Miniature metal detector can be assembled on a single transistor. Scheme the design is a variation of the blocking oscillator. Sensor metal detector - the transformer T1 is mounted on the end of the rod. The magnetic core of the transformer is not shorted, and when approaching metal object changes the inductance of the windings. The oscillation frequency the generator is also changing, the headphones receive the signal of a different tone.
For our detector will need a small output transformer from any tube receiver, radio-gramophone or tape recorder. From core transformer remove all rectangular plate and leave only the W-shaped.
The transformer T1 can be made independently. It is wound on the core section 2 - 5 cm2 of W-shaped plates. The primary winding contains 800-1000 turns of wire PEL of 0.1-0.12 a branch of the middle. The secondary winding has 100-200 turns of the wire enamel insulation diameter 0.25-0.4 mm.
Transistor V1 of the low-frequency, low-power from the MP39-MP42. Power source - battery 3336L or "ruby".
Earphone any type B1 (TM-2, TM-4, etc.).
All parts are mounted in the housing of plastic or in cans, for example, from under the candy. This case will reinforce the wooden or metal handle length 80-100 cm
If the installation is done correctly and all parts are serviceable, after power (switch S1) in the headphones, you should hear a smooth sound - the buzz or squeak. Pitch choose the capacitance change of the capacitor C1.
After achieving a stable operation of the generator, the lower part of the building, where the the projections of a transformer core, closer to the metal object. The tone sound should change. And the more audible the closer is this subject. After setting the lower casing close (for protection dust and dirt) cover of thick cardboard.
Author: I. Efimov