Interesting link I found

[alton]

After viewing all the comments on the Threshold Thread I found this while researching it:

http://www.freepatentsonline.com/4024468.html

It will be interesting to see if anything useful ever makes it to market.

HH Alton

 

[RonNH]

http://www.nexusdetectors.com/

Kind of a novel coil design, might be a little hard swinging in the brush, but I think White's beat them to the analog idea, and what a learning curve. I couldn't do the European conversion, so I don't know what these things cost...not that I'm interested. Ron

 

[John - Maryland]

That Patent is 30 years old.United States Patent 4,024,468
Hirschi May 17, 1977

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Induction balance metal detector with inverse discrimination


Abstract
An induction balance metal detector is described which is capable of discriminating between different types of metal objects, such as bottle caps and coins, by amplitude discrimination of their corresponding receive signals. In addition, the detector is provided with a tuning control which is used to eliminate the effect of ferrous mineral soil on the output signal of the receive coil. The discriminate control changes the phase of the residual output signal of the receive coil to set the quiescent operating point of the metal detector to a selected angle between about zero and -90.degree. relative to the component of the receive coil signal produced by the presence of magnetic mineral soil, and the tuning control adjusts the amplitude of the residual signal to an audio threshold level. As a result, the desired coins or other high conductivity metal objects reduce the amplitude of the received signal below the audio threshold level so that they tend to produce no sound output or a negative indication while all other objects increase the amplitude of such signal above the audio threshold level so they tend to produce a sound output. This inverse discrimination is provided with a positive indication by means of a comparator having one input connected to an audio pulse generator synchronized to the oscillator and its other input connected to the demodulated audio frequency output of the receive coil. The comparator compares the amplitude of the input signals and produces a positive indication signal an audio output only when the received signal is less than the amplitude of the standard audio pulses. The detector may be provided with a transmit coil whose annular loop shaped windings are provided with a re-entrant portion forming a feedback coil so that current flows in opposite directions through such transmit coil and feedback coil.


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Inventors: Hirschi; Richard Edward (Sweet Home, OR)
Assignee: White's Electronics, Inc. (Sweet Home, OR)

Appl. No.: 05/587,844
Filed: June 18, 1975