, 8, 16
47
F 16v electrolytic
C2, 3, 11, 14, 18
100nF ceramic etc.
C4
3n3 polystyrene 5%
C5
10n polystyrene 5%
C6, 7
5n6 polystyrene 5%
C9, 10, 12, 13, 1
5
20n ceramic etc.
C17
470
F 16v electrolytic
A movement
Marley 22mm cold water plumbing (see text)
Bicycle grip
C19
4
7 16v electrolytic
CV1
500p trimmer
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This now forms a dish into which the coils are
fitted. The plastic connector to the handle should
be fitted at this stage.
You’ll now have to find something cylindrical
with a diameter of near enough 140mm (5-1/2in).
A coil will then have to be made of 40 turns of 32
s.w.g. enamelled copper wire. The wire should be
wound close together and kept well bunched and
taped to keep it together when removed from the
former. Two such coils are required, both are
identical.
One of the coils is then fitted into the ‘dish’ and
spot clued in six or eight places using quick setting
epoxy resin: see photograph of the approximate
shape.
L2 is then fitted into place, again spot gluing it
not in the area that it overlaps L1. The cable
connecting the coil to the circuit is then fed
through a hole drilled in the dish and connected to
the four ends. These should be directly wired and
glued in place, obviously taking care that they
don’t short. The cable must be a four-wire type
with individual screens - the screens are left
unconnected at the search head.
You will now need the built up control
box and preferably a scope. The transmit
circuit is connected to L1. The signal
induced into L2 is monitored; at first this
may be very high but by manipulating L2,
bending it in shape etc., the level will be
seen to fall to a very low level. When a very
low level is reached, spot glue L2 until only
a small part is left for bending.
Ensure that when you are doing this that
you are as far away from any metal as
possible but that any metal used to mount
the handle to the head is in place. Small
amounts of metal are acceptable as long as
they are taken into account whilst setting up.
Now connect up the remainder of the
circuit and set RV1 so that it is just passing
through a signal to the speaker. Bring a
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