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It probably won’t find gold nuggets
in the bush but it will demonstrate
how a metal detector works.
A Detector For
Metal Objects
This simple project will demonstrate how metal
detectors work. It uses the principle whereby
the inductance of an air-cored choke changes in
the presence of metal.
By JOHN CLARKE
Metal detectors are used in many
applications. These include motor
vehicle detectors at traffic lights,
detecting unwanted metal objects in
food processing, as process counters
in industry and as treasure locators
for fossickers.
As you might expect, they vary
widely in circuit complexity and
function. A vehicle detector can easily
detect a large piece of metal (ie, a car
or truck) above it but it is quite a lot
harder to detect metal fragments in
food or coins and other items such as
36 Silicon Chip
ring pull tabs from drink cans buried
in beach sand. Some metal detectors
can even discriminate between ferrous metal (ie, those with iron such
as steel, cast iron, wrought iron, etc)
and non-ferrous metals (aluminium,
zinc, tin, lead, copper, mercury, silver,
platinum, gold, etc).
The metal detector presented here
is of the simple variety and it only
detects large items of metal at close
range. It does not discriminate between ferrous and non-ferrous metals.
Most metal detectors depend on the
principle that an air-cored choke will
change its inductance when brought
into close proximity with a piece of
metal. If the metal is ferrous (ie, if it
has magnetic properties), the change
in inductance will be considerably
greater than for a non-ferrous metal
and this fact can be used in circuits
which can discriminate between
metals.
In our circuit, we have used an aircored inductor (choke) as the variable
element in an LC oscillator.
Block diagram
Fig.1 shows the block diagram of
the Metal Detector. There are two
frequency sources, called oscillator 1
and oscillator 2, which are monitored
with a NAND comparator. Oscillator
1 is adjusted using VR1 so that its
frequency is exactly the same as for
oscillator 2 when no metal is close to
ductance increases. When
non-ferrous metals are in
close proximity to L1, the
inductance decreases and
so the frequency increases.
This is the basis of discriminating metal detectors, as
mentioned above.
Circuit diagram
Fig.2 shows the circuit
which is based on two
CMOS logic ICs. NAND
Fig.1: block diagram of the Metal Detector.
gates IC2a and IC2b form
The two oscillators beat together in a NAND
oscillator 1 while IC1a
gate comparator which then drives a loudforms oscillator 2.
-speaker.
Oscillator 1 is a standard
two-gate circuit with the
390pF capacitor alternateinductor L1. When metal is brought in
ly charged and discharged
proximity to L1, the frequency of os- via the 1kΩ resistor and series concillator 2 changes and this is detected
nected trimpot VR1.
in the comparator.
When power is first applied, IC2b’s
The comparator produces a tone in output could be either low or high.
the loudspeaker whenever it detects
When its output is high, IC2a’s output
a difference in frequency between
is low and the 390pF capacitor charges
the two oscillators. The tone varies so that the junction of the two 1kΩ
depending upon how large the metal resistors drops toward 0V. When this
item is and how close it is to induc- junction voltage reaches IC2a’s lower
tor L1.
threshold, its output at pin 11 goes
An interesting property of this high and so IC2b’s output goes low.
type of oscillator is that when metWhen this happens, the 390pF
als containing iron (ie, ferrous) are
capacitor charges in the op
posite
brought close to L1, the frequency direction via the 1kΩ resistor and
of oscillation falls because the in- trimpot. When the upper threshold
of IC2a’s input is reached, its output
goes low again and the cycle repeats.
The frequency is varied by means of
trimpot VR1.
LC oscillator
The LC oscillator works by successively charging and dis
charging
a .022µF capacitor via inductor L1.
When power is first applied, pins 1
& 2 of IC1a will be low (because the
.022µF capacitor is discharged) and
the pin 3 output of IC1a will be high.
The capacitor is then charged through
inductor L1. When the voltage at pins
1 & 2 reaches the upper threshold of
IC1a, its output goes low and discharges the .022µF capacitor via L1. This
cycle repeats endlessly.
The 180pF capacitor at pin 3 of
IC1a makes the oscillator immune to
variations in capacitance across the
inductor. This means that the oscillator is insensitive to hand capacitance.
Without the capacitor, just moving
your hand close to L1 would change
the oscillator frequency.
NAND gate comparator
IC1b inverts and buffers oscillator
2 and then NAND gate IC2c monitors
both oscillators. Pin 2 monitors pin
10 of IC2b while pin 1 of IC2c monitors IC1b. The output of IC2c is then
inverted using IC2d.
Fig.2: the circuit is based on two CMOS logic ICs. IC1a is an LC oscillator using an air-cored inductor.
If metal is brought close to the inductor, the oscillator frequency changes.
May 1998 37
Fig.3: the top trace is the oscillator waveform at pin 10 of IC2b (oscillator 1)
while the second trace is the output of oscillator 2 at pin 4of IC1b. This is what
happens when there is no metal close to the inductor.
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38 Silicon Chip
Fig.4: this is what happens when an metallic object is brought close to
inductor L1. The upper trace frequency remains the same as expected at
299kHz, while oscillator 2 frequency shifts, as shown on the middle trace. The
resulting inverted NAND comparator output on the bottom trace is a varying
pulse width waveform.
A NAND gate has a high output
unless both inputs are high. The oscilloscope waveforms of Figs.3, 4 &
5 show what happens. On Fig.3, the
top trace is the oscillator waveform at
pin 10 of IC2b (oscillator 1) while the
second trace is the output of oscillator
2 at pin 4 of IC1b. The bottoms trace
is an inverted NAND gate output at
pin 4 of IC2d. Note how the oscillator
frequencies and waveforms are virtually the same.
Fig.4 shows what happens when
a metallic object is brought close to
inductor L1. The upper trace frequency remains the same as expected at
299kHz, while oscillator 2 frequency
shifts, as shown on the middle trace.
The resulting inverted NAND com
parator output on the bottom trace
is a varying pulse width waveform.
“So what?”, you might say. Well,
these waveforms do not tell the whole
story because the oscillator frequencies of around 300kHz are totally
inaudible from the loudspeaker. But
when we wind down the timebase
on the oscilloscope we see what is
happen
ing at audible frequencies.
Fig.5 shows the output of IC2d with a
metallic object near inductor L1. See
how it consists of bursts of signal at
a rate of about 1.5ms. This is equivalent to a signal of about 670Hz and is
quite audible although the signal is a
bit weak at this point in the circuit.
Therefore the pulsed signal from
IC2d drives the base of transistor Q1
via a 10kΩ resistor. The transistor in
turn drives the 8Ω loudspeaker via a
100Ω resistor which provides current
limiting.
The circuit is shown as being powered from 12V DC but in practice it can
be powered from a 9V battery or 9V DC
plugpack. Diode D1 protects against
reverse polarity connections while
LED1 indicates when the power is on.
Fig.5: this shows the output waveform from IC2d when a metallic object is
brought near inductor L1. It consists of bursts of signal at a rate of about
1.5ms. This is equivalent to a signal of about 670Hz and is quite audible
through the loudspeaker.
Construction
All the parts are mounted on a PC
board measuring 104 x 69mm and
coded 04405981. If need be, the PC
board can be mounted into a plastic
Table 1: Capacitor Codes
❏
Value IEC
❏ .022µF 22n
❏ 390pF 390p
❏ 180pF 180p
EIA
223
391
181
Fig.6: this is the component layout for the PC board. The loudspeaker’s magnet
is fixed to the PC board using super glue.
Table 2: Resistor Colour Codes
❏
No.
❏ 1
❏ 1
❏ 2
❏ 1
Value
10kΩ
2.2kΩ
1kΩ
100Ω
4-Band Code (1%)
brown black orange brown
red red red brown
brown black red brown
brown black brown brown
5-Band Code (1%)
brown black red brown brown
red red black brown brown
brown black black brown brown
brown black black black brown
May 1998 39
Parts List
1 PC board, code 04405981,
104 x 69mm
1 40mm diameter Mylar 8Ω
loudspeaker
1 DPDT miniature slider switch
(S1)
1 560µH (0.56mH) air-cored
choke (L1)
1 4.7kΩ miniature horizontal
trimpot (VR1)
6 PC stakes
1 40mm length of hookup wire
Semiconductors
2 4011 CMOS NAND gates
(IC1,IC2)
1 BC548 NPN transistor (Q1)
1 1N4004 1A 400V diode (D1)
1 5mm red LED (LED1)
Fig.7: this is the full-size etching pattern for the PC board.
utility case measuring 130 x 68 x
41mm but that won’t be large enough
to accommodate the air-cored choke.
Fig.6 shows the parts layout for
the PC board. Begin construction
by checking the board for shorts or
broken tracks. This done, insert the
PC stakes for the supply input (+12V
and 0V), inductor L1 and for the loudspeaker. The resistors can be inserted
next, using Table 2 as a guide to the
values. Alternatively, check each resistor with your multimeter before it
is soldered into the board.
The capacitors can be installed
next. Take care with the 100µF electrolytic which must be inserted with
the correct polarity. This done, insert
the two ICs making sure that they
are oriented correctly. Trimpot VR1,
LED1, transistor Q1 and diode D1
can then be installed and soldered
in place.
When you mount switch S1 you
will need to crimp its eyelet terminals
so that they will fit into the PC board
holes. The loudspeaker is wired to
its terminals on the board and then
secured with some super glue on the
back of its magnet. Finally, connect
the air-cored choke to the PC board.
Capacitors
1 100µF 16VW PC electrolytic
1 .022µF MKT polyester
1 390pF ceramic
1 180pF ceramic
Resistors (0.25W, 1%)
1 10kΩ
2 1kΩ
1 2.2kΩ
1 100Ω
Testing
Apply power to the circuit and
check that LED1 lights when S1 is
on. Adjust VR1 until no tone is heard
from the loudspeaker. Now bring a
metallic object close to the coil and
check that a tone is heard. You may
need to readjust VR1 for best results.
You may also want to experiment
with a larger search coil of say 150mm
Miscellaneous
Solder, super glue
diameter and about 50 turns. With
the larger diameter coil, the detector
will be more sensitive to smaller
SC
metal items.
14 Model Railway Projects
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