absorption-type metal detector

11114411144111 The action of the detector, which indicates the presence of ferrous as well as non-ferrous metals, depends on the absorption of magnetic energy. An inductor, which forms part of a tuned oscillator circuit, radiates a magnetic field. When a metal object is introduced into this field, enough magnetic energy is absorbed to cause the oscillator to stop working. The Colpitt"s oscillator in figure 1 operates at a frequency of around 70 kHz. Inductor L1also serves as the sensor. Because of the high value of the emitter resistor, fl1, the oscillator only just operates. This is desirable, otherwise any losses in the tuned cir- cuit would easily be replenished by the transistor. The oscillator output is rec- tified by D1and D 2, and the resulting direct voltage is applied to the invert- ing input of Schmitt trigger IC1. If that voltage drops below the level at 2 1 pin 3 (preset by P1), the output becomes logic high, and the relay is energized. The detector is best constructed...