Metal detector comparing the frequencies of two generators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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There are several methods for finding hidden metals:

• bridge - the instrument registers the unbalance of the measuring bridge;
• phase shift method - the phase shift of oscillations of two generators is measured;
• echo sounder method (transmitter-receiver) - a radio wave reflected from a metal object is received.

However, in practice, the frequency beating method is mainly used, based on comparing the frequencies of two generators. It is the most inaccurate and inefficient of all those listed, but it is the easiest to implement, does not require special settings and measures to stabilize the frequency, and therefore is convenient for repetition. The proposed metal detector works exactly according to this method. Frequencies are usually used in the ultrasonic range, since here the generators work more stably and with greater amplitude.

It is not uncommon to choose an operating frequency of 500 kHz (or 465 kHz) to apply ready-made IF circuits to radio receivers. In this device, the oscillators are tuned to a frequency of 200 kHz, although any frequency from 100 to 400 kHz can be chosen with equal success.

scheme metal detector is shown in fig. one.

Fig. 1

On the elements DD1.3, DD1.4, a generator with a search coil L1 is assembled. Its frequency depends on the capacitance of the capacitor C2 and the inductance L1 (the number of turns). Another generator, on the elements DD1.1, DD1.2 - tunable using resistors R1 and R2. It is tuned to the frequency of the searchcoil oscillator to achieve zero beats or smooth adjustment of the difference frequency. Usually it uses an inductor and a variable capacitor (LC circuit). This device uses an RC circuit, which weakened the mutual influence of the generators, increased their stability and simplified the circuit. Resistor R1 changes the frequency roughly, and R2 - smoothly. The signals of both generators through the transition capacitors C3 and C4 are fed to an active mixer-detector, made on the transistor VT1, and from it to the AF amplifier (VT2), the load of which is 100 ohm headphones.

Details. Chip K561LA7 can be replaced by K176LA7. In order for the frequency to “float” less, the ceramic capacitors in the device must be with a small TKE. In this regard, mica capacitors of the "KSO" type of group G are good.

It is best to make three search coils. One - with a diameter of 150 mm, the second - 200 mm, and the third - 260 mm. They are made with a wire of approximately the same length (36 m) and have the corresponding number of turns (76, 58 and 45). Wire - PEV 0,51 mm (from the demagnetization circuit of an old color TV). All three reels are frameless. They are wound on any suitable cylindrical object (pot, jar, etc.), having previously placed a rubber or paper gasket. The finished winding is removed and fastened with threads in several places, and then wrapped with electrical tape or tape.

In the manufacture of coils in [1] it is recommended to shield them from static electricity, in [2] this is not even mentioned. I did not shield my coils, but there were no failures in the operation of the device.

Approximate design metal detector is shown in fig. 2.

Fig. 2

When searching for small metal objects (nails, keys, etc.), a coil with a diameter of 150 mm is turned on. To search for larger objects, a larger coil (200 mm) is required. Something like a sewer manhole or an underground pipe is defined by the largest coil with a diameter of 260 mm.

Setting. First, having unsoldered one end of C3, the generator with the search coil is tuned to the desired frequency. To do this, instead of C2, a variable three-section capacitor (12 ... 495 pF) x3 is temporarily installed and a signal is searched for a broadcast receiver turned on to the long-wave range. And having found, they "bring" it to a frequency of 200 kHz. However, this is not necessary; you can leave the frequency at 150 or, say, 250 kHz. Having determined the value of the capacitance, a constant capacitor is placed instead of a variable. After soldering C3 in place, the tunable oscillator is tuned to the same frequency with the resistor R1. The setting is determined by zero beats in the headphones.

Work. After 10 ... 15 minutes of warming up, the devices bring the search coil closer to the ground at a distance at which the search will be conducted, and adjust to zero beats. When searching, maintain the same distance of the frame from the ground. The appearance of sound in the phones indicates the presence of a metal object near the coil. The closer it is, the more the inductance of the search coil changes and, as a result, the tone of the audio signal.

With this "mine detector" I found a main water pipe (for which, in fact, I had to do all this), laid in the ground at a depth of 70 cm.

Literature

1. Radio, No. 1, 1984.
2. Radio, No. 8, 1968.

Author: E.Ryabichko, st.Kelermesskaya, Adygea.

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