Transistor metal detector can detect a 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.
Scheme of the detector is shown in Fig. 1.
Fig. 1
The resistor R12 has a resistance of 5.6 K.
The principle of operation of the detector based on the change in inductance of the coil when introduction in its field metal object. This coil is L2, which located in the remote detector head and is included in the circuit of the generator changing the frequency, assembled on the transistor T2. In addition to changing generator frequency, the device has a generator of fixed frequency, assembled on the transistor T1.
The output voltages of the generators are taken from the emitters of the transistors and through chain С4R7 and R8C9 served on detector diode D1. The beats are formed on the output of the detector is filtered by a chain of R9C10 and to the input a two-stage amplifier of audio frequency transistors T3 and T4. The output amplifier piezoelectric included headset Phone.
Coil L1 of the contour generator of fixed frequency is wound on the frame 6-mm-diameter wire PEL of 0.3 and has 115 turns taped 15 turns, starting from the end connected to the collector.
Coil L2, a part of the remote detector head wound and the appearance of the frame wire PEL of 0.5. It has 50 turns, with a branch from 10 turns, starting from the end, connected to the capacitor C7. The coils are located at the four pillars, fortified by a plate of insulating material, as shown in Fig. 2. In addition to the coil, a capacitor C6 and a trimmer capacitor C5. The remote connection of the head with the instrument is a three-core cable length about 1.5 M.
Fig. 2
The device is configured so that in the absence of metal objects near head oscillation frequency, audible in the phone, was close to Kul. It is achieved by adjustment of the capacitors C5 and C8. As we approach the remote head to a metal object, the beat frequency should increase. Moving the head of the metal detector along the surface of the earth, to change pitch, audible in the phone, you can judge the location in the land of metal items.
As transistors T1 - T4 and the diode D1 can be used transistors P14 - p and diode DI. Instead of the piezoelectric phone high resistance applied electromagnetic. In this case, the capacitance of the capacitor C15 should be increased to 20 UF.
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