Radio constructor: A simple metal detector on a K561LA7 chip. (021)
This metal detector circuit from all simple circuits showed best results. By using this device it is possible to detect both ferrous metals (fittings in the walls of rooms) and metal objects in the ground (both ferrous and non-ferrous). The detection depth depends on the size of the metal object (small objects are detected at a depth of up to 12 cm). The operation of the circuit is based on the frequency beat of two generators assembled on the basis of the domestic K561LA7 microcircuit, consisting of four 2I-NOT logic elements (K561LA7 can be replaced with K561LE5 or imported analogue CD4011). It can be seen from the diagram that an exemplary generator is assembled on the elements DD1.3 and DD1.4, with the frequency of which the frequency of the search generator assembled on the elements DD1.1 and DD1.2 will be compared. Let's consider how the circuit elements work: The frequency of the exemplary generator is determined by the parameters of the capacitor C1 and the total resistance of the variable resistors R1 and R2 and lies in the range of 200 - 300 kHz. The frequency of the search generator is set by the parameters of the circuit C2, L1 (it is within 100 kHz), that is, it depends on the capacitance of the capacitor and the inductance of the coil and is constant (conditionally, because the frequency stability depends largely on changes in temperature, supply voltage, humidity). During the operation of the search generator, not only the fundamental frequency of 100 kHz is generated, but also multiple harmonics of 200 kHz, 300 kHz, 400 kHz, and so on. The higher the harmonic, the lower its level. When the exemplary generator (OG) is operating at a frequency of 300 kHz, the “necessary” harmonic of the search generator (PG) is the third, that is, also 300 kHz. If we set the OG frequency to 305KHz with resistors R2 and R3, and the PG frequency is 100KHz, then the third harmonic of the PG, equal to 300KHz (frequencies above 20KHz are no longer determined by ear), from the output of capacitor C4 is mixed with the frequency of the OG at the output of capacitor C3. Further, these frequencies are fed to the diode mixer VD1, VD2, assembled according to the voltage doubling scheme (in one half-cycle, the signals from the outputs of the generators pass through the diode VD1 and charge the capacitors C3 and C4, in the second half-cycle, the voltages from the outputs of the generators are added to the voltages of the charged capacitors C3 and C4 and come through the VD2 diode to the headphones T. The diode mixer, acting as a detector, selects the difference frequency 305KHz - 300KHz = 5KHz, which is heard in the headphones in the form of a tone signal. Higher harmonics are significantly inferior in signal strength and are no longer heard in headphones, and lower harmonics do not give such a difference in frequency change - when a metal object enters the zone of the receiving coil, its inductance changes slightly, which affects the frequency of the PG. For example, the frequency became not 100.000 Hz, but 100.003 Hz. x is hardly perceptible, but at the third harmonic 100.003 Hz will be equal to 300.009 Hz, and the difference with the OG frequency will be 9 Hz, which is more noticeable by ear and increases the sensitivity of the device. Diodes VD1, VD2 can be any, but always germanium. C6 serves to bypass the high-frequency components of the signal at the output of the mixer. The headphones of the headphones must be connected in series (the photo shows the terminals of the telephone jacks for serial connection of standard stereo headphones). All these rules allow the most efficient use of the output signal without resorting to additional amplifiers that complicate our design. In our case, the volume of the signal does not affect the sensitivity of the device. The main thing in tuning is to set the beat frequency correctly and focus on its change. Now to the main element of our circuit - the search coil. The ability of the device to detect metal objects will depend on the quality of its manufacture.
The search coil (PC) consists of 50 turns copper wire type PEV, PEL, PELSHO with a diameter of 0.2 - 0.6 mm, wound on a mandrel with a diameter of 12 - 18 cm. There are several ways to make a PC. You can draw a circle with a diameter of 12 - 18 cm on plywood, board, plywood, etc., hammer nails around the circumference, then wind a coil around the nails, tie it firmly in a circle with threads, then pull out the nails. You can wind the coil on any round plastic structure of the appropriate diameter (for example, a piece of plastic sewer pipe, lower part plastic buckets that are thrown away by shops after selling herring, pickles. The excess is cut off. It is advisable to impregnate the coil wound in this way with varnish or paint (only not nitro! The solvent will damage the varnish insulation of the coil wire) in order to fill the cavities between the turns, into which water can later enter. After drying, the coil must be tightly wrapped with electrical tape over the entire surface. For improvement protective properties PC and reduce the influence of external electric fields on it, it must be shielded. You can immediately wind the coil on a copper or aluminum tube bent into a circle and sawn on the outside with a hacksaw or a “grinder” with a thin disk, but it’s easier to take aluminum foil for baking, cut it into strips and wrap the coil from the initial to the final taps with these strips, leaving a gap of about 1 - 2 cm not wound. Otherwise, a short-circuited turn will result, which will not allow the coil to work. Given that not everyone has the opportunity to solder the "earth" wire to the aluminum screen, you can strip 3 - 8 cm of insulation from the wire by wrapping the aluminum screen with the bare end and wrapping it tightly with electrical tape. It is desirable to shield the insulated connecting wires from the coil to the board with aluminum foil, connecting it to the same ground wire in the same way as in the coil. You can start setting up the device after winding the PC before its impregnation and shielding. Everything else is an improvement of the device. If everything is assembled correctly, then after connecting the PC to the circuit and applying power (observe the polarity of the power supply connection and the correct installation of the microcircuit in the socket) in the headphones, while rotating variable resistor R2 "Rough", you will hear the beats of the frequencies of the generators. In the absence of special instruments (oscilloscope, frequency meter), the operation of the generators can be determined by any voltmeter connected instead of headphones. Having soldered capacitor C4 from the diode mixer, the voltmeter will show the work of the exhaust gas in the form of a voltage approximately equal to the supply voltage of the circuit. And vice versa, having unsoldered C3, we will see the operation of the SG according to similar readings of the voltmeter. The work of both is manifested in listening to the tone of the beats in the headphones. Resistor R2 allows you to tune the frequency of the OG in a wide range, which is manifested in repeatedly appearing beats in the headphones. Now we need to carefully check these beats, choose the most “powerful” (resistor R3 should be in the middle position). When checking each of the harmonics, the resistor R2 must be set in such a position that the “voiced” tone of the signal goes down. Further tuning must be carried out with the resistor R3 “Exactly” and ensure that the beat tone turns into wheezing and clicks. This position is the working one with maximum sensitivity. Next, we take an object made of ferrous metal and bring it to the coil - the signal tone should increase. When a non-ferrous metal object (aluminum, copper, brass) is brought to the coil, the signal tone should, on the contrary, decrease or completely break. If this does not happen or happens vice versa, it is necessary to rebuild the exhaust gas to a different harmonic and do it all over again. As soon as you have found the “necessary” harmonic, you need to remember the position of R2 and in the future work only with R3, tuning as much as possible to the working section of the beats. The more accurately you tune in to it, the higher the search results will be. After you understand the principle of operation, you can begin to improve the search coil. When assembling the circuit, the metal parts of the variable resistors R2, R3 must be connected to a common (negative) wire, otherwise the approach of the hand to the handle will affect the beat frequency. It is desirable, to reduce the influence of external factors, to place the device circuit in a metal case connected to a common
Fragments from the book “Do-it-yourself metal detectors. How to search to find coins, jewelry, treasures. Authors S. L. Koryakin-Chernyak and A. P. Semyan.
Continuation
Read the beginning here:
3.1. Compact metal detector on the K175LE5 chip
Purpose
The metal detector is designed to search for metal objects in the ground. It can also be used in determining the location of fittings and concealed wiring during construction works in the house.
circuit diagram
A diagram of a compact metal detector based on a K175LE5 chip is shown in fig. 3.1, a. It contains two generators (reference and search). The search generator is assembled on the elements DD1.1, DD1.2, and the reference generator - on the elements DD1.3 and DD1.4.
The frequency of the search generator, made on the elements DD1.1 and DD1.2, depends on:
- from the capacitance of the capacitor C1;
- from the total resistance of the tuning and variable resistors R1 and R2.
Variable resistor R2 smoothly change the frequency of the search generator in the frequency range set by the tuning resistor R1. The frequency of the generator on the elements DD1.3 and DD1.4 depends on the parameters of the oscillatory circuit L1, C2.
The signals from both generators are fed through the capacitors C3 and C4 to the detector, made according to the voltage doubling circuit on the diodes VD1 and VD2.
The load of the detector is the headphones BF1, on which the difference signal is allocated in the form of a low-frequency component, which is converted by the headphones into sound.
In parallel with the headphones, a capacitor C5 is connected, which shunts them at a high frequency. When the search coil L1 approaches a metal object, the frequency of the generator on the elements DD1.3, DD1.4 changes, as a result, the tone of the sound in the headphones changes. On this basis, it is determined whether there is a metal object in the search area.
Applied parts and replacement options
Trimmer resistor R1 type SP5-2, variable resistor R2 - SPO-0.5. It is permissible to use other types of resistors in the circuit, preferably small ones.
Electrolytic capacitor C6 type K50-12 - for a voltage of at least 10 V. The rest are constant capacitors of the KM-6 type.
Coil L1 is placed in a ring with a diameter of 200 mm, bent from a copper or aluminum tube with an internal diameter of 8 mm. There should be a small insulated gap between the ends of the tube so that there is no short-circuited loop. The coil is wound with PELSHO 0.5 wire.
Headphones TON-1, TON-2 can be used as BF1 headphones.
The metal detector is powered by a Krona battery or other types of 9 V batteries.
In the metal detector circuit, the K176LE5 microcircuit can be replaced with K176LA7, K176PU1, K176PU2, K561LA7, K564LA7, K561LN2 microcircuits.
Mounting the device
The details of the device, except for the inductor, power supply and headphones, can be placed on a printed circuit board cut out of 1 mm thick foil fiberglass (Fig. 3.1, b). It is possible to use another type of printed circuit board.
A handle made of a metal tube is attached to one end of the connector, and a metal ring with an L1 coil is attached to its other end using an adapter made of insulating material.
The general view of the device is shown in fig. 3.1, d, and the placement of the device elements - in fig. 3.1, c.
Setting
Before setting up the metal detector, the tuning and variable resistors must be placed in the middle position and contacts SB1 must be closed. By moving the slider of the tuned resistor R1, achieve the lowest tone in the headphones.
In the absence of sound, the capacitance of the capacitor C2 should be selected. If a malfunction occurs in the operation of the metal detector, a capacitor with a capacity of 0.01 ... 0.1 μF should be soldered between terminals 7 and 14 of the DD1 microcircuit.
Source
Yavorsky V. Metal detector on K176LE5. // Radio, 1999, No. 8, p. 65.
From book S. L. Koryakin-Chernyak, A. P. Semyan. ""
Continue reading
After reading a little amateur radio forums on production of metal detectors found that most people who assemble metal detectors, in my opinion, undeservedly written off beating metal detectors- so called BFO metal detectors. Allegedly, this is the technology of the last century and "children's toys." — Yes, this is a simple and unprofessional device that requires certain skills and experience in handling. It does not have a clear selectivity of metals and requires adjustment during operation. However, even with it, you can make a successful search under certain circumstances. As an option - beach search- perfect beating metal detector option.
Place to search with a metal detector.
With a metal detector, you need to go where people lose something. I'm lucky to have such a place. Not far from my house there is an abandoned river sand quarry, where people drink and swim in the river all the time in the summer. Of course, they always lose something. In my opinion, best place for searching with a metal detectorBFO cannot be imagined. Lost things are instantly self-buried to a shallow depth in dry sand and it is almost impossible to find them manually. Some sort of mystic. I remember, as a child, I dropped the keys to the apartment there in the sand. Here I stand, the keys fell here, but no matter how much I dug up that area, all to no avail. They literally fell through the ground. Just a haunted place. At the same time, on this "golden" beach, I constantly found other people's keys, lighters, coins, jewelry and phones in the sand. And when last trip with a metal detector - female slim Golden ring. It was almost at the surface slightly sprinkled with sand. Perhaps just luck. Actually, it was under this beach that I made my metal detector.
Advantages of a metal detector on beats.
Why exactly BFO? - First of all, this is the most simple metal detector. Secondly, it has at least some signal dynamics depending on the properties of the object. Not that pulse metal detector- "peeping" on everything the same. In no way do I want to belittle advantages of a pulse metal detector. This is also a wonderful device, but it is not suitable for a beach littered with traffic jams and foil. Many will say that beating metal detector does not distinguish the properties of the object, howls and buzzes all the same. However, it is not. After practicing on the beach for a couple of days, I got pretty good at identifying foil as a sharp and deep change in frequency. Caps from beer bottles cause a strictly defined change in frequency, which must be remembered. But the coins emit a weak, "point" signal - a barely perceptible change in frequency. All this comes with experience with patience and a good ear. Beat metal detector- it's still "auditory" metal detector. Here the human is the analyzer and signal processor. Therefore, it is imperative to search for headphones, and not for a speaker. And the best option- big headphones, not "plugs".
The design of the metal detector.
Structurally I decided to make a metal detector foldable and compact. So that he gets into a regular package, so as not to attract the attention of "normal" people. Otherwise, getting to the place of search, you look like an “alien”, or a collector of scrap metal. For this purpose, I bought the smallest (two-meter five-knee) telescopic rod in the store. Left three knees. It turned out to be a fairly compact folding base, on which I and assembled my metal detector.
The entire electronic unit was assembled in a plastic box for wiring 60x40 that I already loved. The end cap, the power compartment partition and the power compartment cover were also made of its plastic. The parts were glued together with superglue and sat on M3 bolts. Fastening electronic block metal detector to the rod is made in the form of a metal bracket, which is inserted into the place of the fishing reel with fishing line and fixed with a regular rod nut. The result is an excellent lightweight and durable design. The power button, a coil jack (a five-pin jack from the “grandfather's” tape recorder), a frequency control and a headphone jack are placed outside the unit.
Metal detector circuit board It was made on the spot by wiring tracks with a waterproof marker. Unfortunately, I can't provide a printout for this. Mounting surface hinged - no holes - "lazy" - my favorite. It is also important after assembling the board to cover it with any varnish to protect it from moisture and debris. In the field, this is very important. For example, I lost one day due to the fact that some debris got inside under the microcircuit. The metal detector just stopped working. And I had to return home, disassemble it, blow through and open the board with varnish.
Scheme of a metal detector on beats.
The circuit itself (see below) was redesigned and optimized by me from two metal detector circuits. This is "" - the magazine "Radio", 1987, No. 01, pp. 4, 49 and " metal detector hypersensitivity "- Radio magazine, 1994, No. 10, p 26.
The result is a simple and functional circuit that provides stable low-frequency resulting beats - just what you need to hear the slightest changes in frequency.
The stability and sensitivity of the metal detector is provided by the following circuit solutions:
Reference and measuring generators are separated- made in separate microcircuit packages - DD1 and DD2. At first glance, this is wastefulness - only one of the four logical elements of the microcircuit package is used. That is, yes, the reference generator is assembled on only one logic element of the microcircuit. The remaining three logical elements of the microcircuit are not involved at all. The measuring generator is built in the same way. It would seem that it makes no sense not to use free logical elements of the microcircuit package. However, this is where it makes the most sense. And it consists in the fact that if, for example, we still assemble two generators in the same microcircuit housing, they will synchronize each other at close frequencies. You will not be able to get the slightest change in the resulting frequency. In practice it will look like abrupt change frequencies only when a massive metal object is close to the measuring coil. In other words, the sensitivity is sharply reduced. metal detector does not react to small objects. The resulting frequency, as it were, "sticks" at zero - up to a certain point there are no beats at all. They also say - dumb metal detector"," dull sensitivity. By the way " Metal detector on a microcircuit"- Radio magazine, 1987, No. 01, pp. 4, 49 is built on just one chip at all. This effect of frequency synchronization is very noticeable there. It is absolutely impossible for him to search for coins and small objects.
Also, both generators must be shielded with separate small tin screens. This increases by an order of magnitude overall stability and sensitivity of the metal detector. It is enough just to solder small partitions made of tin to the minus between the generator microcircuits to make sure that the parameters of the metal detector are improved. The better the screen, the better better sensitivity(the influence of generators on each other is weakened and plus protection from external influence to frequency).
Electronic tuning.
comparator on DD3.2 - DD3.4.
This element of the circuit converts the sinusoidal signal from the output of the mixer DD3.1 into rectangular pulses of doubled frequency.
First, the rectangular pulses are clearly audible at hertz frequencies as distinct clicks. While the sinusoidal signal of hertz frequencies is already hardly distinguishable by ear.
Second, doubling the frequency allows the adjustment to come closer to zero beats. As a result, by adjusting it, you can achieve a “clatter” in the headphones, the change in frequency of which can already be caught when a small coin is brought to the coil at a distance of 30 cm.
Generator Power Stabilizer.
Naturally, in this circuit, the supply voltage significantly affects the frequency of the generators DD1.1 and DD2.1 metal detector. Moreover, each of the generators affects differently. As a result, with the discharge of the battery a little "Floats" and the beat frequency of the metal detector. To prevent this, a five-volt stabilizer DA1 was introduced into the circuit to power generators DD1.1 and DD2.1. As a result, the frequency ceased to "float". However, it should be said that, on the other hand, due to the five-volt power supply of the generators, several decreased sensitivity of the metal detector generally. Therefore, this option should be considered optional and, if desired, generators DD1.1 and DD2.1 can be powered from a crown without a stabilizer DA1. You just have to more often adjust the frequency manually, with a regulator.
The design of the metal detector coil.
(See diagram below).
Since it not a pulse metal detector, butBFO, then the search coil (L2) is not afraid of metal objects in its design. We don't need a plastic bolt. That is, we can safely use a metal (but only open!) frame and an ordinary metal bolt for the hinge for its manufacture. Subsequently, when adjusting the circuit, all the influences of the metal in the structure will be brought to zero by the tuning core of the coil L1. The L2 coil itself contains 32 turns of PEV or PEL wire with a diameter of 0.2 - 0.3 mm. The coil diameter should be about 200 mm. Winding is conveniently done on a small plastic conical bucket. The resulting turns are completely wrapped with electrical tape and tied with a thread. Further, this entire structure is wrapped in foil (cooking foil for baking). Tinned wire is wound on top of the foil in several turns around the entire perimeter of the coil. This wire will be the lead of the coil's foil shield. Once again, everything is wrapped with electrical tape together. The coil itself is ready.
The frame on which the reel will be located and by which it will be attached to the rod is made of steel spring (not soft) wire 3-4 mm. It actually consists of three parts (see figure) - two twisted wire loops of the hinge, which will be bolted together and a wire ring threaded into the dropper tube (the ring should not be a closed loop).
This whole structure, together with the finished wire coil, is also tied together with threads and electrical tape.
The hinge with the reel itself is attached to the rod by tying with nylon threads and gluing with epoxy resin.
It is advisable not to wet the coil during the search process, and even more so not to use it for underwater search. She's not airtight. Moisture that gets inside can eventually destroy it.
Coil L1 (see diagram) is wound on a frame from a small-sized radio receiver with a metal screen and a tuning core. The coil contains 65 turns of PEV wire with a diameter of 0.06mm
Me and Diode. © site.
Metal detector on a microcircuit
A similar device has already been described in an article by I. Nechaev under the same name in "Radio", 1987, N9 1, p. 49 . In contrast, in the proposed version, there is only one inductor and a slightly different circuit design, which also made it possible to do without a variable capacitor.
The scheme of the metal detector is shown in fig. 1. As in the above design, it has two generators: one is made on the elements DD1.1 and DD1.2, and the second - on the elements DD1.3 and DD1.4. The frequency of the first generator (tunable) depends on the capacitance of the capacitor C1 and the total resistance of the resistors R1, R2. The tuning resistor R1 sets the operating range of the generator, and the variable resistor R2 smoothly changes the frequency of the generator in this range. The frequency of the second generator depends on the capacitance of the capacitor C2 and the inductance of the search coil L1.
The signals of both generators are fed through the decoupling capacitors C3 and C4 to the detector, made on the diodes VD1, VD2 according to the voltage doubling scheme. The load of the detector is BF1 headphones - a difference signal is allocated to them in the form of a low-frequency component, which is then converted by the phones into sound. Capacitor C5 shunts the load at higher frequencies, in other words, closes the signals of both generators to a common wire.
When the search coil approaches a metal object, the frequency of the second oscillator changes. As a result, the tone of the sound in the headphones changes. On this basis, metal objects are detected in the search area, for example, a sublayer of soil, snow. A metal detector will be of great help in determining the location of fittings and hidden wiring during construction work in the house.
In addition to that indicated in the diagram, in a metal detector, you can use the K176LA7, K176PU1 K176LU2 microcircuit (the last two microcircuits are the so-called level converters), K561LA7, K174LA7. K561LN2. Trimmer resistor R1 - SP5-2 variable R2 - SPO-0.5. but other small-sized resistors will do. Oxide capacitor - K50-12 or another small-sized one for a rated voltage of at least 10 V, the rest of the capacitors can be, for example, KM 6
Coil L1 is placed in a ring with a diameter of 200 mm from an aluminum or copper tube with an internal diameter of 8 mm. The ends of the tube must be separated from each other, but at some distance, so that a short-circuited coil is not obtained. To wind the coil, use a PELSHO wire (in enamel and silk insulation) with a diameter of 0.5 mm, trying to stretch it inside the tube as possible more turns. This operation may seem laborious, so you can use the technique described in the above article - first lay the wire segments inside the tube, and then bend the tube into a ring and connect the segments in series to obtain a multi-turn coil. The coil leads are subsequently connected to the printed circuit board, and the tube is connected to a common wire.
Headphones BF1 - TA-4 TON-1 or others, with as much resistance as possible. Power source - Krona battery or another, with a voltage of about 9 V.
Fig.2
Fig.3
Fig.4
Most of the metal detector parts are mounted on a figured printed circuit board (Fig. 2 and 3) made of one-sided foil fiberglass. The terminals of the resistors R1 and R2 are connected to the corresponding circuits of the device either using a wire or printed conductors if the installation is carried out on a double-sided foil material. The board is placed inside the L-shaped casing of the ShR connector (Fig. 4) and attached to one of its halves with a nut screwed on the outside of the variable resistor R2. To access the adjusting screw of the tuning resistor R, a hole is sawn in the casing.
The power source is placed inside the handle-case, which can be either plastic or metal (say, a case from a round flashlight). On top of the handle-case, a power button SB1 is attached, and at the bottom, an X1 socket for connecting headphones.
The ring with the coil is fixed in an adapter made of insulating material, and the adapter is already attached to the casing. The result is a compact design that is easy to use.
Setting up a metal detector comes down to selecting the frequency of the first generator. Previously, the engines of the tuning and variable resistors are placed approximately in the middle position and temporarily close the contacts of the SB1 button. By moving the slider of the resistor R1, the lowest tone in the headphones is achieved. If there is no sound, you should choose a capacitor 2. The work will be facilitated if you use an oscilloscope. Its input probe is first connected to pin 11 of the microcircuit and the frequency of the first generator is measured, and then the pin 4 of the microcircuit is touched with the probe and the frequency of the second generator is measured. Comparison of the measurement results will allow you to quickly determine which capacitor C2 (smaller or larger capacity) needs to be installed in the generator.
If interference or malfunction occurs due to mutual influence generators, it is recommended to solder a capacitor with a capacity of 0.01 ... 0.1 microfarads between pins 7 and 14 of the microcircuit.
The technique of working with the device is the same as with the metal detector I. Nechaev.
V. YAVORSKY Kyiv
The same scheme, but on the other printed circuit board and design is described in the article A simple metal detector on a K176LE5 chip books by Adamenko M.V. "Metal detectors" M.2006 (Download the book).
Homemade metal detector on the K176LA7 chip
Many have already written with requests to our site, to post some a simple circuit homemade metal detector. And today, in his free time after passing the exam, the site appears metal detector circuit on 3 microcircuits- K176LA7.
Previously, we reviewed some metal detector schemes on our website.
And now, let's move on to the topic of the article by clicking the button in more detail.
The schema itself:
L1 - wind on a 3-section frame with a tuning core (IF circuit of the Sokol-40 radio receiver) and placed in an armored magnetic circuit with a diameter of 8.8 mm made of 600NN ferrite. The coil contains 200 turns of PEV-2 wire 0.08 ... 0.09 mm.
I used a random aluminum shielded IF coil..
L2 - 18 pieces of wire in reliable insulation are threaded into an aluminum thin-walled tube with a diameter of 6 ... 9 mm and a length of about 950 mm. Then the tube is bent on a mandrel with a diameter of about 15 cm, and the wire segments are connected to each other in series. The inductance of such a coil should be approximately 350 uH. The ends of the tube are left open, but one of them is connected by a common wire. I used a rubber hose with a metal braid inside which I pulled a solid wire in varnish insulation with tweezers. Sponges of the tweezers must be wrapped with electrical tape so as not to damage the insulation. Care must be taken that the winding is as fixed as possible, otherwise the metal detector will falsely work.
The board is placed in a metal, necessarily non-magnetic case.
The wires from the board to the L2 coil must be shielded.
Starting to adjust the metal detector, the capacitor knob is set to the middle position and, by rotating the tuning core L1, zero beats appear in the phones. The setting can be considered correct if, with a small turn of the variable capacitor knob, a low-frequency sound signal appears in the phones. Adjustment should be carried out at a distance of at least a meter from massive metal objects. In my version, it turned out that the sensitivity of the metal detector increased if the core of the tuning coil was screwed into it to the end, and by rotating the variable capacitor it was possible to adjust the absence of beats in two places. At the same time, the sound in the headphones at full volume was quiet. If the sound does not appear at all, then you need to check with an oscilloscope for the presence of a U-shaped signal at pins 4 DD1 and DD2, and a mixture of signals at pins 11 and 8 DD3. In the original, instead of R3 3kOhm, 300kOhm is indicated, but with such resistance, sound did not appear in the headphones. In the absence of 5600pF capacitors, I used 4700pF.
In practice, such a metal detector showed itself well. They can detect a coin at a depth of up to 10 cm, a pan up to 30 cm, manhole up to 60 cm.
The main disadvantage: due to changes in ambient temperature, it is required to adjust the zero beat frequency with a variable capacitor. I would like to see proposals for eliminating this shortcoming in this scheme (preferably with examples).
Note:
1) Recommend pour epoxy into the search coil and let her freeze. This will prevent false positives of the metal detector, since sometimes you have to touch the coil during the search. various items, which causes displacement of the turns inside the coil. Instead of epoxy, melted wax or plasticine can be poured, but then care must be taken that it does not leak out in hot weather. Paraffin should not be poured, because when it hardens, it becomes brittle and not elastic.
2)R3-30kOhm you need to replace it with 300 kOhm and adjust the frequency of the exemplary generator until confident loud clicks appear in the headphones. The lower the click rate, the more sensitive the metal detector. I manage to detect a one-kopeck coin from the times of the USSR at a depth of up to 10 cm, if the coin lies horizontally on the surface.
If you set the click following tone to high, then this allows you to detect objects by changing the tone of the signal.
I don’t know what this is connected with, but having re-assembled another one of the same metal detector, for a long time I could not achieve sound in the headphones. The removal of the capacitor C7 from the circuit helped (replacing it with another or with a smaller capacity did not work). True, the sound volume became a little less, but this made it possible to do without a variable resistor - a volume control. The sensitivity of the metal detector remained at the proper level.
In a radio store you can buy a ready-made plastic case 65x115x45 mm in size inexpensively (31 rubles. PMR), in which you can freely place the circuit of this metal detector. You can shield the circuit like this: cut out a “shirt” from cardboard, wrap it with foil, fix its edges with tape to the cardboard, then attach the conductor with a stapler and connect it to a common wire (minus).
