Metal detector based on the principle of an electronic frequency meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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This is a joint development of the author and an engineer from Donetsk (Ukraine) Yuri Kolokolov (the address of his personal page on the Internet is home.skif.net/-yukol/index.htm), who managed to turn the idea into a finished product based on a programmable single-chip microcontroller . He developed the design and software, as well as carried out full-scale tests.

Despite the simplicity of the design of the proposed metal detector based on the principle of a frequency meter, its manufacture at home can be difficult due to the need to enter a special program into the microcontroller. This can be done only with the appropriate experience and firmware to work with the microcontroller.

At present, the Moscow firm "Master Kit" has mastered the production of kits for radio amateurs for self-assembly of the described metal detector. The kit contains the printed circuit board and electronic components, including the already programmed controller. Perhaps, for many lovers of treasure hunting and relics, the purchase of the NM8041 kit (numbered according to the Master Kit catalog) and its subsequent simple assembly will turn out to be a convenient alternative to purchasing an expensive industrial device or making a metal detector completely on your own.

For those who feel self-confident and are ready to try to make and program a microprocessor-based metal detector, Yuri Kolokolov's personal page on the Internet contains the trial version of the controller firmware in Intel Hex format and other useful information. This version of the firmware differs from the full version, which is stored in the microcontrollers of the NM8041 set, in the absence of a dynamic mode and some other features.

The principle of operation of the considered metal detector is based on measuring the generator frequency with an electronic frequency meter, the circuit of which includes a sensor - an inductor. In this case, it is not the frequency value itself that carries useful information, but its increment, which occurs when the sensor approaches the target, and the sign of this increment.

The metal detector has a detection range that is about one and a half times greater than that of the prototype on beats. At the same time, it has selectivity for metals. Low current consumption and a wide range of possible supply voltages allow for a wide range of options for connecting batteries or batteries. The device automatically adjusts to the initial frequency of the measuring generator. In this case, theoretically, the frequency value can be in the range from about 100 Hz to 200 kHz, which also gives great opportunities for choosing the design of the sensor. In terms of the number of parts, the proposed metal detector is no more difficult than a metal detector with beats. This was achieved thanks to the software implementation of most of the functions in a single-chip microcontroller.

Main Specifications

Structural scheme

The block diagram of a metal detector, made on the principle of an electronic frequency meter, is shown in fig. 12.

Rice. 12. Structural diagram of a metal detector based on the principle of a frequency meter

Actually, the considered metal detector consists only of a measuring generator and an electronic frequency meter. The block diagram is, rather, an illustration of the algorithm of its operation.

And the algorithm of the metal detector is as follows. First, the electronic frequency meter measures the frequency of the measuring oscillator when the sensor is away from metal objects and ferromagnets. This value is stored in a storage register. Then, in real time, the frequency meter measures the frequency of the measuring oscillator. The value of the reference frequency is subtracted from the obtained values ​​and the result is fed to the display device.

Schematic diagram

Schematic diagram of the metal detector is shown in fig. 13.

Rice. 13. Schematic diagram of a metal detector based on the principle of an electronic frequency meter

The measuring generator is built on the integral timer A1 type NE555 (domestic analogue - K1006VI1). This chip is used in a somewhat unusual way - as an LC oscillator. The oscillatory circuit of the generator consists of capacitors C1*, C2* and the sensor inductor L. The resonant frequency is determined as for a conventional oscillatory circuit, while the capacitance of the series-connected capacitors C1* and C2* acts as the circuit capacitance. When using a typical sensor with a diameter of 180 ... 190 mm, containing 100 turns of wire and capacitances of capacitors C1 * = 0,047 μF and C2 * = 0,01 μF, the generation frequency is about 20 kHz. If necessary, the generator frequency can be changed by changing the capacitances of capacitors C1* and C2*. In this case, it is desirable that these containers are in a ratio of approximately (4 ... 6): 1.

The A2 microcontroller is responsible for all other functions for processing the signal of the measuring generator up to the indication. This circuit uses an AT90S2313-10PI microcontroller manufactured by ATMEL. This is an 8-bit low-cost RISC single-chip microcontroller. It has a performance of 10 MIPS at 10 MHz. Contains: 2 KB flash, 128 bytes EEPROM, 15 I/O lines, 32 operating registers, two timers/counters, watchdog timer, analog comparator, universal serial port. To solve the problem, the selected microcontroller has sufficiently high technical characteristics at a relatively low price.

Directly connected to the microcontroller chip are both controls and indications. Variable resistor R6 adjusts the sensitivity of the device. LEDs VD1-VD3 indicate the level of deviation of the frequency of the measuring generator in the case of the predominance of the ferromagnetic effect. LEDs VD5...VD7 - in the case of the predominance of the conduction effect. LED VD4 indicates a zero frequency shift. The earpiece or piezo emitter Y is designed for sound indication of the frequency deviation of the signal of the measuring generator. Using switch S1, the device operation mode is set - static or dynamic. In static mode, the signal, which is a digital code of the frequency difference, is logarithmic and is immediately displayed. Each level of light indication is accompanied by its own sound indication tone.

The dynamic mode is designed to search for targets against the background of interference from soil, minerals, etc. In dynamic mode, the signal is subjected to digital filtering, which separates the useful signal from the background of interfering signals. This device uses optimal matched filtering. In short, its essence lies in the fact that for any signal there is an optimal filter that allows you to get the maximum response at its output. Such a digital filter is implemented for the frequency detuning signal that occurs when the search coil moves over small targets at a speed of 0,5 ... 1 m/s. The filter is implemented programmatically in the microcontroller.

Connector X1 is used to connect a computer at the stage of loading the program into the microcontroller.

Part types and design

The design contains a minimum number of parts. However, there are no special requirements for them.

The A1 timer chip (NE555) can be replaced with KR1006VI1. It is desirable to choose LEDs with increased brightness of the glow. Stabilizer A3 (LP2950) can be used type 1184EN1 or, somewhat worse, 78L05. In the latter case, the minimum allowable battery voltage will be 6,7 V.

The A2 microcontroller is soldered directly into the printed circuit board (since the program is entered through the connector, there is no need to remove it from the board even if it changes), but if desired, the microcontroller can also be installed in the socket. The AT90S2313-10PI chip can be replaced by the AT90S2313-10PC, however, in this case, the manufacturer does not guarantee operation at temperatures below 0 °C (which may well be in the field).

Resistors can be used in a wide variety of types, for a power dissipation of 0,063 ... 0,25 W. Capacitors C1 * and C2 * - it is desirable to use thermally stable ones, especially C2 *. Electrolytic capacitor C4 - any type. The remaining capacitors are ceramic, type K10-17. Quartz resonator types RG-05, RK169, or other small-sized. The sensor is a shielded coil. The design can be taken from this book.

Software

Most of the device functions are assigned to the program executed by the microcontroller and recorded (programmed) in its non-volatile memory. At the time of writing this material, the following device operation algorithm was implemented.

1. After starting the program, by pressing the SO button, the microcontroller roughly measures the frequency of the measuring oscillator for a fixed time interval (about several tens of milliseconds).

2. Then one internal timer of the microcontroller is adjusted so that the division of the input frequency results in a measured interval Ti slightly less than the above fixed interval.

3. Next, a control measurement of the measured interval Ti is performed using the second timer, to which counting pulses are fed with a clock frequency of several megahertz.

4. The measured value of the time interval Ti is stored and subsequently used as a reference Te.

5. The measurement of the Ti interval is repeated in the cycle.

6. The intervals Ti and Te are compared by subtracting one from the other.

7. The result obtained is processed for its convenient perception with the help of light and sound indication.

The software for this device has been created and debugged for more than two years and continues to be constantly improved, as well as the printed circuit board. Perhaps, at the time of your reading this text, the proposed design and software have already undergone significant changes. For the latest information, we recommend that you refer to Yuri Kolokolov's personal page on the Internet, home.skif.net/-yukol/index.htm, which contains information about new functionality.

Working with the device

When switch S1 is closed, the device goes into static mode. In this mode, when the coil approaches the ferromagnetic target, the LEDs VD3, VD2, VD1 start to light up sequentially. If the coil is brought closer to a non-ferromagnetic metal object, the VD5, VD6, VD7 LEDs will light up.

Unfortunately, the device reacts in the same way to iron objects with a large surface area (for example, a tin can). This is due to the fact that when the search coil is exposed to metal ferromagnetic objects, two effects appear at once - the conduction effect and the ferromagnetic effect. At a certain ratio of the object's surface area to volume, the conduction effect begins to dominate.

The device switches to dynamic mode when switch S1 is opened. In this mode, the metal detector has the highest possible sensitivity, but reacts to objects only when the sensor moves - the coil must move above the ground at a speed of approximately 0,5 ... 1 m / s. The location of the object in dynamic mode is found by the method of "artillery fork" when passing the coil over the object twice - from left to right and from right to left. In this mode, it is important to feel the lowest speed at which you can move the coil. It is easily mastered with a short training session. The display in dynamic mode looks a little different. When the coil moves over a ferromagnetic object, the LEDs from the "scale" VD5, VD6, VD7 first light up, and then from the "scale" VD3, VD2, VD1. When moving the coil over a non-ferromagnetic object, the indication works in reverse.

As mentioned above, each LED has its own tone of sound indication. After a short work with a metal detector, the "tunes" characteristic of different types of targets are remembered. This allows you to use predominantly sound indication when searching, which is quite convenient.

Before starting work in both modes, it is necessary to set the optimal sensitivity of the device using a variable resistor R6. It is set to a position when the device starts to indicate false responses. Then slowly rotating the rotor of this resistor, it is necessary to achieve the disappearance of false positives.

Author: Shchedrin A.I.

See other articles Section metal detectors.

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