Do-it-yourself pirate metal detector - detailed instructions. Homemade metal detector on a chip Sound generator on K561la7 for MD pirate

Simple metal detector circuit

Spring is beginning now. Many radio amateurs and ordinary users like to get carried away with searching for metals on the street or in the gardens, be it coins or cartridges from the Second World War. In the article I propose a simple metal detector diagram, which even a beginner can assemble in two evenings, since the scheme is suitable even beginner.

The metal detector is assembled on a K175LE5 microcircuit.

Works on the beats frequencies and basically contains two generators. One generator is assembled on elements DD1.1, DD1.2 and the second - on elements DD1.3. DD1.4.

The frequency of the first tunable oscillator depends on the capacitance of capacitor C1 and the total resistance of resistors R1 and R2. A variable resistor smoothly changes the frequency of the generator in the frequency range set by the trimming resistor. The frequency of another generator depends on the parameters of the search oscillatory circuit L1 C2. Signals from the generators are supplied to a detector made according to a voltage doubling circuit on diodes VD1 and VD2.

The detector load is headphones. They produce a difference signal in the form of sound. Capacitor C5 shunts the headphones at high frequency.

When the search coil approaches to a metal object, the generator frequency changes to DD1.3, DD1.4. This changes the tone of the sound. This change in tone is used to determine whether an iron object is in the search area. In the metal detector circuit, the K176LE5 microcircuit can be replaced with K176LA7, K561LA7, K564LA7 microcircuits. The price of such a microcircuit on the radio market is only 0.2 dollars. Trimmer resistor R1 type SP5-2, variable R2 - SPO-0.5. The search coil is wound with PELSHO wire 0.5-0.8.

For food metal detector circuits battery type used "Krona" at 9 volts or other similar source. Tests have shown fairly good performance of the device, so for beginners in radio electronics this circuit can be safely recommended for repetition.

Author of the article: Shimko S.

After reading a little on amateur radio forums production of metal detectors, found that most people collecting metal detectors, in my opinion, are unfairly written off beat 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 metal selectivity and requires adjustment during operation. However, it is also possible to perform a successful search under certain circumstances. As an option - beach search- ideal option for a metal detector on beats.

Place to search with a metal detector.

You need to go with a metal detector where people lose something. I'm lucky to have a place like this. Not far from my house there is an abandoned river sand quarry, where people constantly relax in the summer, drinking and swimming in the river. It’s clear that they are constantly losing something. In my opinion, the best place for searching with a metal detectorBFO I can't think of it. Lost items are instantly buried at a shallow depth in dry sand and it is almost impossible to find them manually. Some kind of mysticism. I remember when I was a child I dropped my apartment keys in the sand there. Here I am standing, the keys fell here, but no matter how much I dug up that area, it was all to no avail. They literally fell through the ground. Just an enchanted 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 on my last trip with a metal detector, I found a woman’s thin gold ring. It was almost at the surface, slightly sprinkled with sand. Perhaps it was just luck. Actually, it was for this beach that I made my metal detector.

Advantages of a beat metal detector.

Why exactly BFO? - First of all, this is the most simple metal detector option. Secondly, it has at least some signal dynamics depending on the properties of the object. Not really pulse metal detector- “beeping” for everything the same. I in no way 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 corks and foil. Many will say that a beating metal detector does not distinguish the properties of an object, howls and buzzes at everything the same. However, it is not. After practicing on the beach for a couple of days, I became quite good at identifying foil as a sharp and profound change in frequency. Beer bottle caps cause a strictly defined frequency change that needs to be remembered. But the coins emit a weak, “point” signal - a subtle change in frequency. All this comes with experience, patience and good hearing. Beat metal detector- it’s still "auditory" metal detector. The analyzer and signal processor here is a person. For this reason, you must search on headphones, and not on the speaker. Moreover, the best option is large headphones, not earplugs.

Metal detector design.

Structurally I decided to make a metal detector foldable and compact. So that it fits into a regular bag, so as not to attract the attention of “normal” people. Otherwise, when you get to the search site, you look like an “alien” or a scrap metal collector. For this purpose, I bought the smallest (two-meter five-legged) telescopic rod in the store. Left three knees. The result was a fairly compact folding base, on which I assembled my metal detector.

The entire electronic unit was assembled in the 60x40 plastic wiring box I already loved. The end cap, the power compartment partition and the power compartment cover were also made from its plastic. The parts were glued together with superglue and mounted on M3 bolts. Fastening metal detector electronic unit to the rod is made in the form of a metal bracket, which is inserted into place of the fishing reel with fishing line and secured with the standard nut of the rod. The result is an excellent lightweight and durable design. On the outside of the unit there is a power button, a coil connection socket (a five-pin socket from a “grandfather’s” tape recorder), a frequency regulator and a headphone jack.

Metal detector circuit board was made on site by laying out the paths with a waterproof marker. For this reason, unfortunately, I cannot provide a seal. Surface mounting - 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 field conditions this is very important. For example, I lost one day because some debris got inside under the microcircuit. The metal detector just stopped working. And I had to return home, disassemble it, blow it out and open the board with varnish.

Diagram of a beat metal detector.

The circuit itself (see below) was redesigned and optimized by me from two metal detector circuits. This is "" - Radio magazine, 1987, No. 01, pp. 4, 49 and " High sensitivity metal detector" - Radio magazine, 1994, No. 10, page 26.

The result is a simple and functional circuit that provides stable low-frequency resulting beats - what is needed to determine by ear the slightest changes in frequency.

The stability and sensitivity of the metal detector are ensured by the following circuit solutions:

The reference and measuring generators are separated- made in separate microcircuit packages - DD1 and DD2. At first glance, this is wasteful - only one logical element of the microcircuit package is used out of four. That is, yes, the reference generator is assembled on only one logical element of the microcircuit. The remaining three logical elements of the microcircuit are not used at all. The measuring generator is built in exactly the same way. It would seem that it makes no sense not to use the free logical elements of the microcircuit package. However, this is precisely what makes a lot of sense. And it consists in the fact that if, for example, you assemble two generators in one microcircuit package, they will synchronize each other at close frequencies. It will not be possible to obtain the slightest changes in the resulting frequency. In practice, this will look like a sharp change in frequency only when a massive metal object is close to the measuring coil. In other words, sensitivity sharply decreases. Metal detector does not react to small objects. The resulting frequency seems to “stick” to 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 chip" - Radio magazine, 1987, No. 01, pp. 4, 49 is built on just one microcircuit at all. This effect of frequency synchronization is very noticeable there. It is completely impossible for him to search for coins and small objects.

Also, both generators must be shielded with separate small screens made of tin. This increases by an order of magnitude stability and sensitivity of the metal detector as a whole. It is enough to simply solder small partitions made of tin at minus between the generator chips to ensure that the parameters of the metal detector are improved. The better the screen, the better the sensitivity (the influence of the generators on each other is weakened and plus protection from external influences on the frequency).

Electronic tuning.

Comparator on DD3.2 – DD3.4.

This circuit element converts the sinusoidal signal from the output of the DD3.1 mixer into rectangular pulses of double frequency.

Firstly, rectangular pulses are clearly audible at hertz frequencies as clear clicks. While a sinusoidal signal of hertz frequencies is already difficult to distinguish by ear.

Secondly, doubling the frequency allows the adjustment to come closer to zero beats. As a result, by adjusting you can achieve a “clicking” sound in the headphones, the change in frequency of which can already be detected when you bring a small coin to the coil at a distance of 30 cm.

Generator power stabilizer.

Naturally, in this circuit, the supply voltage noticeably affects the frequency of generators DD1.1 and DD2.1 metal detector. Moreover, each of the generators is affected differently. As a result, with the battery draining a little The beat frequency of the metal detector also “floats”. 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 stopped “floating”. However, it should be said that on the other hand, due to the five-volt power supply of the generators, several The sensitivity of the metal detector has decreased generally. Therefore, this option should be considered optional and, if desired, generators DD1.1 and DD2.1 can be powered from the crown without a DA1 stabilizer. You just have to adjust the frequency manually more often using a regulator.

Metal detector coil design.

(See diagram below).

Since this 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 a regular metal bolt for the hinge to make it. Subsequently, when setting up the circuit, all the influences of the metal in the structure will be brought to zero by the tuning core of the L1 coil. The L2 coil itself contains 32 turns of PEV or PEL wire with a diameter of 0.2 - 0.3 mm. The diameter of the coil should be about 200 mm. It is convenient to wind on a small plastic conical bucket. The resulting turns are completely wrapped with electrical tape and tied with thread. Next, 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 output of the foil screen of the coil. Once again everything is wrapped together with electrical tape. The coil itself is ready.

The frame on which the reel will be located and with which it will be attached to the fishing rod is made of springy steel (not soft) wire 3-4 mm. It actually consists of three parts (see figure) - two twisted wire loops of the hinge, which will be connected by a bolt to each other and a wire ring threaded into the tube from the dropper (the ring should not be a closed turn).

This entire structure, together with the finished wire spool, is also tied together with threads and electrical tape.

The joint itself with the reel is attached to the rod by tying it with nylon threads and gluing it with epoxy resin.

It is advisable not to wet the coil during the search process, and especially not to use it for underwater searching. It is not airtight. Moisture that gets inside can destroy it over time.

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.

(Beat Frequency Oscillation)

The metal detector diagram from the book “Encyclopedia of Radio Amateurs” (second edition, supplemented, V.M. Pestrikov, Science and Technology, 2001, p. 204, ISBN 5-94387-039-3) was taken as a basis. Initially, the article was published in the magazine "Radio" 1989, No. 8, pp. 65, 66. Author V. Yavorsky, Kiev.

The original circuit does not have a supply voltage stabilizer. This fact affects the frequency drift of the generator, and without a stabilizer on a battery set to 7.5V, the frequency has to be adjusted every 10 seconds! One of the important disadvantages is the use of high-impedance headphones (TON-1, TON-2 or TA-1), which are difficult to obtain. The operating frequency is too high to search for various metals. Also, the generation of the optimal frequency for searching for metals (7.5 kHz) of the tunable generator does not itself begin when the power is connected. And the most important drawback of the scheme is the very principle of its operation, which does not allow achieving parameters sufficient for practical use in nature. In the circuit, the reference oscillator is adjusted to the frequency of the tunable one. The proposed coil manufacturing technology is too complex. The performance of the rectifier as a voltage doubler is questionable.

In the proposed circuit, a voltage stabilizer is added, the operating frequency is changed to 7.5 kHz, the output stage is replaced with a single-transistor one with a headphone connection. The main advantages of the circuit are simplicity and low current consumption - 3mA without headphones, and 7.5mA with headphones. All experiments were carried out by measuring the triggering distance to a specific object in centimeters using a ruler.
Reworked final circuit of the metal detector:

Having looked at the signal on the tunable generator with an oscilloscope, I discovered that the signal has high-frequency oscillations at the moments of switching elements:

To smooth them out, I placed a 3.3nF capacitor between pins 3,5,6 of the microcircuit and ground. At the same time, the noise level noticeably decreased, and the tunable generator began to start itself when the power was turned on.

In practice, no difference was noticed between the use of the signal mixer shown in the circuit using capacitors and the mixer using field-effect transistors.
The use of low-pass filters did not give a noticeable effect with a noticeable attenuation of the signal.

The main ways to improve the performance of metal detectors on beats are to reduce interference (eliminating sources of interference and filtering), stabilizing the frequency of the reference oscillator, and using a flat coil. It also makes sense to increase the voltage on the coil and use several operating frequencies.

Coil diameter 180mm, 105 turns. The coil was used without a screen. The screen can be made of foil if desired. The coil is wrapped with electrical tape, then with foil, then with electrical tape again. The maximum sensitivity of this circuit with the described coil is a thick aluminum plate 4x4 cm at a distance of 11 cm.
Instead of the K176LE5 microcircuit, you can try using K176LA7, K176PU1, K176PU2, K561LA7, K564LA7, K561LN2.
A variable resistor can be used combined with a switch. I used KT315 transistors.

Viewing the reaction of generators to metal with an oscilloscope gave an important result. With detuned generators, when an aluminum plate was brought to the coil, an increase in the frequency of the tunable generator was observed, while the frequency of the reference did not change. This was observed both with and without a condenser mixer. However, for generators tuned to the same frequency, when an aluminum plate was brought in, an increase in the frequency of both generators was observed, both with and without a capacitor mixer. This observation confirms the presence of a positive connection between the generators, and confirms that the capacitor mixer is not involved in this.

Radio constructor: A simple metal detector based on the K561LA7 chip. (021)

This metal detector circuit showed the best results among all the simple circuits. Using this device, you can detect both ferrous metals (reinforcement in the walls of premises) and metal objects in the ground (both ferrous and non-ferrous). 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 an imported analog CD4011). From the diagram it can be seen that a model generator is assembled on elements DD1.3 and DD1.4, with the frequency of which the frequency of the search generator assembled on elements DD1.1 and DD1.2 will be compared. Let's consider how the elements of the circuit work: The frequency of the model generator is determined by the parameters of capacitor C1 and the total resistance of 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 (located within 100 KHz), that is, it depends on the capacitance of the capacitor and the inductance of the coil and is constant (conditionally, since frequency stability depends largely on changes in temperature, supply voltage, humidity). When the search generator operates, not only the fundamental frequency of 100 KHz is generated, but also its multiple harmonics of 200 KHz, 300 KHz, 400 KHz, and so on. The higher the harmonic, the lower its level. When the standard oscillator (OG) operates at a frequency of 300 KHz, the “needed” harmonic of the search oscillator (PG) is the third, that is, also 300 KHz. If we set the exhaust gas frequency to 305 KHz with resistors R2 and R3, and the exhaust gas frequency is equal to 100 kHz, then the third harmonic of the exhaust gas generator, equal to 300 kHz (frequencies above 20 kHz can no longer be determined by ear), from the output of capacitor C4 is mixed with the exhaust gas frequency at the output of capacitor C3. Next, these frequencies are supplied to the diode mixer VD1, VD2, assembled according to a voltage doubling circuit (in one half-cycle, the signals from the outputs of the generators pass through the diode VD1 and charge 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 are supplied through the diode VD2 to the headphones T. The diode mixer, acting as a detector, selects a difference frequency of 305 KHz - 300 KHz = 5 KHz, which is heard in the headphones in the form of a tone signal. Why was this ratio of generator frequencies 300 KHz to 100 KHz chosen? This is the most optimal ratio. Higher harmonics are significantly inferior in signal strength and can no longer be heard in headphones, and lower harmonics do not make such a difference in the frequency change - when a metal object enters the receiving coil area, its inductance changes slightly, which affects the frequency of the PG. For example, the frequency became not 100.000Hz, but 100.003Hz. A difference of 3 hertz is hardly perceptible by ear, but at the third harmonic 100.003 Hz will be equal to 300.009 Hz, and the difference with the exhaust gas frequency will be equal to 9 Hz, which is more noticeable by ear and increases the sensitivity of the device. Diodes VD1, VD2 can be anything, but must be germanium. C6 is used to shunt high-frequency signal components at the mixer output. The headphone headphones must be connected in series (the photo shows the outputs of telephone jacks for serial connection of standard stereo headphones). All these rules allow us to use the output signal most efficiently, 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 setting 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 (SC) consists of 50 turns of copper wire such as 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, boards, plywood, etc., hammer nails around the circle, then wind a coil around the nails, tie it tightly in a circle with threads, then pull out the nails. You can wind the coil on any round plastic structure of appropriate diameter (for example, a piece of plastic sewer pipe, the bottom part of a plastic bucket, which are thrown away by stores after selling herring and pickles. The excess part is cut off. It is advisable to soak the coil wound in this way with varnish or paint (not nitro! The solvent will damage the varnish insulation of the coil wire) to fill the cavities between the turns, into which water can subsequently get in. After drying, the coil must be tightly wrapped with electrical tape over the entire surface. To improve the protective properties of the 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 sawed along the outside with a hacksaw or a grinder with a thin disk, or it’s easier to take aluminum foil for baking, cut it into strips and wrap these strips around the coil from the initial to the final taps, leaving a gap of about 1 - 2 cm unwound. Otherwise, you will get a short-circuited turn that will not allow the coil to work. Considering that not everyone has the opportunity to solder the “ground” wire to an aluminum screen, you can strip 3 - 8 cm of insulation from the wire by wrapping the bare end around the aluminum screen and wrapping it tightly with electrical tape. It is advisable to also shield the insulated connecting wires from the coil to the board with aluminum foil, connecting it to the same ground wire using the same method as in the coil. You can begin setting up the device after winding the PC before it is impregnated and shielded. Everything else is an improvement of the device. If everything is assembled correctly, then after connecting the PC to the circuit and supplying power (observe the polarity of connecting the power source and the correct installation of the microcircuit in the socket), the beats of the generator frequencies will be heard in the headphones when the variable resistor R2 is rotated “Rough”. In the absence of special instruments (oscilloscope, frequency meter), the operation of the generators can be determined with any voltmeter connected instead of headphones. Having unsoldered capacitor C4 from the diode mixer, the voltmeter will show the operation 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 PG based on similar voltmeter readings. Both work by listening to the beat tone in headphones. Resistor R2 allows you to tune the exhaust gas frequency over a wide range, which manifests itself in repeatedly appearing beats in the headphones. Now you need to carefully check these beats, select the most “powerful” ones (resistor R3 should be in the middle position). When checking each of the harmonics, resistor R2 must be set in such a position that the “ringing” tone of the signal goes to a lower tone. Further adjustment must be made with resistor R3 “Precisely” and ensure that the beat tone turns into wheezing and clicks. This position is the working position with maximum sensitivity. Next, we take an object made of ferrous metal and bring it to the coil - the tone of the signal should increase. When you bring an object made of non-ferrous metal (aluminum, copper, brass) to the coil, the tone of the signal should, on the contrary, decrease or break completely. If this does not happen or happens the other way around, it is necessary to rebuild the exhaust gas to another harmonic and do it all over again. Once you have found the “right” 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 area of ​​the beats. The more precisely you tune in to it, the higher the search results will be. Once 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 the common (negative) wire, otherwise the approach of the hand to the handle will affect the beat frequency. It is advisable, to reduce the influence of external factors, to place the device circuit in a metal case connected to a common

L1 - wound 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 IF coil with an aluminum shield.
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 approximately 15 cm, and the pieces of wire are connected to each other in series. The inductance of such a coil should be approximately 350 μH. The ends of the tube are left open, but one of them is connected with a common wire. I used a rubber hose with a metal braid inside which I pulled through a solid wire in varnish insulation using tweezers. The tweezer jaws should be wrapped with electrical tape to avoid damaging the insulation. You need to make sure that the winding is as fixed as possible, otherwise the metal detector will trigger falsely.
The board is placed in a metal, necessarily non-magnetic case.
The wires from the board to the L2 coil must be shielded.
When starting to set up the metal detector, set the capacitor handle to the middle position and, by rotating the tuning core L1, achieve zero beats in the phones. The setting can be considered correct if, when turning the variable capacitor knob slightly, a low-frequency sound signal appears in the phones. The adjustment should be made 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 completely, 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 use an oscilloscope to check the presence of a U-shaped signal at pins 4 of DD1 and DD2, and a mixture of signals at pins 11 and 8 of DD3. In the original, instead of R3 3kOhm, 300kOhm is indicated, but with such resistance the sound did not appear in the headphones. Due to lack of availability, instead of 5600pF capacitors, I used 4700pF.
In practice, this metal detector has proven itself well. They can detect a coin at a depth of up to 10 cm, a pan up to 30 cm, and a sewer hatch up to 60 cm.
The main disadvantage: due to changes in ambient temperature, it is necessary 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).

Fig. 1 Metal detector diagram