Metal detector with integral ULF. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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Schematic diagram
The scheme shown in fig. 2.48 is a classic metal detector. The operation of the circuit is based on the principle of superheterodyne frequency conversion, which is usually used in a superheterodyne receiver.
Rice. 2.48. Schematic diagram of a metal detector with integrated ULF
Two radio frequency generators are used, the frequencies of which are 5,5 MHz.
The first RF oscillator is assembled on a BF1 type T494 transistor, the oscillator frequency is stabilized by quartz. A 5 MHz ceramic filter is used, which is usually used in the IF path of the TV sound.
The second radio frequency generator according to the "Colpitz" scheme is assembled on a T3 transistor. The frequency is set by the parameters of the inductance L1 and the capacitor VC1.
The frequencies of the two RF generators are mixed in a mixer assembled on the transistor T2. The difference frequency signal from the collector of transistor T2 is fed to a detector assembled on diodes D1 and D2. Voltage ripples are smoothed out by a low-pass filter assembled on elements R12, C6, C10. Through the level control, the signal is then fed to a low-frequency amplifier.
The inductor L1 contains 15 turns of PEL 0,25 wire, wound on a mandrel with a diameter of 10 cm and fixed with varnish.
Setting up the metal detector
After tuning the frequency of both generators in the absence of metal objects, the frequencies must exactly match in order to obtain zero beats of the frequency difference at the ULF output.
The setting is carried out by selecting the value of the trimmer capacitor VC1. When properly tuned, there is no difference frequency tone in the loudspeaker.
When metal objects appear near the coil L1, the frequency of the LC generator changes, and a difference frequency tone is heard in the loudspeaker. Its volume is regulated by a variable resistor VR1.
Author: Akundinov A., UA3VVM; publication: electronic-lab.com
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