metal detector

In contrast to the other metal detector in this issue, the present one works on the principle that the frequency of an LC oscillator changes when the inductance is altered. Any metal object brought near the inductor will modify the inductance. The degree by which the frequency changes depends on the nature of the metal and on the frequency. If the fre- quency is very high, a metal object will act as a shorted turn, which lowers the inductance, so that the fre- quency increases. If the frequency is low enough for eddy-current losses to be ignored, it is possible to distinguish ferrous from non-ferrous metals. The inductance required for an oscil- lator frequency of not greater than 200 Hz would be pretty difficult to make, and the oscillator in the present circuit, therefore, works at about 300 kHz. The inductance then needed is quite easy to make and consists of a single turn of coaxial cable as shown in the accompanying diagram. The circuit consists of oscillator T i, frequency-t...