There are several methods to search for hidden metals:
- bridge - a device is registered with the unbalance of the measuring bridge;
- method of phase shift is measured by the phase shift of the oscillations of the two oscillators;
- method finder (transmitter-receiver) is reflected from the metal the subject of the radio wave.
However, in practice, mostly used method of beating frequencies, based on comparison of the frequencies of the two generators. He is the most inaccurate and inefficient of all listed, but the most simple to implement, requires no special settings and measures to stabilize the frequency, and therefore it is easy to repeat. The proposed detector works by this method. Frequency usually use the ultrasonic range, because here the generators operate more steadily and with greater amplitude.
Often choose an operating frequency of 500 kHz (or 465 kHz), ready to apply the inverter circuits of radios. In this device the generators are tuned to the frequency 200 kHz, although with the same success it is possible to select any frequency from 100 to 400 kHz.
Diagram of the detector shown in Fig. 1.
Fig. 1
On the elements DD1.3, DD1.4 assembled generator search with the search coil L1. Its frequency depends on the capacitance of the capacitor C2 and inductor L1 (number of turns). Another generator on the elements DD1.1, DD1.2 is tunable via the resistors R1 and R2. It is configured on the oscillator frequency with a search coil to obtain a zero beat or smooth adjustment of the difference frequency. Usually there is used an inductor and variable capacitor (LC-circuit). This device uses a RC-chain, what has weakened the mutual influence generators, increased stability and simplified scheme. The resistor R1 changes the frequency roughly, a R2 - smoothly. The signals of both generators through the transitional capacitors C3 and C4 act on active the mixer-detector, performed on the transistor VT1, and with him - to the AF amplifier (VT2), the load of which are headphones with impedance of 100 Ohms.
Details. Chip CLA can be replaced by CLA. To have less "float" the frequency ceramic capacitors in the device must be low TKE. In this good mica capacitors type "CSR" group G.
Search coils is better to make three. One with a diameter of 150 mm, the second 200 mm, and the third - 260 mm. They are made by wire approximately the same length (36 m) and have a corresponding number of coils (76, 58, and 45). Wire - sew 0.51 (from loop demagnetizer old color TV). All three coil - frameless. They wound on any suitable cylindrical item (pot, jar, etc.), after putting a strip of rubber or paper. The finished coil is removed and in several places fasten the thread, and then wrapped with electrical tape or duct tape.
Coils of in [1] it is recommended to shield them from static electricity, in [2] it was not even mentioned. I your coils are not screened, but failures in the device was not.
The approximate design of the detector is shown in Fig. 2.
Fig. 2
While searching for small objects made of metal (nails, keys, etc.) turns on the coil with a diameter of 150 mm. to search For larger items require bigger coil (200 mm). Something like a manhole or underground pipes is determined by the big coil with a diameter of 260 mm.
Configure. First, hotpaw one end of C3, configures the generator with a search coil at the desired frequency. To do this, instead of C2 temporarily put three-section AC condenser (12...495 pF )X3 and are searching for a broadcast signal the receiver included in the long-wave range. And finding, "lead" on it the frequency of 200 kHz. However, it is not necessary, you can leave the frequency of 150 or say, 250 kHz. Determining the capacitance, instead of the variable put permanent capacitor. Sapaw C3 in place, the resistor R1 is set to the same the frequency tunable oscillator. The setting is determined by a zero beat in headphones.
Work. After 10...15 minute warm-up devices lower the search coil to the earth at a distance, which will be sought, and set to zero beats. When searching stand the same distance of the frame from the ground. The appearance the sound in the phones indicates the presence of a metal object near the coil. Than it is closer, the more changes the inductance of the search coil and, as a consequence, the tone of the audio signal.
This "mine detector" I found the main water supply pipe (which, in fact, I had to do it all), laid in the ground at a depth of 70 cm.
Literature
Author: E. Rebecca, senior Kelermesskaya, Adygea.