Circuit Circus Charles D.

Rakes
From “Circuit Circus” by Charles D. Rakes, Popular Electronics, March, June, August 1991 Copyright © Gernsback Publications, reproduce for personal use only

Here are some metal detector circuits from three separate Circuit Circus columns.

METAL DETECTOR (March 1991) output of Q2 (taken from its emitter) is ics can be built on perfboard and should
The next entry is one of my favorite then converted to DC by D1, and from be housed in a metal cabinet.
gadgets - a simple two-transistor metal there, is applied to M1 (a 50- to 100-µA Capacitor C1 can be any variable
detector - which you can put together in meter). capacitor that you happen to find in
an evening or two and enjoy using for With The oscillator operating at, or your junkbox or one removed from an
hours on end. The circuit (see Fig. 3) very near the filter’s center frequency, old broadcast-radio receiver. The 50-µA
probably won’t lead you to a pot of the meter will read somewhere in the meter movement can come from an
gold, or any other treasure for that mat- vicinity of mid-scale. But when any older volt-ohm meter or from some
ter. But it can help locate wiring in the metal object larger than a BB is brought other piece of retired gear. Several dif-
walls or pipes in the floor, and will cost near the loop, the meter’s reading will ferent 455-kHz ceramic filters were
you next to nothing to build. And if you either increase or decrease, depending tried in the circuit and all seemed to
happen to have a youngster under foot on The type of metal. The circuit will work just fine. If you can’t locate a
with nothing to do, this circuit just detect a penny two inches away or a ceramic filter, just send an SASE. (self-
might be the one gadget that will get “D”-cell battery at about five inches in addressed, stamped envelope) to me at
junior outdoors and into a fun hobby. open air. “Circuit Circus,” Popular Electronics
In Fig. 3, transistor Q1 (a 2N3904 The search loop is wound on a small Magazine, 500-B Bi-County Blvd.,
NPN device) is connected in a simple diameter form that’s best suited for Farmingdale, NY 11735 and I’ll send
LC oscillator circuit with the values of locating smaller objects at close range, you one. [Note: Popular Electronics is
L1, C3, C4, and C9 determining the cir- but a larger loop may be built to detect no longer in business. For ceramic fil-
cuit’s operating frequency. The larger objects located at greater dis- ters, try Digi-key Corp.]
oscillator’s output is fed through C1 and tances. A plastic end cap for a 4-inch The loop should be located at least
R4 to a 455-kHz ceramic filter. When PVC sewer pipe (which can be pur- one foot away from the locator’s cabi-
the oscillator is tuned to the filter’s cen- chased at just about any plumbing- net, separated by a non-metal support.
ter frequency, the filter acts like a supply shop) can be used as the coil A wood dowel rod is a good choice.
parallel tuned circuit and produces a form for The search loop. The search Run a twisted pair of unshielded wires
high level 455-kHz signal at the junc- loop should be ten close-wound turns of between The loop and the circuit board.
tion of R3 and R4. The 455-kHz signal number-26 enamel-coated copper wire If for some reason you don’t get a
is fed to the base of Q2, which is con- wound around the bottom of the end cap meter reading when turning C9 through
figured as an emitter follower. The and taped firmly in place. The electron- its rotation, it could be that the oscilla-

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 tor just isn’t tuning to the filter’s If the oscillator’s frequency is too high, in tuning the locator for detecting the
frequency. A frequency counter can be add capacitance across C9. If the fre- size and type of desired metal objects.
connected to the emitter of Q1 to see quency is too low decrease C3 and C4. The circuit is more sensitive when the
From “Circuit Circus” by Charles D. Rakes, Popular Electronics, March, June, August 1991 Copyright © Gernsback Publications, reproduce for personal use only

what signal (if any) is present. Or, if a Also if the meter won’t quite make it to tuning is adjusted so that the meter is at
counter isn’t available, use a standard full scale, R4 can be reduced in value; if about half scale when no metals are
BC receiver and tune to the oscillator’s the needle bangs full scale, R4 can be present; at that setting, the circuit will
second harmonic. If the oscillator is increased. indicate ferrous and non-ferrous metals
operating at 500 kHz, tune your radio to Through a little experimenting, you’ll by causing the meter to increase with
1 MHz and you should hear the carrier. soon determine the best method to use one and decrease with the other.

METAL DETECTOR (June 1991) on both the transmitter and receiver por- The VLF receiver, see Fig. 4, is built
A while ago we discussed a simple tions of our detector are slightly over 4 around an LM1458 dual op-amp and a
metal detector circuit and, judging from inches in diameter and are separated by single 2N3904 general-purpose NPN
the response, it was obvious that a num- about 12 inches. The operation of the silicon transistor. Coil L1, the pick-up
ber of you were very enthusiastic about TX/RX metal detector is based on the device, is a homebrew inductor (100-
the subject. So the next circuit that we’ll directional properties of the magnetic turn loop) that is tuned to approxi-
discuss is one that is designed to do the field produced by the transmitter loop mately 7 kHz by C6. Any 7-kHz signal
same job, but in a different way. and the reception properties of the picked up by the loop is fed to U1-a,
One of the most sensitive and inex- receiver loop. which provides a gain of 100. The sec-
pensive metal detectors that you can In such circuits, the majority of the ond op-amp is also configured for a
build is a variation of the VLF TX/RX magnetic energy flows from the trans- gain of 100. The two op-amps produce
(very-low frequency transmitter/ mitter loop in an edgewise direction a combined gain of 10,000, depending
receiver) detector, which is a two part with almost no radiation perpendicular of the setting of R8. The output of U1-b
apparatus. Such double-box detectors to the loop. The receiver loop offers the at pin 7 is fed to a rectifier circuit that
— which would not respond to any- same directional properties as the trans- converts the 7-kHz signal into a posi-
thing smaller than a pound coffee can mitter’s loop, but since it is positioned tive DC voltage.
— were generally designed to detect perpendicular to the transmitter loop, That DC voltage is then fed to the
large metal objects buried deep, beneath almost no energy is detected. When a base of Q1 through R5, causing Q1 to
the ground. metal object is placed within the field of turn on. With Q1 turned on, BZ1 sounds
Our’s is a mini-version that can detect either loop, the loop’s magnetic field is to indicate that metal has been detected.
coin-sized objects from a few inches slightly distorted, allowing the receiver Power for the receiver is supplied by a
away or larger objects at a distance of to detect a small part of the redirected single 9-volt transistor radio battery.
over two feet. The sensing loops (coils) energy. The transmitter portion of the circuit

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From “Circuit Circus” by Charles D. Rakes, Popular Electronics, March, June, August 1991 Copyright © Gernsback Publications, reproduce for personal use only

(see Fig. 5) is built around a single tran- are then mounted to opposite ends of a increasing the receiver’s gain and repo-
sistor that’s configured as a Colpitts wood dowel (about 12 inches), and ori- sitioning the transmitter for the deepest
oscillator. The transmitter’s sensing ented perpendicular to each other. null. If everything is working correctly
coil, L1 (another 100-turn loop), is The receiver and transmitter circuitry the null (at full receiver gain) will be
tuned to about 7 kHz by capacitors can be built on perfboard and mounted sharp. If not, the receiver and transmit-
C2—C4. Transmitter power is supplied inside the end caps on which the loops ter may not be tuned to the same
by a 9-volt battery. are formed, or placed in separate plas- frequency.
Assembling the circuit is a snap. The tic enclosures and positioned away from To tune the receiver to the transmit-
loops are wound on plastic end caps the dowel mounted loops. ter’s frequency connect a DC voltmeter
(that are made to fit on 4-inch plastic Tuning up and checking out the to the cathode of D1 and vary C6 for the
pipe) with an outside diameter of 4-1/2 detector is easy. Turn both units on; the maximum output voltage at the diode; 4
inches. The coil is made by jumble- buzzer (BZ1) should sound. Turn the to 5 volts is normal.
winding 100 turns of number-26 receiver’s gain down until the sound The detector is most sensitive when
enamel-covered copper wire around the just about ceases, and then slowly rock the circuit is operating at maximum
center of each end cap. The ends of the the transmitter’s loop back and forth gain and off null just enough to pro-
coil are then taped in place. The loops until a perfect null is obtained. Keep duce a low level output from BZ1.

METAL DETECTOR (August 1991) and the local oscillator is operating at the best operating stability, the metal
Our next entry see Fig 2, places the 250.5 kHz, the audio tone would be the detector circuit should be neatly assem-
NE602 at the center of a simple yet sen- difference of the two, or 500 Hz. bled (keeping the component leads as
sitive metal detector. Transistor Q1, a The audio tone passes through a low- short as possible) and housed in a metal
2N3904 general-purpose NPN transis- pass filter, made up of L3 and C8, and cabinet. Transistor Q1 and its associ-
tor, is connected as a Colpitts oscillator, then travels to the headphone jack (J1) ated components should be located
operating at a frequency of about 250 through coupling capacitor C12. When away from U1 and its support compo-
kHz. The oscillator’s inductor, L1, the sense loop is passed over a metal nents, so that the two oscillators won’t
serves as the metal sensor. When the object, the Colpitts oscillator’s fre- lock together when the circuit is tuned
loop is brought near a metal object, the quency is shifted, causing the audio for a very low-frequency, audio-output
loop’s inductance changes, causing a tone to change, thereby indicating that tone. A standard 9-volt transistor radio
shift in oscillator frequency which is metal has been detected. battery will do for the power source.
transmitted to pin 1 of U1. Inductor L1 is a homemade coil, To use the circuit, position the search
Integrated circuit U1’s internal oscil- made from 5 turns of #20 enamel- loop away from any metal object and
lator is also operating at a frequency of coated copper wire wound on a 9-inch adjust C9 for a low-frequency audio
about 250 kHz. When the two oscilla- diameter wood or plastic form. After tone. It is much easier to detect metal
tors are operating at, or about the same winding the coil, tape the windings in objects at greater distances from the
frequency U1’s mixer output at pin 4, is place and attach a non-metallic handle search loop if the output-tone’s fre-
an audio tone that equals the difference to the search loop. The coil should be quency is very low. That’s because it is
frequency of the two oscillators. If the connected to the circuit via shielded much easier to detect a two- or three-
loop oscillator is operating at 250 kHz mike or mini-coaxial cable. To obtain hertz change at 15 Hz than at 150 Hz.

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From “Circuit Circus” by Charles D. Rakes, Popular Electronics, March, June, August 1991 Copyright © Gernsback Publications, reproduce for personal use only

Therefore, it is wise to set C9 for the

lowest possible output frequency for
maximum sensitivity. When searching
for buried objects, position the search
loop parallel to the ground and about
one inch above its surface. Then simply
sweep over the desired area.

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