The detector, whose scheme is shown in the figure, one can collect only a few minutes. It consists of two almost identical LC oscillators, performed on the elements DD1.1-DD1.4, the detector according to the scheme of doubling the rectified voltage on the diodes VD1, VD2 and high impedance (2 ohms) headphones BF1, change the tone of the sound which indicates the presence of under the coil antenna of a metal object.
Generator, assembled on the elements DD1.1 and DD1.2 snowspeeders at the resonance frequency of the series resonant circuit L1CI configured on the frequency of 465 kHz(used elements of the if filter of a superheterodyne receiver). The frequency of the second generator (DD1.3. DD1.4) is determined by the inductance of the coil antenna L2 (30 turns of wire PEL of 0.4 mandrel with a diameter of 200 mm and a capacitance of the variable capacitor C2, before allowing the search to configure the detector to the detection of objects of a certain mass.
The beats resulting from the mixing of oscillations Wallpaper generators, is detected by diodes VD1, VD2, filtered by capacitor C5 and fed to the headphones BF1.
All the device is assembled on a small printed circuit Board that allows when powered by a flat battery flashlight make it very compact and easy to handle.
The repetition of the metal detector, you can use the chip TO DS,any high frequency germanium diodes and KPE from radio "Mountaineer".
Author: Janeczek A; Publication: N. Bolshakov, rf.atnn.ru