Transistor metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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The transistor metal detector can detect massive metal objects buried in the ground at a depth of about 60 cm, small metal objects the size of a small coin can be detected at a depth of about 5 cm.
The scheme of the metal detector is shown in fig. 1.
Rice. 1 (click to enlarge)
Resistor R12 has a resistance of 5,6 k.
The principle of operation of the metal detector is based on a change in the inductance of the coil when a metal object is introduced into its field. Such a coil is L2, which is located in the remote head of the metal detector and enters the circuit of a variable frequency generator assembled on a transistor T2. In addition to the variable frequency generator, the device has a fixed frequency generator, assembled on the transistor T1.
The output voltages of the generators are taken from the emitters of the transistors and fed through the C4R7 and R8C9 chains to the diode detector D1. The beats generated at the output of the detector are filtered out by the R9C10 circuit and fed to the input of a two-stage audio frequency amplifier based on transistors T3 and T4. A piezoelectric headset is switched on at the output of the amplifier.
Coil L1 of the fixed frequency generator circuit is wound on a frame with a diameter of 6 mm with PEL 0,3 wire and has 115 turns with a tap from 15 turns, counting from the end connected to the collector.
Coil L2, which is included in the remote head of the metal detector, is wound in the form of a frame with PEL 0,5 wire. It has 50 turns, with a tap from 10 turns, counting from the end connected to the capacitor C7. Its coils are located on four posts mounted on a plate of insulating material, as shown in Fig. 2. In addition to the coil, capacitor C6 and trimmer capacitor C5 are located here. The remote head is connected to the device with a three-wire cable about 1,5 m long.
Fig. 2
The device is adjusted so that in the absence of metal objects near the head, the frequency of vibrations heard in the phone is close to the cool. This is achieved by adjusting the capacitors C5 and C8. As the remote head approaches a metal object, the beat frequency should increase. By moving the head of the metal detector along the surface of the earth, by changing the pitch heard in the phone, one can judge the location of metal objects in the earth.
As transistors T1 - T4 and diode D1, transistors P14 - P16 and diode D2Zh can be used. Instead of a piezoelectric telephone, a high-resistance electromagnetic one is used. In this case, the capacitance of the capacitor C15 should be increased to 20 microfarads.
Literature
- Wireless world, 1966, No. 1
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