Selective metal detector circuit having dual tuned resonant circuits

Claim 1

In a metal detector of the type to be passed over the ground to detect metallic objects near the surface of the ground, the combination comprising: a. a search coil; b. a transistor oscillator for driving said search coil and having a parallel resonant tank circuit and a series resonant feedback circuit, each tuned to a different frequency; c. said parallel resonant tank circuit including an inductor inductively coupled to said search coil and including a pair of series connected capacitors connected in parallel with said inductor; d. a transistor having an emitter, collector and a base; e. means connecting said parallel tank circuit to said emitter; f. means connecting said series resonant feedback circuit between the junction of said capacitors and said base; g. amplifying means, means connecting said amplifying means to said collector; and h. indicator means connected to said amplifying means for receiving the amplified signal therefrom and presenting an indication of the amplitude thereof to an operator.