Schema and electronic assembly of a highly sensitive metal detector

The metal detector (or "frying", because of the shape of its "head" researcher) that we propose to build does not cost a fortune so far, but it has a sensitivity as you do not raking acres of land of your election (beach, countryside, ruins, etc.).
for nothing. You have had, even if you do not see the Golden Goat or the treasury of the Knights Templar on the first try, at least the pleasure to introduce you to the theory of detecting underground and build yourself a great device ... and why not, with recovery equipment such as a cane for the handle English? The most critical part of this metal detector is receiving the head (consisting of three balanced chokes), we found a rise and already set: you can be assured that your device will work right away, without having to go summer to try to develop! Holidays are always too short, too, at least, to let them spoil the stewardship! But enough preliminaries, see at once how the metal detectors in general and ours in particular. There are four types

See the block diagram Figure 1 fi: it has a self detector (the "pan") located at the end of a handle. The signal produced by the self is mixed with an identical frequency taken from a second oscillator. The mixing of two frequencies produces a third equal to their difference. If the self sensing ranges of 100 kHz, the inductance of the internal oscillator must also oscillate on 100 kHz and mixing of these two frequencies gives a third equal to the difference, namely:
100 000 - 0 = 100,000 Hz

If a metal object approaches the sensing self, its frequency decreases in proportion to the size of the object and inversely proportional to the distance to the head. If, for example, the frequency produced down to 99 700 Hz, the mixture of this frequency with that of 100 000 Hz produced by the internal oscillator will give a difference:
100 000 - 99 700 = 300 Hz
Wiring diagram of the metal detector coils to balance. The circuit is powered by two 9 V batteries 6F22,
inserted into the two side brackets used to secure the small speaker in the plastic housing:

metals amplitude variation are of high quality semi-professional and, unlike the others, they are insensitive to temperature variations. If, for example, the oscillator stage produces a signal amplitude of 1 V, it does enough to approach the self sensing a small metal object so that this voltage drops sharply to 0.9 V. Then, if you approach a medium-sized metal object, the voltage drops below 0.1 V. This AC signal is rectified to derive a voltage applied to the inputs of an amplifier with differential: on the one input of the amplifier is applied to operating an electrolytic capacitor (fi gure 2). It goes without saying that the electrolytic capacitor is loaded with the maximum amplitude value, a V in our example. When the detector is self infl uencée by any metal object on the two inputs of the amplifier differential voltage
1 V is present and therefore the output voltage is:
1 - 1 = 0 V.
Once the detector is self infl uencée by a metal object, instantly signal the amplitude of its decline and, if such it reaches 0.9 V, we find that tension only on the input of the amplifier to which differential the electrolytic capacitor is not connected. Indeed, on the entrance opposite to that containing the electrolytic capacitor is always present a voltage of 1 V, because the electrolytic capacitor has not had time to discharge. Therefore, on an input of this amplifier with differential we have a voltage of 0.9 V and the other a voltage of 1 V. If this amplifier with differential is calculated for a gain of 15 times, we find out
voltage:
(1 - 0.9) x 15 = 1.5 V.
This voltage is used to excite an oscillator stage BF producing an audible note in the headphones or speaker. The metal detector pulse See the block diagram fi gure 3: Normally in the minds of metal detectors pulse is a self operated alternately transmitting and receiving. When it is transmitting, the self is excited by a series of square wave pulse so that when it is receiving, it captures the pulse refl Echia the basement. If the pulses are infl uencées by metal objects, their leading edge is changed and this difference is used to
excite an oscillator stage produi BF-ing an audible note in the headphones
or speaker. Although these detectors
metals are high cost and they are considered as semi-professional, we believe they have a
less sensitive than those with amplitude variation

The metal detector coils to balanced
See the block diagram fi gure 4: in the head with metal detectors balanced inductors are three coils, but outside you can only see two. The outer self, larger diameter, is used as
transmitter and the other two, mounted in opposite phase, are used as receivers. Recent
are placed at the center of the head, so as to cancel the signal produced by the issuing self. When the head of an object is approximated

Figure 9a: Layout diagram of the components of the platinum metal detector coils balanced. On pins
the connector on the detector head are marked with numbers 4-2 and 1-3 designed to avoid wiring errors. The circuit
is printed double-sided plated through holes, if you realize yourself, be sure all connections indispensablesentre both sides.

List of components:


R1 ......... 2.2 kOhms
R2 ......... 10 k
R3 ......... 5.6 kOhms
R4 ......... 10 k
R5 ......... 12 kOhms
R6 ......... 4.7 kOhms
R7 ......... 27 kOhms
R8 ......... 10 k
R9 ......... 100 kOhms
R10 ....... 1 MW pot. lin.
R11 ....... 100 kOhms
R12 ....... 100 kOhms
R13 ....... 10 k
R14 ....... 22 kOhms
R15 ....... 100 kOhms
R16 ....... 10 k
R17 ....... 10 k
R18 ....... 10 k
R19 ....... 10 k
R20 ....... 10 k
R21 ....... 10 k
R22 ....... 100 kOhms
R23 ....... 4.7 kOhms
R24 ....... 10 k
R25 ....... 1 M
R26 ....... 10 k pot. lin.
R27 ....... 270 kOhms
R28 ....... 100 kOhms
R29 ....... 12 kOhms
R30 ....... 100 kOhms trimmer
R31 ....... 2.2 kOhms
R32 ....... 1 k
R33 ....... 10 k pot. lin.
R34 ....... 33 kOhms
R35 ....... 10 k
R36 ....... 15 kOhms
R37 ....... 47 kOhms
R38 ....... 100 kOhms
R39 ...... 100 kOhms
R40 ....... 10 k
R41 ....... 22 kOhms
R42 ....... 2.2 kOhms
R43 ....... 1 M
R44 ....... 10 Ω
C1 ......... 820 nF polyester
C2 ......... 680 nF polyester
......... C3 100 nF polyester
C4 ......... 10 nF polyester
C5 ......... 22 nF polyester
......... C6 47 nF polyester
C7 ......... 100 nF polyester
C8 ......... 220 pF ceramic
C9 ......... 100 pF ceramic
C10 ....... 10 nF polyester
C11 ....... 100 nF polyester
C12 ....... 100 nF polyester
C13 ....... 100 nF polyester
C14 ....... 100 nF polyester
C15 ....... 47 uF electrolytic
C16 ....... 100 nF polyester
C17 ....... 47 nF polyester
C18 ....... 47 nF polyester
C19 ....... 10 nF polyester
C20 ....... 1 uF polyester
C21 ....... 100 nF polyester
C22 ....... 47 uF electrolytic
C23 ....... 100 nF polyester
C24 ....... 100 nF polyester
C25 ....... 100 nF polyester
C26 ....... 100 nF polyester
C27 ....... 100 uF electrolytic
C28 ....... 100 nF polyester
C29 ....... 100 nF polyester
C30 ....... 220 uF electrolytic
C31 ....... 100 nF polyester
C32 ....... 100 uF electrolytic
C33 ....... 100 uF electrolytic
DS1 ....... Diode 1N4148
DS2 ....... Diode 1N4007
TR1 ........ PNP BC557
TR2 ........ NPN BC547
TR3 ........ NPN BC547
TR4 ........ PNP BC557
IC1 ........ Integrated NE5532
IC2 ........ Integrated NE5532
IC3 ........ CMOS 4017
IC4 ........ Integrated NE5532
IC5 ........ CMOS 4053
IC6 ........ Integrated NE5532
IC7 ........ Integrated CA3130E
IC8 ........ Integrated MC78L12
P1 ......... Tappet
S1 ......... Switch
HP ......... 0.2 W speaker

Figure 9b-1: Design, scale 1, the double circuit
face plated holes, side components.
Figure 9b-2: Design, scale 1, the double circuit
face plated holes, solder side.
Figure 11: On the front are set in
over the loudspeaker, the three potentiometers,
push-button P1 and the switch
S1. Against the two brackets are
place both batteries 6F22 (Figure 12).
Figure 10: To fix
the speaker in front
before, use two
4 brackets and braces
consist of bolts
with two nuts

Start with the NPN TR1, mounted oscillator to excite the self issuing the outer circle of the head. With the values ​​chosen for C1 and C2, the self should oscillate at 5500 Hz and produce a signal of about 10 Vpp amplitude. The frequency is not critical and therefore if it oscillates at 5300 Hz or 5700, because of tolerance capacitors, characteristics and sensitivity of metal detectors will not be affected at all. The signal produced is captured by the two coils in opposite phase to the inside of the head and the low signal at the output, about 0.004 Vpp when the self is infl uencée by any metal object, is applied to the input non-inverting amplifier of the first - er operational IC2-A. This amplifier with operational amplifier signal e 22 times and it is therefore an output of 0.09 V. C10 delivers the signal to the inverting inputs (- sign) of the two operational amplifiers indicators
IC2 and IC4-B-A. The first operational amplifier with an amplifier IC2-B signal about 4.5 times, but with a phase inversion of 360 °. These signals, phase shifted by 180 ° and 360 ° are applied to the inputs of the electronic switch IC5-A, acting as a dual rectifier demionde and therefore we find he left halves of positive vibes to 11 000 Hz or at a frequency double that of 5500 Hz produced by the oscillator stage TR1. The more we approach a metal head and halves the amplitude of the positive vibes coming out of the electronic switch IC5-A increases. These waves halves, fi lters by R22 and C16, allows us to obtain a voltage
continues that, applied to the non-inverting input of the amplifier operating IC4-B, ed is amplified 100 times. If you turn the slider of potentiometer R33 to the output of the amplifier operating IC4-B, we obtain the maximum sensitivity, minimum sensitivity to R32. Blood taken from the potentiometer R33 is applied to the non-inverting input of the amplifier operating IC6-B and collected at its output to be applied to the electronic switch IC5-C. This switch, opening and closing at a frequency of 550 Hz, produced a note that modulated, amplified ed by TR2, TR3 and TR4, a speaker driver or casque.Plus greater the value of the voltage at the amplifier output cateuropérationnel IC6-B and higher is the intensity of the note. The last operational amplifier with
IC7, bottom right of the wiring diagram is used to balance the output signal so as to mute the amplifier in the absence of metal objects. The potentiometer wiper reset R27 is rotated so as to eliminate completely the low notes could be heard in the absence of metal objects, when the cursor of the potentiometer R33 sensitivity is turned up. Knowing that a lot of land containing metal dust, we have inserted in the circuit the P1 button to automatically correct small imbalances can occur when you explore this type of land with the metal detector set to maximum sensitivity.

Figure 13: Before attaching the three
potentiometers on the front, you
must shorten the lines to
buttons to set the correct
(A few mm of the panel).
The washer and nuts are
to hold off the panel
L brackets and attach the speaker.

operational amplifier IC3, the CMOS 4017. The frequency of 5500 Hz, applied
on pin 14 of IC 3, is taken
on pin 3 divided by 10
and we find ourselves on this pin
a square wave at 550 Hz used
to drive the electronic switch
IC5-C. This metal detector has no measuring instrument because, besides the fact that this would have complicated the pattern, its presence would have been to the detriment of the sensitivity of the device. On top of this galvanometer is not strictly necessary because, when exploring a field hearing is more natural and spontaneous visual observation of the small displacement of a needle on a dial. Indeed, one look at the dial of the galvanometer after hearing the note in the headphones and it is not
indispensable.
Food
To feed the metal detector, use two 9 volt batteries 6F22en series, making a total voltage of 18 V. This tension is only used to power the amplifier stage comprises indicator BF TR2, TR3 and TR4. All other levels of the metal detector are fed with a voltage of 12 V stabilized by the integrated controller MC78L12 IC8.

The practical realization of metal detector:
If you follow carefully the fi gures 8-12 and in particular the fi gure 9a, you should have no problem to mount the metal detector: proceed in order, afi n do not forget anything, do not interchange components is like, not to reverse the polarity of the polarized components and not to make by welding or short circuit between tracks and pads or cold junction stuck. When you are in possession of double-sided circuit board with metallized holes (a scale drawing of two faces fi gure 9b 1 and 2), mount all components as shown in fi gure 9a. Place first 7 media circuits
ez integrated and verified that you have forgotten to solder any pin. Assemble all resistance, by carefully monitoring their values ​​(rank them first), and the R30 trimmer. Now mount the silicon diodes, rings black marker-keyed shot in the direction of the fi gure 9a. Then mount all ceramic capacitors and polyester, pressing out their cases on the surface of printed circuit and the electrolyte, although respecting polarity + / - of the latter (the longest leg is the + and - is registered the side of the cylindrical case). . Mount fi nally transistors TR1 to TR4 and regulator IC8, repèredétrompeurs turned flat in the directions shown by the fi gure 9a.
Mount the top left terminal block 4-pole: you then screw the two decision-6F22 batteries. Insert and solder the top and bottom of the plate, all the pins for external connections you make when mounting in the housing produced. Make sure you have not forgotten anything. Insert now (unless, purists, you'd rather wait for the installation of the fi n in the case and that the latest welding has cooled!) Integrated circuits, benchmark-oriented U-keyed in the direction shown in Figure 9a, or all down. The assembly in the housing no problem if you look closely
the fi gures from 10 to 15 and particularly the fi gure 12. The plastic handle of the handle fi xez aluminum back panel forming the bottom of the case console with metal spacers of 5 mm (fi gure 15): These spacers are also used to hold the PCB to 5 mm of the base metal . At the end of the rear panel, if xez also plug socket which will then fit
the female prong cord going to the detector head (fi gure 15). Then take the front and aluminum fi xez the 3 knobs, push-button P1, the switch S1 M / A and the speaker (using for this
last two squares in L: fi gures 10 and 11). These two brackets are also used to fi xer 6F22 both batteries inside the plastic housing (fi gure 12). The front panel is aluminum fi xer on the plastic box with 4 small bolts, having completed the course four holes in the plastic. Profi-in for Tez
perform another hole in the plastic wall of the small enclosure for passage of the female jack (headphone jack), see fi gure 14. Unless you decide to settle for the speaker. Fi with pieces of the plastic insulated, connect all the pins of the edges of printed circuit to the terminals of the potentiometers, the push of the switch, the jack and speaker, by ordering them in a bundle if possible.
Wire as the connector going to the detector head. Remember the two decision-batteries with good
polarity (fi gure 9). Before fi xer the plastic housing on the handle of the metal detector, turn the trimmer
Setting:
The metal detector operates on the first try, but for maximum sensitivity, you still need to adjust the trimmer "offset" R30. Before performing this adjustment, put the head on a table or a stool non-metallic. After turning the knob potentiometers R10, R26 and R33 at halfway, you must connect TP1 and the mass function of a multimeter and DC range 10 to 15 V full scale.
Hold the button pressed and slowly turn the P1 cursor trimmer R30 to read a voltage of 6 V exactly. Remove the meter, close the plastic housing and are looking for a field you want to explore the basement: the field experience you will also gain some practice, because only by using your device you can locate in fi you ant to changes in sound, more or metal objects buried in the ground moinsgros

How to try it? :
Before venturing on any terrain in search of "treasures" of your dreams, we advise you to do a little hands on a field near your home (your garden, for example). Ask a friend to bury at a depth of 10 cm three objects:
- A piece of one euro
- A tablespoon,
- A can of beer or soda metal
you will then have to find ant modifies the sensitivity of metal detectors by adjusting the potentiometer R33. After turning the button on the sensitivity potentiometer R33 to the desired position or minimum - average - maximum, press the head (sensor!) On the ground (after checking e is not there no metal) and the loudspeaker emitting a note, slowly turn the knob of potentiometer R26 to reset the position for which the note continues. If the note is still faintly audible,
turn the potentiometer R10 balancing to cancel. Do not be surprised if, in turning the R33 for maximum sensitivity, it is more diffi cult to completely cancel the note. To achieve this we recommend - press P1 and turn the knob to zero to find the R26
position in which the note fades maximum - P1 release and turn the balance control R10 until you find the position for which the note disappears completely. If a residue remains, just t
slightly reduce the sensitivity with the knob R33. When you hear a low note, the sensitivity of
metal detector will be set up and it will be so easy to find a room the size of a euro at a depth of 17-18 cm. To discover the three metal objects hidden in the ground, we recommend that you select three different sensibilities in order to understand how the note varies depending on the depth where the object. When this test is completed, go to any land to get on with the research itself: it goes without saying that you will
hold the detector head close to the ground to be able to distinguish small metal objects buried at shallow depths of large metal masses to greater depths.
All you need to know if you explore a land zigzagging randomly, you will succeed diffi cult to find something. For a more successful, you must act consistently by traversing the ground as do the farmers for planting (fi gure 18), and each square meter will be visited by the head of the unit. If you find a oppidum prehistoric or medieval castrum an old coin, a ring or a bule fi, you know that you are
in an area of ​​interest to your findings and therefore do not forget every inch of the billet. Land
freshly plowed are fruiting, because the Social brings to the surfaceles objects buried deeper. Remember that prior to enter into
private land you must obtain permission from the owner and that it has rights in the objects may be discovered, as well as archaeological services in offi cial and they also have the duty and to prevent you from Ledroit rampage a site, even if you are the inventor
by hasard.Par against the beaches in the early morning or evening in the cool, you will probably find little sesterces or ducats, but watches and jewelry lost by swimmers (I'll spare you the capsules cans) : report them to the lost property office (no caps!) and, if unclaimed after one year and one day these things will be
you outright.