A simple metal detector on a K155LA3 chip. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple metal detector on a K155LA3 chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Beginning radio amateurs can be recommended to repeat the design of a simple metal detector, the basis for which was a circuit that was repeatedly published in the late 70s of the last century in various domestic and foreign specialized publications. This metal detector, made on just one K155LA3 chip, can be assembled in a few minutes.

Schematic diagram

The proposed design is one of the many options for BFO (Beat Frequency Oscillator) metal detectors, that is, it is a device based on the principle of analyzing the beats of two signals that are close in frequency (Fig. 3.1). At the same time, in this design, the assessment of the change in the beat frequency is carried out by ear.

A simple metal detector on a K155LA3 chip

A simple metal detector on a K155LA3 chip
Rice. 3.1. Schematic diagram of a metal detector on a K155LA3 chip

The device is based on measuring and reference oscillators, an RF oscillation detector, an indication circuit, and a supply voltage stabilizer.

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

The reference oscillator circuit is formed by a 1 pF capacitor C200 and a coil L1. The measurement generator circuit uses a variable capacitor C2 with a maximum capacitance of approximately 300 pF, as well as a search coil L2. In this case, both generators are tuned to an operating frequency of approximately 465 kHz. The outputs of the generators through decoupling capacitors C3 and C4 are connected to the RF oscillation detector, made on diodes D1 and D2 according to the rectified voltage doubling circuit. The load of the detector is BF1 headphones, on which the signal of the low-frequency component is extracted. In this case, the capacitor C5 shunts the load at higher frequencies.

When approaching the search coil L2 of the oscillatory circuit of the tunable generator to a metal object, its inductance changes, which causes a change in the operating frequency of this generator. In this case, if there is an object made of ferrous metal (ferromagnet) near the coil L2, its inductance increases, which leads to a decrease in the frequency of the tunable oscillator. The non-ferrous metal reduces the inductance of the L2 coil, and increases the operating frequency of the generator. The RF signal formed as a result of mixing the signals of the measuring and reference generators after passing through the capacitors C3 and C4 is fed to the detector. In this case, the amplitude of the RF signal changes with the beat frequency.

The low-frequency envelope of the RF signal is isolated by a detector made on diodes D1 and D2. Capacitor C5 provides filtering of the high-frequency component of the signal. Next, the beat signal is sent to the BF1 headphones. Power is supplied to IC1 from a 1V source B9 through a voltage regulator formed by a zener diode D3, a ballast resistor R3 and a regulating transistor T1.

Details and construction

For the manufacture of the considered metal detector, you can use any prototyping board. Therefore, the used parts are not subject to any restrictions related to overall dimensions. Installation can be both hinged and printed.

When repeating the metal detector, you can use the K155LA3 chip, consisting of four 2I-NOT logic elements, powered by a common DC source. As capacitor C2, you can use the tuning capacitor from a portable radio. Diodes D1 and D2 can be replaced with any high frequency germanium diodes.

Coil L1 of the reference oscillator circuit should have an inductance of about 500 μH. As such a coil, it is recommended to use, for example, the IF filter coil of a superheterodyne receiver.

Measuring coil L2 contains 30 turns of PEL wire with a diameter of 0,4 mm and is made in the form of a torus with a diameter of 200 mm. This coil is easier to make on a rigid frame, but you can do without it. In this case, any suitable round object, such as a jar, can be used as a temporary frame. The turns of the coil are wound in bulk, after which they are removed from the frame and shielded with an electrostatic screen, which is an open aluminum foil tape wound over a bundle of turns. The gap between the beginning and the end of the tape winding (the gap between the ends of the screen) must be at least 15 mm.

In the manufacture of coil L2, it is especially necessary to ensure that the ends of the shielding tape do not close, since in this case a short-circuited coil is formed. In order to increase the mechanical strength, the coil can be impregnated with epoxy glue. For the source of sound signals, high-impedance headphones with as high a resistance as possible (about 2000 ohms) should be used. Suitable, for example, the well-known telephone TA-4 or TON-2.

As a power source V1, you can use, for example, a Krona battery or two 3336L batteries connected in series.

In a voltage stabilizer, the capacitance of the electrolytic capacitor C6 can be from 20 to 50 microfarads, and the capacitance of C7 can be from 3 to 300 pF. The voltage at the output of the stabilizer, equal to 68 V, is set by the trimming resistor R000. This voltage will be maintained unchanged even when the batteries are significantly discharged.

It should be noted that the K155LAZ chip is designed to be powered from a 5 V DC source. Therefore, if desired, the voltage stabilizer unit can be excluded from the circuit and one 3336L battery or similar can be used as a power source, which allows you to assemble a compact design. However, the discharge of this battery will very quickly affect the functionality of this metal detector. That is why you need a power supply that provides the formation of a stable voltage of 5 V.

It should be recognized that the author used four large imported imported round batteries connected in series as a power source. In this case, a voltage of 5 V was formed by an integral stabilizer of the 7805 type. The board with the elements located on it and the power source are placed in any suitable plastic or wooden case. A variable capacitor C2, a switch S1, as well as connectors for connecting a search coil L2 and headphones BF1 are installed on the housing cover (these connectors and switch S1 are not indicated on the circuit diagram).

Establishment

As with the adjustment of other metal detectors, this device should be adjusted in conditions where metal objects are removed from the L2 search coil at a distance of at least one meter.

First, using a frequency meter or oscilloscope, you need to adjust the operating frequencies of the reference and measuring oscillators. The frequency of the reference oscillator is set to approximately 465 kHz by adjusting the core of the coil L1 and, if necessary, by selecting the capacitance of the capacitor C1. Before adjustment, you will need to disconnect the corresponding terminal of the capacitor C3 from the diodes of the detector and capacitor C4. Next, you need to disconnect the corresponding terminal of the capacitor C4 from the diodes of the detector and from the capacitor C3 and adjust the capacitor C2 to set the frequency of the measuring generator so that its value differs from the frequency of the reference generator by about 1 kHz.

After all connections are restored, the metal detector is ready for operation.

Operating procedure

Carrying out search operations with the help of the considered metal detector does not have any features. In practical use of the device, the necessary frequency of the beat signal should be maintained by the variable capacitor C2, which changes when the battery is discharged, the ambient temperature changes, or the deviation of the magnetic properties of the soil.

If the frequency of the signal in the headphones changes during operation, this indicates the presence of a metal object in the area of the search coil L2. When approaching some metals, the frequency of the beat signal will increase, and when approaching others, it will decrease. By changing the tone of the beat signal, having a certain experience, one can easily determine what metal, magnetic or non-magnetic, the detected object is made of.

Author: Adamenko M.V.

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