Consider a simple metal detector based on a K561LA7 microcircuit and a sound amplifier. Power is supplied with a voltage of 9 volts. Since the current consumption is small, the crown battery lasts for a long time. In terms of characteristics, the device has average detection depths, worthy for such a simple circuit. There are similar metal detectors on the K561LA9 microcircuits, but they do not give a significant increase in performance, therefore, we give preference to the assembly of this simplified circuit.
In metal detection, the main role is played by the sensor, which consists of a round coil, housing and a connecting wire to the control circuit (Fig. 1).
The appearance of a metal in the range of the sensor is reflected in the inductance of the coil, which, in turn, affects the frequency of the search circuit on the microcontroller. The final logic element of the microcircuit compares the reference value of the frequency and the frequency of the search circuit and, through the amplifier, outputs the difference in the form of a tonal sound in the speaker.
Sensor manufacturing
Metal detector circuits for different devices are completely different from each other. However, a well-assembled sensor can be used as a universal one for various metal detectors working according to the same principle of operation.
For winding the sensor, we use a varnished wire PEV or PEL with a diameter of 0.5 - 0.7 mm, which can be found without any problems in a store or old CRT TVs and monitors (Fig. 2).
With a coil diameter of 20 cm, we wind 100 turns of wire. For other diameters, we change the number of turns, calculating that at 25 and 15 cm in diameter, 80 and 120 turns are wound, respectively. After completing the winding, wrap it tightly with electrical tape, leaving the beginning and end of the wire with a margin.
We make a Faraday shield to eliminate various interference in the coil and microcontrollers. It is necessary to wrap the coil over the electrical tape with cling foil. At the end of the winding, we do not connect the foil and leave a gap of 2-3 cm. On top of the foil we wrap a little uninsulated wire of small cross-section randomly (Fig. 3).
Wire and foil can be soldered in several places. We wrap all this with electrical tape again.
After the steps taken, we should have an insulated coil with two winding leads and a shield lead. We connect them with a shielded cable from video or audio equipment. We connect the cable shield to the wire from the foil, and the cable cores to the wires from the coil. We solder all this and reliably insulate with electrical tape. At the end of the cable we attach a plug with high-quality contacts. The best option if they are gold plated or silver. The plug can be found in cables for various equipment, and we also take the connector there.
It remains to make the body for the coil. You can use two round discs made of dielectric material - plywood, thick cardboard or plastic. Place the winding between the discs. Then, using plastic fasteners, which can be purchased at a plumbing store, we tightly fasten these two discs. For searching in an aqueous environment, you can seal the sensor with epoxy resin or special sealants.
On the upper disc we fasten or glue ears made of plastic or other dielectric material. They will be needed to attach to the bar (Fig. 4).
Accessories for the circuit
Below are the main details and requirements for them necessary for a high-quality assembly of the circuit:
- It is recommended to buy capacitors from a radio store, but if you want to get them for free from old circuits, then measure the capacity before use. The main requirement for them is temperature stability, this will save you from constant metal detector failures. Ceramic or mica are great. When assembling, do not forget to take into account the polarity of the electrolytic capacitors - one or several stripes are drawn on the barrel towards the minus side (Fig. 5). You will need the following capacitors: electrolytic 100 μF x 16 V - 1 pc .; 1000 pF - 3 pcs.; 22 nF - 2 pcs.; 300 pF - 1 pc.
- Fixed resistors can be used with old ones, as they do not lose their characteristics over time. Variables are best bought new to provide precise frequency tuning on the chips. Particular attention should be paid to the contacts of a variable resistor, since according to the scheme, two contacts must be connected to each other, and experience shows that many beginners do not notice this. It is also necessary to ground their case to eliminate interference during adjustment. You will need 5 fixed resistors of 22 Ohm, 1kOhm, 4.7kOhm, 10kOhm, 470kOhm and 3 variable resistors of 1, 5 and 20kOhm.
- Microcircuit K561LA7 in DIP package. The countdown of the legs on the microcircuits starts from the top counterclockwise from the key - a special recess on the case. As an analogue, you can make a metal detector on the K561LE5 or CD4011 microcircuit.
- The KT315 transistor is very common in old radio equipment. But it can be replaced with many other transistors: KT3102, BC546, 2SC639 and similar low-power low-frequency transistors. We carefully study the terminals of the transistor before soldering, for KT315 they are located from left to right from the front part - emitter, collector, base (Fig. 6):
- We choose any low-power diode from domestic or foreign manufacturers - kd522B, kd105, kd106, in4148, in4001 and others. Before soldering, nickname it with a multimeter, so as not to confuse the anode and cathode in places.
- Standard headphones from a phone or an mp3 player, or a miniature speaker from old technology. In the case of using headphones, you can use the jack or direct soldering.
- Crown battery 9 V and contacts for it (Fig. 7):
- We select the connector for the sensor cable plug in advance, when manufacturing the sensor.
After assembling all the necessary parts, you can safely proceed with their installation according to the scheme described below.
Control circuit installation
The electrical circuit consists of a K561LA7 microcircuit, its strapping for adjustment, an amplifier, power supply and a speaker. The microcircuit has 4 logical elements. Two of them create the desired frequency, the third plays the role of a search part. The final gate compares both frequencies and, at different values, outputs a positive signal to the amplifier, which feeds the amplified signal to the speaker.
The circuit of the metal detector on the microcircuit described above is shown in Figure 8.
It is very convenient to assemble electrical schematic diagrams on a breadboard with holes (Figure 9). Or we make a homemade printed circuit board, shown in Figure 10. You can make a board using the laser-ironing method or ordinary drawing. We carry out the poisoning by any known method.
We solder parts and solder all external parts with wiring - regulators, headphone jack, sensor and batteries.
After assembling the circuit, we fix it in the case. We put the battery there. Plastic, mounting, home-made wooden boxes and other boxes of your choice are suitable as a case (Fig. 11).
For the three regulators and the sensor connector, make the appropriate holes. You can add a switch in series to the battery and also take it out to the case. Provide small holes for the speaker, or, in the case of headphones, secure the connector tightly.
The main condition for assembling the case is accessibility, for example for changing the battery, and, at the same time, tightness - from sudden rain. You can attach pretty caps to the regulators, paint the box, and label the regulators with a switch.
Assembling and configuring the device
When the sensor and control unit are ready, you need to link them into a finished metal detector. This requires a barbell. It can be made from PVC pipes and adapters, which, by heating, can be bent to the desired size and shape. You can also use a regular wooden pole, crutch or telescopic fishing rod. Which material to choose depends on your preference - consider weight, flexibility and length. For convenience, you can build a handle and an armrest, as well as make a collapsible bar (Fig. 12).
Next, we fix the sensor with ready-made ears to the bar. Use plastic fasteners, durable glue, or plumbing adapters. We fix the control unit in the same way.
To make the setting, connect the battery and the sensor. Since metal detectors are sensitive devices, it is necessary to remove all metal objects in the vicinity for correct adjustment. We turn it on and observe one of two options:
If after switching on there is perfect silence or a barely audible squeak, then there are two options:
a) Generators operate at the same frequency. Such cases are rare, but they do happen. Try turning the smooth R7 and coarse R8 controls. If the silence changes to a loud tonal sound, then the circuit works. We return the controls to their initial position and try to achieve the best results with the smooth control R7, for example, a complete absence of sound;
b) Circuit malfunction. We carefully recheck the entire circuit and radio components.
If there is a hum or high tone after switching on, then we try to decrease it by turning the coarse adjustment knob R8, and having achieved the best result, we adjust R7. If the metal detector does not respond to the rotation of the regulators, then the frequency of the reference generator is too different from the frequency of the search circuit. In this case, we try to catch the desired frequency by changing the capacitor C6 and the resistor R6.
The entire setup can be greatly simplified by an oscilloscope. The essence of the setup is to achieve the same or similar frequency of pins 5 and 6 on the microcontroller. Frequency adjustment can be done in the above described ways.
If you mastered the assembly of this device, you can safely try to assemble a more complex metal detector on three microcircuits or a microcontroller.
Searching for treasures, ancient relics and other interesting things for many is quite a popular hobby, along with fishing or hunting. This type of recreation can also be considered active, and for some, a metal detector is quite a good tool for making money, because in the ground you can find a fairly large amount of ferrous metals, which are valued today. After all, there is a proverb that "we walk on money."
In the store, even for a not very powerful metal detector, they sometimes ask for decent money. This article will discuss how you can assemble a metal detector with your own hands. This requires minimal skills in the field of working with electronics and small (in comparison with buying a new metal detector scanty) investment.
Materials and tools for assembly:
- microcircuit K561LA7 or its analog;
- low-power low-frequency transistor (KT315, KT312, KT3102 are suitable, analogs: BC546, BC945, 2SC639, 2SC1815 and so on)
- any low-power diode (for example, kd522B, kd105, kd106 ...);
- three variable resistors (4.7 kOm, 6.8 kOm, 10 kOm with a switch);
- five fixed resistors (22 Ohm, 4.7 kOm, 1.0 kOm, 10 kOm, 470 kOm);]
- five ceramic or mica capacitors (1000 pF - 2 pcs., 22 nF -2 pcs., 300 pF);
- one electrolytic capacitor (100.0 microfarads x 16V);
- wire type PEV or PEL with a diameter of 0.6-0.8 mm;
- headphones from the player (or any low impedance);
- 9V battery.
Metal detector manufacturing process:
Step one. The body and appearance of the device
Due to the fact that searches often take place among branches, grass or in wet weather, the device must be reliably protected from the effects of all these factors. As a housing for electronics, you can use a soap or shoe polish box. The main thing is that the electronic part is reliably protected.
It is important to know that if you do not connect variable resistors (their cases) to the minus of the board, the device will generate interference. If everything is done correctly and a high-quality coil is made, no problems arise during the operation of the device. When you turn on the metal detector, a characteristic squeak should immediately appear in the headphones, it should respond to the frequency control knob. If this is not observed, then it is necessary to choose a 10 kΩ resistor, which is in series with the regulator, or select a 300 pF capacitor in this generator. As a result, you need to align the frequencies of the search and reference oscillators.
You will need an oscilloscope to determine what frequencies the generator is producing. In total, the operating frequency can be in the range of 80-200 kHz. Measurements are made at pins 5 and 6 of the K561LA7 microcontroller.
The system also has a protective diode. It is needed in order to protect the electronics from incorrect battery inclusion.
Step two. Making a search coil
The coils are wound on mandrels with a diameter of about 15-25 cm. As a form, you can use a bucket or shuttle made of wire or plywood. The smaller the coil is, the less sensitivity it will have, it all depends on what purpose the metal detector will be used for.
As for the wire, it can be a wire in varnish insulation such as PEV or PEL with a diameter of 0.5 to 0.7 mm. This wire can be found in older CRT TVs. In total, the coil contains 100 turns, you can wind from 80 to 120. From above, the whole thing is tightly wrapped with electrical tape.
When the coil is wound, a winding of a strip of foil is made over it, while leaving an area of \u200b\u200b2-3 centimeters unwound. Foil can be found in some types of cables, and it can also be obtained from chocolates by cutting into pieces.
It is not insulated wire that is wound over the foil, but best of all is tinned. The beginning of the wire ends up on the coil, and the other end is soldered to the body. From above, all this is again well wrapped with electrical tape.
Subsequently, the coil is attached to a dielectric, as an option, non-foil textolite is suitable. Now the coil can be attached to the holder.
To connect the coil to the circuit, a shielded wire must be used, the shield is connected to the body. Similar wires can be used to re-record music from a tape recorder. You can also use the bass cord to connect various devices to the TV.
Step three. Metal detector check
When the device is turned on, you can hear a characteristic noise in the headphones, the frequency must be adjusted with the regulator. As you bring the coil close to metal, the noise in the headphones will change.
You can also remake the circuit in such a way that the metal detector is silent during operation, and the signal will appear only when metal appears under the coil. In this case, the frequency of the noise will tell you what size the object is and at what depth it is. But, according to the author, this approach greatly reduces the sensitivity of the metal detector, and it picks up only very large objects.
To obtain zero beats, two frequencies must be aligned.
Metal detector on a microcircuit
A similar device has already been described in the article by I. Nechaev under the same title in "Radio", 1987, N9 1, p. 49. In contrast, in the proposed version there is only one inductor and a slightly different design of the circuit, which also made it possible to do without a variable capacitor.
The diagram 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 go through decoupling capacitors C3 and C4 to a detector made on diodes VD1, VD2 according to the voltage doubling circuit. The detector is loaded with BF1 headphones - a difference signal is allocated on 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, it closes the signals of both generators to the 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 found in the search area, for example, an underlayer of soil, snow. A metal detector will be of great help in determining the place of laying fittings and hidden wiring during construction work in the house.
In addition to the one indicated in the diagram, the K176LA7, K176PU1 K176LU2 microcircuit (the last two microcircuits are the so-called level converters), K561LA7, K174LA7 can be used in the metal detector. K561LN2. Trimmer resistor R1 - SP5-2 variable R2 - SPO-0.5. but other small resistors will work as well. Oxide capacitor - K50-12 or other small-sized for a rated voltage of at least 10 V, other capacitors can be, for example, KM 6
The L1 coil is placed in a 200 mm diameter ring made of aluminum or copper tube with an inner diameter of 8 mm. The ends of the tube should be spaced apart from each other at some distance, so that a short-circuited turn does not turn out. For winding the coil, a PELSHO wire (in enamel and silk insulation) with a diameter of 0.5 mm is used, trying to stretch the largest possible number of turns inside the tube. This operation may seem time consuming, 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 then connected to the printed circuit board, and the tube is connected to a common wire.
Headphones BF1 - TA-4 TON-1 or others, with the highest possible resistance Power source - battery "Krona" or another, 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-clad fiberglass. The terminals of the resistors R1 and R2 are connected to the corresponding circuits of the device either by means of a wire or by printed conductors if the installation is carried out on double-sided foil material. The board is placed inside the L-shaped about the casing of the ShR connector (Figure 4) and fixed to one of its halves with a nut screwed from the outside onto the variable resistor R2. To access the adjustment screw of the trim resistor R, a hole is cut in the casing.
The power source is placed inside the handle-case, which can be either plastic or metal (for example, a case from a round flashlight). On top of the handle-case, the SB1 power button is attached, and on the bottom there is an X1 socket for connecting headphones.
The ring with the coil is fixed in the 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 is reduced to the selection of the frequency of the first generator. Preliminarily, the trimmer and variable resistor sliders are set 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 easier if you use an oscilloscope. Its input probe is connected first to pin 11 of the microcircuit and the frequency of the first generator is measured, and then the probe is touched to pin 4 of the microcircuit 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.
In the event of interference or malfunctions in the operation of the device due to the mutual influence of generators, it is recommended to solder a capacitor with a capacity of 0.01 ... 0.1 μF between pins 7 and 14 of the microcircuit.
The method of working with the device is the same as with I. Nechaev's metal detector.
V. YAVORSKY Kiev
The same circuit, but with a different printed circuit board and design, is described in the article Simple metal detector on the K176LE5 microcircuit books by Adamenko M.V. "Metal detectors" М.2006 (Download the book).
This metal detector, despite the small number of parts and ease of manufacture, has a fairly high sensitivity. It can detect large metal objects, such as a heating battery, at a distance of up to 60 cm, while small ones, for example, a coin with a diameter of 25 mm, at a distance of 15 cm.
The principle of operation of the device is based on a change in the frequency in the measuring generator under the influence of nearby metals and the release of a difference frequency (beats) between the measuring and exemplary generator.
Since this frequency is in the audio range, it can be heard in headphones.
Schematic diagram
The schematic diagram of the metal detector is shown in Figure 1. In this diagram, the frequency of the reference oscillator made on DD1.1 is stabilized using a piezoelectric element. As a piezoelectric element, a piezofilter (ZQ1) was used for an intermediate frequency (465 kHz), which is available in any household superheterodyne radio receiver.
Such elements are widespread and much cheaper than quartz resonators. The use of a piezoelectric element makes it possible to increase the frequency stability of the reference oscillator in comparison with conventional LC or RC oscillators, and, therefore, to increase the detection range of metal objects.
Figure: 1. Schematic diagram of a homemade metal detector with a piezofilter.
The measuring generator is assembled on a logical element DD1.2 and contains a coil (L1) in the form of a frame, which is a sensor. When the coil approaches the metal, its inductance changes, which leads to a change in the frequency of the oscillator.
The initial frequency of the oscillator is determined by the elements C1C2C3L1 and is adjusted using an adjustable capacitor C1, close to the frequency of the reference oscillator (slightly more or less than 465 kHz).
On element DD1.3, the signals of the two generators are mixed. The output signal DD1.3 contains a differential harmonic, and in order to separate it from high-frequency pulses, an R3C5 filter is installed. The low-frequency signal is amplified by the VT2 field-effect transistor and fed to the sound emitter - BF1 BF2 headphones.
The use of logic elements of a CMOS microcircuit in autogenerators, due to their large input impedance, makes it possible to obtain a high quality factor in the oscillatory circuit of a search generator, which increases its frequency stability.
This makes it possible to work at low beatings and thus increase the sensitivity of the metal detector.
Details and construction
The power supply of the autogenerators is stabilized using a precision Zener diode KS166V. Only parametric stabilizers for a voltage of about 6 V have a voltage drift close to zero when the ambient temperature changes.
The metal detector circuit remains operational when the voltage drops to 5 V, but in this case there will be no stabilization of the supply voltage.
The current consumed by the metal detector (and, therefore, the duration of operation) strongly depends on the resistance of the headphones connected at the output. For this reason, their resistance should be as high as possible (\u003e 100 Ohm), for which the phones in the headphones are connected in series.
Resistor R7 limits the maximum current of transistor VT2 during a short circuit in the headphones, and resistor R6 allows you to adjust the sound volume. For convenience, this resistor is combined with the SA1 power switch.
Headphones are connected via any standard socket X1.
The X2 socket is intended for connecting the mains charger for the G1 battery. This will allow recharging the batteries without removing them from the case.
The C1 tuning capacitor can be taken from any miniature radio receiver (for example, KP-180). C2 and SZ must be with a minimum negative TKE (M47, M75), C4 and C5 from the K10 (K10-17) series, C6 - K53-1 at 16 V.
Variable resistor R6 - SPZ-ZbM (it provides for horizontal installation on the board and has a built-in switch SA1), adjusted resistor R5, type SPZ-19a, the rest will fit any small.
The piezoresonator (piezofilter Z01) will most likely fit any of the FP1P1-61 series (-01, -02, etc.), and you can also try many other types of piezofilters from Chinese receivers that have three outputs.
Details of the device can be located on a one-sided printed circuit board made of fiberglass with a thickness of 1.5 mm and dimensions of 75 x 40 mm (Fig. 2, a-b).
Figure: 2. Printed circuit board of a self-made sensitive metal detector on a microcircuit.
It is desirable to place the board near the L1 sensor coil. The place where the board with the elements is fixed does not need to be shielded.
The sensor coil of the L1 metal detector has the form of a toroidal frame. It is wound with a PEV copper wire with a diameter of 1.2 mm, on any suitable mandrel with a diameter of 20 cm, for example, cut out of foam.
Figure: 3. The design and parameters of the search coil of a homemade metal detector.
Winding should be done in bulk, 30 turns (the inductance is about 480 μH). After winding the coil, the frame must be wrapped with any dielectric tape (varnished cloth or electrical tape), and then with thin aluminum foil.
Copper foil can also be used. At the point of the coil terminals, there is no need to cover a section of about 10 mm with foil (a gap is left between the ends of the screen, as shown in Fig.).
The use of a thick wire in the manufacture of L1 provides a higher quality factor for the coil and makes the frame more rigid without the use of additional fasteners.
Most metal detectors are built according to a zero beat scheme, this is when there are 2 generators with a constant and a search frequency, while the frequency of the search generator depends on the inductance of the search coil. It is possible to increase the sensitivity of a metal detector built using this method if the reference frequency is raised by about 10 times the frequency of the search generator. Using this method, it was possible to obtain a metal detector capable of detecting a penny coin at a depth of 1 m.
The diagram of such a metal detector is shown in the figure. It uses 2 microcircuits K561LA7, a search generator and an output amplifier are made on D1, a reference generator (with a square resonator) is made on D2.
The search generator uses 2 elements D1.1 D1.2. The generation frequency is set by a circuit consisting of a search coil L1 and capacitors C1 C2 VD1. Varicap VD1 serves to adjust the frequency within a small range during operation. The adjustment itself is made by the resistor R3, which changes the voltage across the varicap.
Generating mode (PIC) is set by elements R2 C4 C5 C6. From the outputs of both generators, the pulses are fed to D2.4 on which the mixer is made, and a frequency beat signal appears at its output. This signal goes to the D1.4 power amplifier and then goes to the sound emitter (headphones or Chinese headphones). Volume control R6.
The electronic part of the circuit is mounted in a foil-clad PCB housing. In this case, the case must have a shielded partition between the microcircuits (between the generators). The search coil is wound on a ring made of cambric with an outer diameter of 15 mm. A plastic tube or a soft plastic rod of this diameter can be used. In any case, the diameter of the ring should be 200mm. 50 turns of PELSHO 0.27 or PEL 0.27-0.35 are wound on the ring. After that, the winding is wrapped with electrical tape and then the coil is shielded with aluminum foil.
The tuning is reduced to tuning the L1C1 circuit at 100 kHz with the middle position of R3 (frequency control at pin 10 of D1.3).
Literature - RK
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