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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A simple metal detector with high sensitivity. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors Metal detector features The presented metal detector is relatively simple to manufacture, does not contain scarce elements, but at the same time has a fairly high sensitivity. With it, you can detect a coin buried in the ground to a depth of 15-20 cm. Operating principle The search for metal objects in the ground is based mainly on two physical phenomena. One of them is the effect of the magnetic properties of an object on the inductance of a coil, or on the coupling coefficient between two coils. These values may increase or decrease depending on the relative magnetic permeability of the material. All known substances are divided into three groups according to their magnetic properties:
The differences in the permeability of diamagnets and paramagnets are very insignificant. But ferromagnetic materials stand out very strongly on this basis. The second phenomenon is eddy currents that occur in electrically conductive objects in an alternating magnetic field. The intensity of the currents depends both on the size and shape of the object, and on the specific electrical resistance of its material. In a flat metal plate, eddy currents are much stronger than in an object of complex shape with an uneven surface. The position of an object in a magnetic field (the number of lines of force penetrating it) also matters. If we also take into account the dependence of the effect on distance and the influence of the ground, it becomes obvious how difficult the task is. The described simple metal detector works according to the TR / IB principle. This abbreviation stands for Transmit-Receive / Induction Balance - transmission, reception and induction balance. In this case, two coils are installed in the sensor, transmitting and receiving. The presence of metal changes the inductive coupling between them, which affects the received signal. Its sensor consists of two coils. The device contains only two transistors and one microcircuit. Schematic diagram Shown in fig. 2.20 the scheme is quite simple. The generator on the transistor VT1 operates in intermittent oscillation mode. It generates two frequencies at the same time - high and low, and high-frequency oscillations are additionally modulated by low-frequency ones. With the start of generation, the capacitor C2 is charged through the diode VD1. Upon reaching a certain voltage at (2), the high-frequency oscillations break down, and the capacitor is discharged through the resistor R1. After a while, the oscillations reappear, and the cycle repeats.
Transmitting coils L1-L1 are connected between the collector and the base of the transistor VT3, which are designed in such a way that the capacitive effects of surrounding objects on them cancel each other out. Capacitor C5, located in close proximity to the coils, determines the generated frequency. Coils L4 and L5 are receivers, they are placed next to the transmitters. The zones covered by the turns of the receiving and transmitting coils partially overlap. In the absence of metal objects, the signal induced in the receiving coils is compensated by the signal coming directly from the generator through the variable capacitor Sat. The metal that appeared nearby upsets the balance. The signal is fed to the inverting input of the comparator DA1, which compares it with a constant voltage. The latter is set by variable resistors R5 (GROUGH) and R6 (FINE). Diode VD2 is required so that only positive voltage is applied to the input of the comparator. If the signal is above the set threshold, a voltage appears at the output of the comparator, opening the transistor VT2. Since the peaks of the signal flashes pass through the comparator, a sound is heard in the acoustic head BA1 (power - 0,1 W, resistance - at least 8 ohms). Changing the coupling between the transmitting and receiving coils affects the amplitude of the received signal, which, in turn, leads to a change in the pulse width at the output of the comparator. By ear, this is perceived as a change in the volume and timbre of the sound. The negative voltage rectified by the VD3 diode, proportional to the signal level at the collector of the transistor VT2, is returned to the input of the comparator. This achieves an automatic threshold adjustment that compensates for slow changes in the signal induced in the receiving coils. At the output of the device there is a microammeter RA1 with a total deflection drain of 100-250 μA. Therefore, it is possible to judge the presence of metal objects near the coils by the vibrations of its arrow. When the SB1 button is pressed, using a microammeter, you can check the battery voltage. Construction of the metal detector Good operation of the metal detector can be achieved only with the correct manufacture of the sensor, the drawing of which is shown in Fig. 2.21. Coil frames are two plates of organic glass or other durable dielectric material. Wood in this case is not suitable because of its hygroscopicity. Along the perimeter of each plate in its edge with a cutter or file, you need to make a groove for laying the windings.
All coils must be wound with copper wire with a diameter of 0,3 mm with enamel insulation. The beginning of the wire should be fixed with a drop of glue at point A of the frame, wind 22 turns of the coil L2 clockwise. The end of the winding also needs to be glued to the plate at point A. Without cutting the wire, it is advisable to clean its small section of insulation and solder the beginning of another piece of wire to it, with which to wind four turns counterclockwise - coil L1. After that, with the wire that the L2 coil was wound, you need to make another 22 turns clockwise - the L3 coil. The conclusions of all windings should be firmly glued to the frame plate at point A. Coils L4 and L5 are proposed to be placed on the second plate. Each of them starts and ends at point B and contains 36 turns wound in the same direction. Capacitors C5 and C7 must be soldered directly to the coil terminals in accordance with the diagram and glued to the plates. The winding plates should be stacked on top of each other and fastened together with bolts of insulating material (eg nylon) as shown in fig. 2.21. An arcuate cut in one of the plates allows you to fix the plates in the optimal position when setting up the device. It is best to fasten the entire assembly to the end of a wooden or plastic rod 1-1,5 m long. Under no circumstances should metal screws, screws or nuts be used to secure the assembly. At the opposite (upper) end of the rod, you need to install a case (can be metal) with a printed circuit board of the device. The handles of the controls (SA1, SB1, C6, R5 and R6) should preferably be brought to the front panel of the case. Coils L1 - L5 must be connected to the metal detector board with shielded wires. Setting up the metal detector Before setting up the metal detector, both plates with coils should be turned at the maximum angle without fixing them. Do not install jumper wires A and B yet. After turning on the power, at a certain position of the variable resistors R5 and R6, a sound should be heard in the BA1 head. Slowly move the coils (there should be no metal near them) until the volume of the sound begins to decrease. By rotating the axes of variable resistors and moving the coils, it is necessary to achieve a minimum volume. Sometimes this takes several tries. Having found the minimum, move the coils a little more (less than 1 mm) and fix them. Next, the metal detector can be turned off and wire jumper A installed. After turning on the power, you need to try to find the position of the rotor of the capacitor C6, in which no sound is heard. If this fails, jumper A will have to be removed, jumper B installed, and try again. If that doesn't help, the position of the coils has probably been found wrong. The last opportunity to tune the metal detector is to connect a 470 pF capacitor in parallel with C6 and try again to find the balance point. In case of failure, you will have to make other coils, strictly following all the recommendations. When starting to adjust the supply voltage control unit, set the latter to 9 V. Using R5 and R6, ensure that no sound is heard from the BA1 head, and the arrow of the PA1 microammeter does not deviate. By pressing the SB1 button, use R14 to set the arrow to the last division of the scale. By reducing the supply voltage to 7 V, mark the position of the arrow corresponding to the minimum allowable voltage. It remains to adjust the sensitivity of the device with a variable resistor R12 at your own discretion. Sometimes the sound heard in the BA1 head is accompanied by a hum with a frequency of 100-150 Hz. To get rid of the hum, you should connect a tuned 1 kΩ resistor in series with R50 and select the position of its slider. Beginning of work Before you start working with a metal detector, you need to make sure that at the slightest deviation of the rotor of the capacitor C6 from the balance point in the BA1 head, a sound is heard. Having gained some experience, it is possible, by setting C6 in various positions, even to distinguish diamagnetic materials from paramagnetic ones. References:
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