Simple metal detector circuit

Metal detector, proposed by Thomas Scarborough, made on a low-cost IC Schmitt trigger. As in many other constructions metal detectors, it uses the principle of "beat-generator", but the role of the second generator, and plays a sound indicator pocket radio with a range of medium wave. IS1.A - master oscillator operating at a frequency of 200 ... 300 kHz frequency control coil L1, which is wound on a frame of 120 mm diameter and contains 70 turns of PEL-0,35.IS1.V - buffer, the output of which a small segment of shielded cable is connected to the input of the receiver. Turn the power of both devices, set up the receiver in the range of average waves at the frequency at which the dynamics is clearly a loud whistle. If you then bring to the coil L1 a metallic object such as a coin, the inductance L1 and the frequency of generation change that is clearly defined by ear. If a ferromagnetic metal, the frequency increases, and if paramagnetic, then decreases. The sensitivity of the device is sufficient to detect the coin at a distance of 80-90 cm