Abstract: A metal detector circuit derives a first sine wave signal in response to metal objects in proximity to the search head. A second sine wave signal at the operating frequency of the circuit is applied to a center tapped inductor. The initial and final turns of the inductor are connected through a system of capacitors, resistors, and diodes so that conduction occurs during a brief period of each cycle of the applied sine wave. The electrical switching action thus afforded is used to charge a capacitor to the instantaneous voltage of a specified component of the first sine wave. A phase shift network provides a means of adjustment of the phase angle at which switching occurs, to permit a predetermined component of the first sine wave to be detected. When the proper component of the first sine wave is detected, a selective or discriminatory response may be derived, resulting in signals of opposite polarity for high Q objects, such as coins, and low Q objects, such as aluminum pull tabs.

Abstract: A metal detector circuit is disclosed utilizing a search coil inductively coupled to two parallel connected oscillator circuits operating at different frequencies for selectively responding to metal objects in the vicinity of the search coil. Each oscillator circuit may include a dual tuned resonant circuit for neutralizing the effects of ferromagnetic minerals in the vicinity of the search coil.

Abstract: A metal detector circuit is described utilizing a search coil inductively coupled to a parallel resonant circuit. This circuit replaces the tapped parallel resonant circuit of a Hartley oscillator. Feedback to sustain oscillation is provided by a series resonant circuit driven by the parallel resonant circuit, with the series resonant circuit having a higher resonant frequency than the parallel resonant circuit.