BASIC CIRCUITRY Charles D.

Rakes

METAL-DETECTING
CIRCUITS II
Gernsback Publications, reproduce for personal use only

Hello, Circuiteers, are you ready for dyne signal. An AM receiver detecting AM broadcast stations is rock solid,
some more metal detector circuitry fun? two RF signals, which are very close in only our search oscillator will produce a
If so, stick around. We ended our last frequency, usually causes this condi- shift in frequency. The end result is a
visit with a simple two-transistor Beat- tion. If the two RF frequencies are less detector that operates like our two-tran-
Frequency Oscillator (BFO) detector than a few kHz apart, an audio tone sistor circuit, but requires less parts and
circuit, and we’re starting out this time (difference frequency) will be heard. time to construct.
with an even simpler single-transistor This is basically how our single-transis- The oscillator circuit in Fig. 1 is very
BFO detector circuit. How do we do tor detector circuit operates. similar to the oscillators used in our pre-
that? Read on and find out. In our single-transistor circuit, see vious circuit. Transistor Q1 is
Fig. 1, only one RF oscillator circuit is connected in a Colpitts oscillator cir-
Single Transistor Circuit used. The other RF signal is supplied by
Before getting into the circuitry, we one of many AM broadcast radio sta-
had better take a quick look at how the tions. A portable transistor AM radio
single-transistor detector system oper- receives the two RF signals and outputs
ates. I’m sure that at some time you’ve an audible tone. The mixing and audio
heard a whistle or tone while tuning amplification is handled by the transis-
your AM broadcast receiver or, even tor radio. If either RF signal shifts in
more likely, when listening to an AM frequency, the audio tone will increase
short-wave broadcast station. In radio or decrease by the same amount. Since
circles, this is referred to as a hetero- the frequency stability of all licensed

Poptronics, Sept 2001 — Copyright

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Gernsback Publications, reproduce for personal use only

method offered here should get you lator is not operating near the desired
headed in the right direction. Refer to frequency. Now, how do we determine
Fig. 2 as a guide for constructing the if the oscillator’s frequency is too low
loop. The loop form should be con- or too high? Naturally, a frequency
structed from non-metallic and non- counter would be the easiest way to
cuit with components C2, C3, C5, C6, moisture-absorbent material. A sealed determine the oscillator’s frequency. If
and L1 making up the oscillator’s tuned wood form will do, and it can be either one is not available, what then? A short-
circuit. Changing any one or any combi- solid or hoop-like. The form should be wave receiver that runes both below and
nation of these components will vary ¼ to 1 inch wide to allow room for the above the standard AM broadcast band
the oscillator’s operating frequency. coil windings. Close wind six turns of can be used to ferret out the oscillator’s
Increasing the value of any capacitor #20 enameled or insulated wire on the frequency.
will lower the oscillator’s frequency and form. Wrap the windings with at least Once the oscillator’s frequency is
decreasing the value will increase the two layers of good quality plastic elec- determined, adjustments can be made to
frequency. Increasing L1’s inductance trical tape. Put the loop aside and
will also cause a decrease in frequency
 and vice versa.
construct the oscillator circuit on a
piece of multipurpose PC board with
Poptronics, Sept 2001 — Copyright

pre-drilled holes. Stability is one of the

Building The Loop most important considerations in build-
The search loop may be constructed ing any stable oscillator circuit, so keep
in several different ways; however, the all component leads short and solidly
mounted.
The two variable capacitors should be
mounted in a manner that allows tuning
from outside the enclosure. In order to
achieve the best results, the circuit
should be housed in a metal cabinet to
which the circuit ground is connected.
Temporarily connect the loop to the cir-
cuitry with about 30 inches of shielded
microphone cable or 2-conductor inter-
com wire. Any wire gauge from #18 to
#24 will do. Actually two insulated
wires may be twisted together by hand
and used.
Place the loop away from any metal
object and apply power to the circuit.
Locate a transistor radio near by and
tune in a station somewhere near the
middle of the dial. Adjust both C5 and
C6 to a frequency that will heterodyne
with the broadcast station. If nothing
happens, it is most likely that the oscil-

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 formed, add a 4-inch length bare wire
under the foil at one end and glue the
shield in place. Place the loop on a flat
surface and place a solid object on top
to secure the foil to the loop form. After
the glue dries, connect the other end of
the bare wire to the loop’s ground-end
connection.
An old broom handle or dowel rod is
attached to the middle of the loop and
serves as the handle and support for the
loop and detector circuit. See Fig. 4.
The AM radio may be attached to the
handle as well or carried separately.
Position the loop over the area to be
Gernsback Publications, reproduce for personal use only

searched and tune the oscillator to pro-

duce an audible beat frequency tone.
Maximum sensitivity is achieved when
the oscillator is within a few cycles of
the broadcast station. This detector will
detect all types of metal, so be ready to
dig, and then dig some more.

Crystal-Filter Detector
Our next entry is a version of one of
my favorite metal-detector circuits. A
loop and an oscillator circuit similar to
the one in our previous detector are the
basic ingredients used in the crystal-fil-
ter detector. The addition of an emitter
move the frequency into the broadcast follower gives isolation to the oscillator
band. Reducing the total capacitance of and supplies a low-impedance source
the oscillator’s tuned circuit or lower- for the crystal. The output is rectified by
ing the inductance of the loop will raise Dl and D2 and fed to the meter. Take a
the frequency. Lowering the frequency look at Fig. 5, as you continue to read
 is accomplished by increasing the the circuit description.
Here’s a brief description of how the
Poptronics, Sept 2001 — Copyright

capacitance of the tuned circuit or by

increasing the inductance of the loop. crystal-filter metal-detector circuit oper-
Removing or adding a turn to the loop ates. The oscillator is tuned to the series
is a good method to use if the oscillator resonance frequency of the crystal,
is way off frequency. which can be any frequency from 100
kHz to over 1 MHz. However, in our
Adding A Faraday Shield circuit, a 1-MHz crystal is used. When
reading when the oscillator is operating
The search loop normally scans the the oscillator is operating at the crys-
at the series-resonance frequency of the
ground in a parallel manner in search of tal’s frequency, the output at the meter
crystal.
metal objects. The loop’s parallel posi- is at maximum.
tion to the ground forms a capacitance Any shift in the oscillator’s fre- Transmitter/Receiver Detector
to ground, which shifts the oscillator’s quency will cause a reduction in the
meter reading. The circuit is very sensi- Our last detector circuit is suitable for
frequency. As the loop moves up and locating large metal objects at greater
down above the ground, the oscillator’s tive to small frequency shifts due to the
crystal’s narrow band-pass characteris- depths—feet instead of inches. This
frequency shifts in a like manner. Add- two-box detector has been around for
ing a Faraday shield to the loop will tics in the series mode. The basic loop
construction used in the previous detec- about 75 years and is still one of the
help in reducing the ground-effect fre- most popular deep-searching detectors.
quency-shift problem. tor circuit may be used here as well.
This detector’s circuitry should be The basic system is shown in Fig. 6.
The Faraday shield is a metal shroud Two non-metallic boxes serve as the
that is formed around the loop with an constructed in the same manner as our
previous circuit. If any component 2 housing for the electronics and the
insulating gap in the middle. A shield forms for the loops. The transmitter and
can be formed out of aluminum foil by moves or vibrates during use, the meter
will falsely indicate a detected object. receiver boxes are mounted on a 3-foot-
cutting a length that’s 3 inches wide and long wood handle, with the receiver
long enough to go almost completely Build it solid. The choice of the meter
used for M1 can vary from a sensitive placed in a horizontal position and the
around the edge of the loop while leav- transmitter in a vertical position. This
ing a gap of 1 to 2 inches in the middle, 50-µA to a 1-mA movement. The value
of R4 is selected for a full-scale meter 90-degree relationship between the
see Fig. 3. Once the aluminum foil is transmitter and receiver allows for mini-
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 inches of wire from each end of the
loop to the inside of the housing for cir-
cuit connections. Tape the winding in
place with plastic electrical tape.
The operating frequency will be
somewhere between 35 kHz and 50
kHz. The capacitor values for C1 and
C2 are .1-µF for the transmitter and .05-
µF for C1 in the receiver circuit. Less
turns or smaller loops may be used for
higher frequency operation. Try and
keep the operating frequency below 200
kHz, as this type of metal locator works
best at low frequencies.
Gernsback Publications, reproduce for personal use only

Building The Receiver

The receiver (see Fig. 8) is a simple
two-transistor RF amplifier circuit with
an isolated emitter follower input. The
between the two loops causes the trans- RF signal is picked up by the loop and
mitter’s field to distort, allowing some coupled through Q1 to the input of the
of the signal to reach the receiver’s first RF amplifier stage, Q2. Transistor
loop. The signal is amplified by the Q2’s RF gain is set by R10. The signal
receiver and indicated on the meter as from Q2’s collector is fed to the base of
metal detected. Q3, and Q3’s output is coupled to a
two-diode detector circuit. The DC out-
Building The Transmitter put is indicated by M1.
The receiver circuitry will fit on a 2-
We’ll start with the transmitter cir- X 3-inch piece of multipurpose PC
cuit first, (see Fig. 7) because it is the board material. Mount the components
simpler of the two units. The transmit-
close to the board with short leads and
ter circuit is very similar to our previous
keep the input components away from
two oscillator circuits, with a slight
the output circuitry. The meter can be
variation in the base bypass circuit. The
any DC type with sensitivity of 50-µA
values of frequency-determined capaci- to 1-mA. If a 50-µA meter is used, R11
 tors, C1 and C2, are the same.
Depending on the size of the loop, they
may need to be increased to a 10K
potentiometer. Mount the circuit in the
Poptronics, Sept 2001 — Copyright

can vary from .01 to .1-µF.

receiver box and connect the loop.
The receiver loop normally requires a
Mount the transmitter box on one end
capacitor equal to ½ the value of C1 or
of the wood handle and the receiver on
C2 in the transmitter circuit. The trans- the other. The receiver will need to be
mitter loop is tuned with C1 and C2, mounted so that it can be tilted up and
which are always the same value. The
down to obtain a balance between the
actual value of capacitance across the
two loops. This can be accomplished by
transmitter loop is ½ the value of either
using a small hinge attached to the end
C1 or C2. It is most important that both
of the handle and the receiver housing.
loops are tuned to the same frequency. Once the balance point is found, the
About any loop size from 8 to 12 receiver can be mounted in that
square inches will do, but we’ll stick to
position.
the 12-inch box and offer values for that
Hopefully, you will find some use for
size. The loops are formed by close
at least one of our metal-detector cir-
mum transfer of signal between the two winding 20 turns of #24 to #26 wire
cuits. Until next time, may all of your
loops. Placing a large metal object around each housing. Run about 8 circuits work!

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