Select your language

Novice Amateur radio operators can be recommended to repeat the simple design of the metal detector, the basis for which was the scheme, repeatedly published in the late 70-ies of the last century in various domestic and foreign specialized editions. This metal detector made for only on a single chip type K155LA3, you can collect for a few minutes.

Schematic diagram

The proposed design represents one of many options of detectors type BFO (Beat Frequency Oscillator), that is, device, based on the principle analysis of the beating of the two signals, close in frequency (Fig. 3.1). Thus in this design the change the frequency of the beats is by listening.

Fig. 3.1. Schematic diagram of the detector on the chip K155LA3

The instrument consists of a measuring and reference oscillators, the oscillation detector RF readout scheme, as well as the voltage stabilizer.

In this design used two simple LC oscillator, made on the chip IC1. Circuit solutions of these generators are almost identical. The first generator, which is the reference, assembles the elements of the IC1.1 and IC1.2, and the second, measuring or tunable the generator performed on the elements of the IC1.3 and IC1.4.

The reference oscillator circuit is formed by capacitor C1, the capacitance is 200 pF and the coil L1. In the circuit of the measuring oscillator is used, the variable capacitor capacitor C2 with a maximum capacitance of about 300 pF, and the search coil, in theory ka L2. Both generator tuned to the operating frequency of about 465 kHz. The outputs of the generators through the decoupling capacitors C3 and C4 connected to the oscillation detector RF performed on the diodes D1 and D2 according to the scheme of doubling the rectified voltage. Load detector head phones are BF1, which is allocated the signal of the low-frequency component. When the capacitor C5 bypasses the load at higher frequencies.

When approaching the search coil L2 tunable resonant circuit generator a metal object, its inductance is changed, which causes changing the operating frequency of this generator. In this case, if near the coil L2 is the subject of black metal (ferromagnetic), its inductance increases, which leads to reduction of the frequency tunable oscillator. Colored metal reduces the inductance of the coil L2, and the operating frequency of the generator increases. The radio frequency signal generated by mixing the signals of measuring and the reference generator after passing through the capacitors C3 and C4, served on detector. The amplitude of the RF signal varies with the beat frequency.

Low-frequency envelope of the high frequency detector signal is performed on the diodes D1 and D2. Capacitor C5 filters the high frequency component signal. Further, the beat signal is fed to the headphones BF1. Power to the chip IC1 is supplied from the source B1 voltage through 9 In a voltage regulator formed by the Zener diode D3, the ballast resistor R3 and the control transistor T1.

Details and design

For the manufacture of the considered detector, you can use any the breadboard. So used to detail without facing any limitations associated with dimensions. Installation can be either hinged, they print.

The repetition of the detector it is possible to use chip K155LA3 comprising of the four logical elements 2I-NOT fed from a common source DC. As the capacitor C2 is possible to use a capacitor settings from a portable radio. Diodes D1 and D2 can be replaced by any high-frequency germanium diodes.

Coil L1 of the contour of the reference oscillator should have an inductance of about 500 mcg. As this coil is recommended, for example, the coil filter FC superheterodyne receiver.

Sensing coil L2 contains 30 turns of the wire enamel insulation diameter of 0.4 mm and made in the form of a torus with a diameter of 200 mm. This reel is easier to produce hard the frame, however, you can do without it. In this case, as a temporary frame can be used any suitable size round object, such as a Bank. The turns of the coil wound up in bulk, and then removed from the frame and screened electrostatic screen, which is an open-loop tape from aluminium foil is wound on top of the harness turns. The gap between the beginning and the end winding of the tape (the gap between the ends of the screen) should be at least 15 mm.

In the manufacture of the coil L2, it is necessary to ensure that did not happen ~ the closure ends of the shielding tape, as in this case, a closed loop is formed. In order to improve the mechanical strength of the coil can to impregnate with epoxy glue. For the source of sound signals should be used high impedance headphones with possibly a high resistance (about 2000 Ohms). Suitable, for example, is widely known phone TA-4 or TONE-2.

As the power source B1 can be used, for example, the battery "Krona" or two batteries type 3336L connected in series.

In the voltage regulator capacitance of the electrolytic capacitor C6 may to be from 20 to 50 microfarads and capacitor C7 from 3 300 to 68 000 pF. The output voltage of the stabilizer is equal to 5 V, set the trimpot resistor R4. This voltage will be maintained constant even when significant discharge of the batteries.

It should be noted that the chip 155 is powered by a source direct current voltage of 5 V. Therefore, if desired, from the schema can be deleted the unit of the voltage regulator and use the powered one battery type 3336L or equivalent, which allows you to build compact design. However, discharge the battery very quickly affect the functionality of this detector. It is therefore necessary the power supply, ensuring the formation of a stable voltage of 5 V.

It should be recognized that as the power source, the author used four large round batteries imported, connected in series. The voltage 5 V formed integral regulator type 7805. Board with the controls it contains and the power source are placed in any a suitable plastic or wooden case. On the housing cover installed variable capacitor C2, the switch S1, and also connectors for connect the search coil L2 and headphones BF1 (these connectors and switch S1 on the circuit diagram are not listed).

Establishing

As with the adjustment of other detectors, this device should be configured in conditions when metal objects are removed from the search coil L2 on the a minimum distance of one meter.

First, using a frequency counter or oscilloscope, you must configure business the frequency of the reference and test oscillators. The frequency of the reference oscillator is set equal to about 465 kHz by adjusting the core of the coil L1 and if necessary, the selection of the capacitance of the capacitor C1. Before adjusting will need to disconnect the corresponding output of the capacitor C3 from the diodes detector and capacitor C4. Next you need to disconnect the corresponding output of the capacitor C4 from the detector diodes and the capacitor C3 and adjustment of the capacitor C2 to set the frequency of the measuring oscillator so that its value is different from the frequency of the reference oscillator is about 1 kHz.

After restoring all connections the detector is ready for use.

The order of work

The search operations using the metal detector does not have any special features. In practical use of the appliance variable capacitor C2 to maintain the desired frequency of the beat signal, that changed when the battery temperature changing environment or the deviation of the magnetic properties of the soil.

If in the process, the frequency of the signal to the head phones will change it indicates the presence in the area of the search coil L2 of any metal object. When approaching some metals signal frequency beat will increase, and when approaching others to decrease. On to change the tone of the beat signal, having some experience, you can easily see the kind of metal, magnetic or non-magnetic, made the detected object.

Author: M. V. Adamenko