Selective/ground neutralizing metal detector

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. In addition, a metal detector system is presented wherein two of the detector circuits of the invention are advantageously combined so that the output is both selective and ground neutralized. In this system, a first detector circuit provides a selective response to metal objects. A second detector circuit provides a response to metal objects, while excluding ground effects. A clamping circuit controlled by the second detector circuit removes ground effects from the output of the first detector circuit.

BACKGROUND OF THE INVENTION 
In recent years, numerous metal detector circuits have been developed to 
provide a selective or discriminatory response to metal objects in 
proximity to the search head. As a result of the greater resistive 
component of the signals produced by low Q objects, such as aluminum pull 
tabs and the small resistive component of the signals produced by high Q 
objects, such as coins, a circuit which detects the proper component of 
the received signal will produce a response of opposite polarity for these 
objects. 
In addition, numerous circuits have been developed for the purpose of 
excluding the effects of ferromagnetic minerals, such as magnetite, 
resulting in a ground neutralizing detector system. In these systems, the 
circuit is adjusted so that the component of the received signal caused by 
ferromagnetic minerals is not detected, resulting in a ground neutralized 
response. 
The design problems inherent to both selective and ground neutralizing 
detector circuit are similar. Of primary concern is the ability of the 
circuit to provide a consistent response despite changes in the quiescent 
input signal within the detector circuit. These changes may be caused by 
temperature changes, aging of components, or large quantities of 
ferromagnetic minerals in proximity to the search head. In addition, the 
circuit should not require critical adjustment or component selection to 
enable ease of construction and to insure reliability. 
It is therefore an object of the present invention to provide a selective 
or ground neutralizing detector circuit which will operate reliably, which 
may be constructed economically and which will not require critical 
adjustment. 
It is another object of the present invention to provide a metal detector 
circuit incorporating a switching circuit to enable the detector to 
neutralize the effects of ferromagnetic minerals. 
It is still another object of the present invention to provide a metal 
detector having a switching circuit which may be utilized to provide 
selectively for the identification of detected objects in accordance with 
the Q of such objects. 
It is still another object of the present invention to provide a metal 
detector system utilizing a clamping circuit, controlled by a ground 
neutralizing detector circuit, for the purpose of removing the effects of 
ferromagnetic minerals from the response of a selective detector circuit. 
These and other advantages of the present invention will become apparent to 
those skilled in the art as the description thereof proceeds.

Referring now to the drawings, and particularly to FIG. 1, a transmitter 
coil 1 and a receiver coil 2 are located in a conventional manner typical 
in metal detector art. 
Oscillator 3 supplies a sine wave signal with a frequency of 6 kHz to 
transmitter coil 1. Both transmitter coil and receiver coil 2 are tuned to 
resonance at this frequency of 6 kHz. The two coils are placed in an 
overlapping position where the signals induced by transmitter coil 1 into 
receiver coil 2 are minimal. 
A nulling signal source 4 derives from oscillator 3 a signal which is 
combined with the signal from the receiver coil 2, and adjusted in phase 
and amplitude so that the resultant signal is diminished to zero. This 
output will be a zero or null signal when no metal objects or 
ferromagnetic minerals are near the coils. 
When metal objects or ferromagnetic minerals enter the magnetic flux field 
linking the coils, there will be a change in the signal of receiver coil 
2. The signal obtained by combining the receiver coil 2 signal and the 
nulling signal will now be a sine wave signal representing only the 
effects of metal objects or ferromagnetic minerals near the coils. 
The output thus obtained is applied to a pre-amplifier 5 where the signal 
is amplified sufficiently to operate the remaining elements of the 
circuit. Pre-amplifier 5 does not need to be of special design, since it 
is required only to amplify the sine wave signal without causing 
distortion of the waveform. 
The output of pre-amplifier 5 is connected to the base of transistor 6 via 
coupling capacitor 7. A potentiometer 8 is connected from circuit ground 
to the voltage supply, and the wiper of the potentiometer is connected to 
the base of transistor 6 through resistor 9, thus providing an adjustable 
biasing voltage. The collector of transistor 6 is connected to the voltage 
supply, and the emitter is connected to circuit ground via resistor 10. 
A center tapped winding 11 is provided (which may be contained within a 
30.times.19 mm cup core); within this cup core is also contained a second 
winding 12, with this second winding being tuned to resonance at the 
circuit operating frequency of 6 kHz via capacitor 13. The first turn 14 
of winding 11 is connected to the cathode of diode 15 via the parallel 
combination of resistor 16 and capacitor 17. The anode of diode 15 is 
connected to the cathode of diode 18. The anode of diode 18 is connected 
to the last turn 19 of winding 11 via the parallel combination of resistor 
20 and capacitor 21. 
The junction of diode 15 and diode 18 is connected to the junction of 
transistor 6 and resistor 10. The center tap 22 of winding 11 is connected 
to one terminal of a capacitor 23, with the second terminal of this 
capacitor being connected to circuit ground. Center tap 22 is also 
connected to the base of transistor 24, with the collector of this 
transistor being connected to the voltage supply, and the emitter being 
connected to circuit ground via resistor 25. 
The junction of transistor 24 and resistor 25 is connected to amplifier 26. 
This is a DC level amplifier, and does not need to be of special design. 
It is required only to amplify changes in the voltage of capacitor 23, and 
to function as a low impedance source to drive the meter and audio stages 
which follow. 
Amplifier 26 is connected to a meter 27 and audio stage 28. The audio stage 
being of a conventional type which provides a change of sound volume or 
frequency in response to a change in the DC level of the input signal. 
A sine wave signal at the operating frequency is obtained from oscillator 
3, and is applied via conductor 29 to one terminal of resistor 30, with 
the second terminal of this resistor being connected to circuit ground by 
capacitor 31. Conductor 29 is also connected to one terminal of capacitor 
32, with the second terminal of this capacitor being connected to circuit 
ground by resistor 33. Potentiometer 34 is used to connect the junction of 
resistor 30, an capacitor 31, to the junction of capacitor 32 and resistor 
33, thus forming a bridge network. 
The wiper of potentiometer 34 is connected to the base of transistor 35 by 
capacitor 36. The base of transistor 35 is connected to the voltage supply 
by resistor 37 and to circuit ground by resistor 38. The emitter of 
transistor 35 is connected to circuit ground by resistor 39, and the 
collector is connected to the voltage supply by winding 40 (this winding 
may also be contained within a 30.times.19 mm cup core). Capacitor 41 is 
used to tune winding 40 to resonance at the operating frequency of 6 kHz. 
Capacitor 42 is used to connect the junction of transistor 35, and winding 
40, to one terminal of resistor 43, with the second terminal of this 
resistor being connected to circuit ground by capacitor 44. Capacitor 42 
is also connected to one terminal of capacitor 45, with the second 
terminal of this capacitor being connected to circuit ground by resistor 
46. Potentiometer 47 is used to connect the junction of resistor 43, and 
capacitor 44, to the junction of capacitor 45 and resistor 46, thus 
forming a bridge network. 
The wiper of potentiometer 47 is connected to the base of transistor 48 by 
capacitor 49. The base of transistor 48 is connected to the voltage supply 
by resistor 50, and to circuit ground by resistor 51. The emitter of 
transistor 48 is connected to circuit ground by resistor 52, and the 
collector is connected to the voltage supply through winding 12. 
The values of the various components may, of course, vary; however, the 
following components have been found to satisfactorily operate: 
______________________________________ 
6 transistor 2N5828A 
7 capacitor 1.mu.f 
8 potentiometer 5K 
9 resistor 15K 
10 resistor 2.7K 
11 center tapped 
winding 100 turns 
12 winding 50 turns 
15 diode 1N4148 
16 resistor 22K 
17 capacitor 5.mu.f 
18 diode 1N4148 
20 resistor 22K 
21 capacitor 5.mu.f 
23 capacitor 15.mu.f 
24 transistor 2N5828A 
25 resistor 2.7K 
30 resistor 2.7K 
31 capacitor .01.mu.f 
32 capacitor .01.mu.f 
33 resistor 2.7K 
34 potentiometer 25K 
35 transistor 2N5828A 
36 capacitor 1.mu.f 
37 resistor 470K 
38 resistor 100K 
39 resistor 1K 
40 winding 50 turns 
42 capacitor 1.mu.f 
43 resistor 2.7K 
44 capacitor .01.mu.f 
45 capacitor .01.mu.f 
46 resistor 2.7K 
47 potentiometer 25K 
48 transistor 2N5828A 
49 capacitor 1.mu.f 
50 resistor 470K 
51 resistor 100K 
52 resistor 1K 
voltage supply +6 volts 
______________________________________ 
The operation of the circuit so constructed will now be described. A signal 
is obtained from oscillator 3, being a sine wave at the operating 
frequency of 6 kHz, and in the system incorporating the above values being 
of 4 volts peak to peak amplitude; this signal is applied to the bridge 
network containing potentiometer 34. The phase of the signal at the 
junction of resistor 30, and capacitor 31 lags the applied signal by 
45.degree. and the phase of the signal at the junction of capacitor 32 and 
resistor 33 leads the applied signal by 45.degree.. With potentiometer 34 
connected from the junction with the lagging phase angle, to the junction 
with the leading phase angle, a signal may be obtained at the wiper of the 
potentiometer, and the phase of this signal may be adjusted through a 
range of 90.degree. (.+-.45.degree.). 
The signal obtained at the wiper of potentiometer 34 is applied through 
capacitor 36 to the base of transistor 35 with biasing for this transistor 
being provided by resistor 37 and resistor 38. The emitter of transistor 
35 is connected to circuit ground through resistor 39, and the collector 
is connected to the +6 volt supply by winding 40, with this winding being 
tuned to resonance at the operating frequency of 6 kHz by capacitor 41. 
Thus connected, transistor 35 serves to restore the amplitude of the phase 
shifted signal, and to provide a low impedance source to drive the 
following stage. The parallel resonant circuit composed of winding 40, and 
capacitor 41 is used to insure that the waveform of the output will be 
sinusoidal. 
The output signal of the junction of transistor 35 and winding 40 will be a 
sine wave, and is applied to the bridge network containing potentiometer 
47 by means of capacitor 42. The phase of the signal at the junction of 
resistor 43, and capacitor 44, lags the applied signal by 45.degree., and 
the phase of the signal at the junction of capacitor 45, and resistor 46 
leads the applied signal by 45.degree.. With potentiometer 47 connected 
from the junction with the lagging phase angle to the junction with the 
leading phase angle, a signal may be obtained at the wiper of the 
potentiometer, and the phase of this signal may be adjusted through a 
range of 90.degree. (.+-.45.degree.). 
Since the phase shift of the signal at the wiper of potentiometer 34 may be 
adjusted through a range of 90.degree., and the phase shift of the signal 
at the wiper of potentiometer 47 may be adjusted through a range of 
90.degree., the total adjustment available by using both potentiometers 
covers a range of 180.degree.. The phase shift network 60, including both 
of the previously described bridge networks, therefore provides a means 
for shifting the phase of the oscillator signal. 
The signal obtained at the wiper of potentiometer 47 is applied through 
capacitor 49 to the base of transistor 48, with biasing for this 
transistor being supplied by resistor 50 and resistor 51. The emitter of 
transistor 48 is connected to circuit ground through resistor 52, and the 
collector is connected to the +6 volts supply through winding 12. Winding 
12 is tuned at the circuit operating frequency by capacitor 13, and so the 
sine wave signal applied to the base of transistor 48 appears across the 
parallel resonant circuit composed of winding 12, and capacitor 13, in the 
circuit of the embodiment of FIG. 1 having the above components, the 
signal will be 3 volts peak to peak amplitude. 
Thus, it can be seen that a sine wave signal at the operating frequency of 
6 kHz appears across winding 12, with this signal being shifted in phase 
relative to the signal of oscillator 3 by an amount determined by the 
adjustment of potentiometer 34 and potentiometer 47. (It should be noted 
that a significant phase shift does not occur across capacitor 36, 
capacitor 42, or capacitor 49, since these capacitors are of large value.) 
The signal of winding 12 appears across winding 11 with the amplitude of 
the signal of winding 11 being greater than that of winding 12. The first 
and last turns of winding 11 are connected together by the series 
connected combination of resistor 16, diode 15, diode 18, and resistor 20. 
Conduction through this combination of components occurs during the time 
when the last turn 19 of winding 11 is positive with respect to the first 
turn 14. Capacitor 17 is placed in parallel with resistor 16, and 
capacitor 21 is placed in parallel with resistor 20 to insure that 
conduction occurs only during the brief period when the voltage between 
the first and last turns of winding 11 is at or nearly at a maximum. 
The adjustment of the signal amplitude appearing between the terminals of 
winding 11 is not critical with this arrangement of components, since 
capacitor 17, and capacitor 21 will automatically charge to a voltage 
where conduction occurs through diode 15, and diode 18, only during a 
brief period when the signal amplitude is at or nearly at a maximum. 
During the time when conduction occurs through diode 15, and diode 18, the 
voltage at the junction of the diodes will be the same as the voltage of 
the center tap 22 of winding 11. That portion of the circuit shown in 
broken line 61 may be considered a switch that "closes" at a predetermined 
time during each cycle to apply the voltage at the junction of diodes 15 
and 18 to the center tap 22. 
Oscillator 3 supplies a sine wave signal at the frequency of 6 kHz to a 
tuned transmitter coil 1 which, in the embodiment chosen for illustration, 
is four volts peak-to-peak. Receiver coil 2 is tuned to resonance at the 
frequency of the transmitter coil 1, and is placed in a position, 
overlapping the transmitter coil 1, where the induced signal is minimal. A 
null signal source 4 derives from oscillator 3 a signal, and provides 
means of adjusting the signal in phase and amplitude, this null signal 
source being of the type commonly used in transmitter receiver detection 
systems. 
The signal thus obtained is added to the signal of the receiver coil 2, and 
adjusted so that the final signal is diminished to zero with no metal 
object being in proximity to the search head. When metal objects or 
ferromagnetic minerals are placed near the search head, the final signal 
will be a sine wave representing only the effect of these materials. The 
signal thus obtained is amplified by means of pre-amplifier 5, this being 
a conventional AC amplifier, and is required only to amplify the sine wave 
input signal without distorting the waveform. The output signal of 
pre-amplifier 5 is applied via capacitor 7 to the base of transistor 6. 
Biasing for transistor 6 is provided by the voltage obtained at the wiper 
of potentiometer 8, applied through resistor 9 with the position of wiper 
of potentiometer 8 determining the biasing voltage applied to the base of 
transistor 6; this control provides a means for adjusting the audio signal 
level and the meter position. 
The signal of pre-amplifier 5 applied through capacitor 7 will be imposed 
upon the DC voltage level supplied by potentiometer 8 and applied to the 
base of transistor 6. Since transistor 6 is in an emitter follower 
configuration, the AC signal appearing at the junction of the emitter of 
transistor 6 and resistor 10 will be identical to the AC signal applied to 
the base, and the DC level will be slightly lower than the DC voltage 
supplied by potentiometer 8 through resistor 9. Transistor 6 thus 
functions as a low impedance source, supplying an AC signal imposed upon a 
DC level which is determined by the adjustment of potentiometer 8. 
As described previously, the portion of the circuit contained within broken 
line 61 functions an an electrical switch which closes at a predetermined 
time during each cycle. The electrical switching action thus afforded 
causes the voltage at the center tap 22 of winding 11 to be clamped to the 
voltage at the junction of diode 15 and diode 18 during the brief instant 
when conduction occurs through diodes 15 and 18. 
The ungrounded terminal of capacitor 23 is connected to center tap 22; the 
junction of diode 15 and diode 18 is connected to the emitter of 
transistor 6. By this means, a voltage is caused to occur across capacitor 
23, with this voltage being determined by the instantaneous value of the 
AC signal appearing at the emitter of transistor 6 at the time when 
switching occurs, and also by the adjustment of potentiometer 8. 
The voltage level of capacitor 23 is applied to the base of transistor 24 
which is connected in the emitter follower configuration. The output of 
this stage is the junction of the emitter of transistor 24 and resistor 
25; the stage is used to provide a low impedance source to drive the 
amplifier 26 which follows. Amplifier 26 is a DC level amplifier, used to 
amplify the change in signal level, and thus provide increased sensitivity 
and response. This amplifier is of conventional design, with the only 
requirement being the ability to avoid ringing or recoil effects following 
response to a change in signal level. 
The output of amplifier 26 is applied to a meter 27 and audio stage 28. The 
audio stage should be of the conventional prior art type which provides a 
change in volume, or frequency, in response to a change in the DC level of 
the input signal. 
It is necessary to maintain meter 27 and audio stage 28 within their useful 
operating range; to achieve this, potentiometer 8 is used to adjust the 
quiescent level of the signal applied to meter 27 and audio stage 28. 
The circuit just described may be adjusted to operate either as a selective 
detector, or as a ground neutralizing detector, by causing switching to 
occur at the proper instant of each cycle of the waveform supplied by 
transistor 6. 
To attain ground neutralized operation, the instant of switching should be 
adjusted to occur at a 90.degree. lagging phase angle to the signal caused 
by a sample of magnetite positioned near the search head. 
Referring to FIG. 2, the phasor diagram and graph of the waveform supplied 
by transistor 6 illustrate the phasing which should exist between the 
signal caused by magnetite and the instant of switching in order to obtain 
ground neutralizing operation. It should be noted that switching occurs 
during the negative going, zero crossing instant of the magnetite induced 
waveform. As the graph of the waveforms indicates, there will be a 
substantial component of the waveforms of the penny and the pull tab 
present during the instant of switching, so that the signals of these 
objects will be detected and cause a change in the voltage appearing 
between the terminals of capacitor 23. 
The phase shifting network 60 is used to adjust the phasing between the 
sine wave signal supplied by transistor 6 and the instant of switching. To 
do this, a sample of magnetite or black sand is positioned near the 
surface of the search head, thereby causing a sine wave signal to appear 
at the junction of transistor 6, and resistor 10. Using a dual trace 
oscilloscope (not shown), the oscilloscope ground is connected to circuit 
ground, and the first input of the oscilloscope is connected to the 
junction of the emitter of transistor 6 and resistor 10, and set to 
trigger on this signal. 
Thus connected, the waveform displayed will represent the signal supplied 
by the receiver coil in proximity to magnetite as it appears at the 
emitter of transistor 6 (magnetite waveform of FIG. 2). 
The second input of the oscilloscope is connected to the last turn 19 of 
winding 11. Thus connected, the waveform displayed will represent the 
signal of the sine wave voltage appearing at terminal 19 of the winding 11 
of the switching circuit. A small flattened region will be noted at the 
instant of the peak positive alternation of the waveform; the flattened 
region represents the instant of switching. Using potentiometer 34 and 
potentiometer 47, the instant of switching should be adjusted to occur 
simultaneously with the negative going, zero crossing instant of the 
waveform appearing at the emitter of transistor 6. Since the range of 
adjustment provided by the potentiometers is 180.degree. or one-half 
cycle, the adjustment needed may lie outside of this range. In this case, 
the connections to the winding 12 should be reversed. The range of 
adjustment should then contain the setting where switching occurs at the 
correct instant. Since a final adjustment will be required to suit actual 
ground conditions, either potentiometer 34 or potentiometer 47 should be 
adjustable externally to any enclosure used to contain the circuitry. 
As an alternative, switching may be adjusted to occur at a 90.degree. 
leading phase angle to the signal caused by magnetite in place of the 
90.degree. lagging angle just described. To do this, switching should 
occur at the positive going, zero crossing instant of the waveform. In 
this case, the operational characteristics of the circuit will remain the 
same, except that the polarity of the response to metal objects will be 
reversed. 
To attain selective operation for the selective detection of high Q and low 
Q objects, such as copper coins and aluminum pull tabs, the instant of 
switching may be adjusted to occur at a 140.degree. leading phase angle to 
the signal caused by a sample of magnetite positioned near the search 
head. 
In FIG. 3, the phasor diagram and graph of the waveform illustrate the 
phasing which should exist between the signal caused by magnetite, and the 
instant of switching in order to obtain this selective operation. It 
should be noted that this phasing is used to obtain equal responses of 
opposite polarity for the signals of the penny and the pull tab. The 
selective characteristics may be changed by changing the phase angle to 
meet other requirements. 
As was the case with ground neutralized operation, the instant of switching 
is adjusted by means of potentiometer 34 and potentiometer 47. With the 
sample of magnetite positioned near the search head, and the oscilloscope 
connected as before, switching should be adjusted to occur at a 
140.degree. leading angle to the peak positive instant of the waveform 
appearing at the emitter of transistor 6. 
As an alternative, switching may be adjusted to occur at a 40.degree. 
lagging phase angle to the signal induced by magnetite, this angle being 
diametrically opposite to the 140.degree. leading angle. In this case, the 
operational characteristic of the circuit will remain the same, except 
that the polarity of the response to the penny and the pull tab will be 
reversed. 
The concept of the present invention may be employed in a metal detector 
system incorporating a selective detector circuit, and a ground 
neutralizing detector circuit. A block diagram of such a system is shown 
in FIG. 4, wherein a clamping circuit is utilized, and wherein the 
clamping circuit restrains the response of the selective detector circuit, 
except during those times when the clamping circuit is deactivated by the 
ground neutralizing detector circuit. 
Referring now to FIG. 4, a transmitter coil 101, corresponding to the 
transmitter coil 1 of FIG. 1, and a receiver coil 102, corresponding to 
the receiver coil 2 of FIG. 1, are placed in an overlapping position. 
Oscillator 103, corresponding to oscillator 3 of FIG. 1, supplies a sine 
wave signal with a frequency of 6 kHz to transmitter coil 101. As was done 
before, the coils are positioned so that the received signal is minimal, 
and a nulling signal source 104, corresponding to nulling signal source 4 
of FIG. 1, is used to diminish the signal of receiver coil 102 to zero. 
The output thus obtained is applied to a preamplifier 105, corresponding 
to the pre-amplifier 5 of FIG. 1, where the signals are amplified 
sufficiently to operate the remaining elements of the circuit. 
Specifically, the output of pre-amplifier 105 of FIG. 4, corresponds to 
the output of pre-amplifier 5 in FIG. 1. 
The output of pre-amplifier 105 is connected to a selective detector 
circuit 106. Selective detector 106 comprises the circuit of FIG. 1, but 
does not include the transmitter coil 1, receiver coil 2, oscillator 3, 
null signal source 4, or pre-amplifier 5 of FIG. 1, nor does it include 
the amplifier 26, meter 27, audio system 28, or conductor 29 of FIG. 1. 
Pre-amplifier 105 is connected to selective detector 106 via the coupling 
capacitor contained within selection detector 106 which is equivalent to 
capacitor 7 of FIG. 1. A reference signal is provided over conductor 107 
(corresponding to conductor 29 of FIG. 1) for the operation of selective 
detector 106, and is a sine wave signal supplied by oscillator 103. 
Selective detector 106 should be adjusted, as described previously, to 
detect the component of the received signal where the effect of high Q 
objects in proximity to the search head will be opposite to the effect of 
low Q objects near the search head. 
For the system of FIG. 4, operating at 6 kHz, the selective detector 106 
should detect that component of the received signal which is at a 
140.degree. leading phase angle to the signal caused by a sample of 
magnetite placed near the search head; when detection occurs at this 
angle, the effects of a penny and a pull tab will be approximately equal 
and opposite. The output of the selective detector 106 is a DC (direct 
current) voltage level. The response of the selective detector 106 to high 
Q and low Q objects is a change in the output voltage level, with the two 
classes of objects producing changes of opposite polarity. 
The output of selective detector 106 is connected to amplifier 108, a DC 
level amplifier. The function of amplifier 108 is to provide increased 
sensitivity and response to weak signals. In addition, amplifier 108 
should have a low output inpedance to operate into the clamping circuit 
109 to be described. Amplifier 108 is connected to clamping circuit 109 
through 15 .mu.f capacitor 110. 
In addition to being connected to selective detector 106, the output of 
pre-amplifier 105 is also connected to ground neutralizing detector 
circuit 111. The circuit of ground neutralizing detector 111 is identical 
to the circuit of selective detector 106, however ground neutralizing 
detector 111 is adjusted to exclude the response to ferromagnetic 
minerals. 
Pre-amplifier 105 is connected to ground neutralizing detector 111 via the 
coupling capacitor contained within ground neutralizing detector 111 which 
is equivalent to capacitor 7 of FIG. 1. A reference signal is provided 
over conductor 112 (corresponding to conductor 29 of FIG. 1) for the 
operation of ground neutralizing detector 111, and is a sine wave signal 
supplied by oscillator 103. 
Ground neutralizing detector 111 should be adjusted, as described 
previously, to detect the component of the received signal where the 
effect of ferromagnetic minerals in proximity to the search head will be 
minimal. To accomplish this, the circuit should detect that component of 
the received signal which is at a 90.degree. lagging phase angle to the 
signal caused by a sample of magnetite placed near the search head. The 
output of the ground neutralizing detector 111 is a DC voltage level. The 
response of the ground neutralizing detector 111 to all metal objects is a 
change of the same polarity in the output voltage level. 
The output of ground neutralizing detector 111 is connected to amplifier 
113, a DC level amplifier. The function of amplifier 113 is to provide 
increased sensitivity and response to weak signals. In addition, the 
output of amplifier 113 should not incur recoil or ringing effects after a 
metal object has passed through the field of the search head. To 
accomplish this, suitable damping circuits may be incorporated into the 
design of amplifier 113. Amplifier 113 is connected to clamping circuit 
109. 
Clamping circuit 109 is used to clamp the output signals of selective 
detector circuit 106 to a fixed voltage level except during those times 
when a change in signal level occurs in the output of ground neutralizing 
detector 111. The junction of capacitor 110 and clamping circuit 109 is 
connected to meter 114 and audio system 115 through buffer 116. Meter 114 
and audio system 115 are used to provide indication of the response of the 
system to metal objects in proximity to the search head, while buffer 116 
is used to isolate the clamping circuit 109 from signals generated by 
audio system 115. 
The operation of the system of the embodiment will now be described. FIG. 5 
contains waveforms of the responses of the system as the search head is 
moved over mineralized ground containing a penny (high Q object), and an 
aluminum pull tab (low Q object). The response of the output of the 
selective detector 106 is shown in the upper waveform of FIG. 5. It can be 
seen that the penny causes a positive going signal of short duration, and 
the pull tab causes a negative going response of short duration. In 
addition, the mineralized ground will produce random fluctuations in the 
output of selective detector 106 which will be similar to the responses to 
the metal objects. 
The center waveform of FIG. 5 shows the responses of the output of ground 
neutralizing detector 111 to the same situation. Here it can be seen that 
both the penny and the pull tab produce positive going signals, while the 
response caused by ground effects is insignificant. 
The lower waveform of FIG. 5 is of the signal of the selective detector 
106, with ground effects removed by the clamping circuit 109, which is 
applied to the meter 114 and audio system 115. During the times when metal 
objects cause the ground neutralizing circuit 111 to supply a signal, the 
clamping circuit 109 will be deactivated, and the response of the 
selective detector 106 will be transmitted to the meter 114 and audio 
system 115. Since the clamp, when deactivated, does not oppose a signal 
change of either polarity, both the positive going response caused by the 
penny, and the negative going response caused by the pull tab will be 
transmitted to the meter 114 and audio system 115. During the time when no 
metal object is near the search head, the clamping circuit 109 will block 
changes in the output of the selective detector 106 so that the response 
of the selective detector to ground effects will not be transmitted to 
meter 114 and audio system 115. Thus, meter 114 and audio system 115 will 
provide indication of the response to metal objects while excluding the 
response to ground effects. 
Capacitive coupling, provided by capacitor 110 in FIG. 4, is used in place 
of resistive or direct coupling between amplifier 108 and clamping circuit 
109. Therefore, signal level changes in the output of amplifier 108 which 
occur during times when clamping circuit 109 is in the ON state (clamped), 
will result in changes in the voltage differential across capacitor 110. 
Signal level changes in the output of amplifier 108 which occur during 
times when clamping circuit 109 is deactivated (unclamped) will be 
transmitted to meter 114 and audio system 115 via buffer 116 with the 
voltage differential across capacitor 110 remaining constant. Therefore, 
meter 114 and audio system 115 will provide indications only of signal 
level changes which occur during unclamped time, and will not be 
influenced by signal level changes which occur prior to unclamping. By 
this means, the correct selective response is assured in the situation 
where the brief response of the selective detector 106 to a metal object 
occurs coincidentally with a longer duration response in opposition caused 
by mineralized ground. 
A detailed description of the clamping circuit 109 will now be provided. It 
has been found that if the base to emitter junction of a transistor is in 
forward conduction, and the transistor is in saturation, then the 
collector will clamp to the emitter voltage level and that the collector, 
so clamped, will oppose or block an applied signal of either polarity. If 
forward conduction base to emitter ceases, a signal of either polarity 
applied to the collector will not be opposed or blocked. It should be 
understood that this is an unconventional application of a transistor, the 
collector is connected only to the applied signal source. The transistor 
is, in effect, being used as a switch, with collector to emitter 
conductance being controlled by base to emitter current. 
Referring now to FIG. 6, a schematic circuit diagram of a clamping circuit 
having a single transistor 117 is shown utilizing the principle just 
described. Current to maintain the base to emitter junction of transistor 
117 in forward conduction is supplied by resistor 118 connected to a +6 
volt source. Terminal 119 is connected to the collector of transistor 117. 
The junction of capacitor 110 of FIG. 4 and buffer 116 of FIG. 4 is then 
connected to terminal 119. The output of amplifier 113 of FIG. 4 driven by 
ground neutralizing detector 111 of FIG. 4 is connected to terminal 120, 
and then is coupled by capacitor 121 to the base of transistor 117. 
Proper operation of the clamping circuit of FIG. 6 requires that a negative 
going signal be applied to terminal 120 to cause unclamping. For this 
reason, the design of amplifier 113 should be such that a signal of 
negative polarity is applied to terminal 120 when a metal object passes 
through the field of the search head of the detector. 
The operation of the circuit, so connected, will now be described. Current 
supplied by resistor 118 to the base of transistor 117 will maintain the 
transistor in the ON or clamped state, unless a negative going signal is 
applied to terminal 120. Such a negative going signal, applied by 
capacitor 121 to the base of transistor 117 will result in a momentary 
loss of forward conduction, base to emitter, causing transistor 117 to be 
in the OFF or unclamped state. Thus, in the absence of a negative going 
signal applied to terminal 120, transistor 117 will be in the clamped 
state, and the signals of amplifier 108 will be dissipated across 
capacitor 110 and will not appear at terminal 119. However, when a 
negative going signal is applied to terminal 120, transistor 117 will be 
in the unclamped state, and the signals of amplifier 108 will appear at 
terminal 119. 
The circuit of FIG. 6, operating as described, will therefore prevent the 
signals of the selective detector 106 from appearing at terminal 119, 
unless the ground neutralizing detector 111 causes a negative going signal 
to be applied to terminal 120. Suitable values for the components of FIG. 
6 are as follows: 
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117 transistor 2N5828A 
118 resistor 47K 
121 capacitor 100.mu.f 
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Improved performance and smoother operation will result if the clamping 
circuit of FIG. 6 is replaced with a clamping and limiting circuit, as 
shown in FIG. 7. In this type of circuit, the signals of the selective 
detector 106, which are supplied to the meter and audio system during 
unclamped time, are limited by the amplitude of the signals supplied by 
the ground neutralized detector 111. FIG. 7 is a schematic of such a 
clamping and limiting circuit. Here, the junction of capacitor 110 of FIG. 
4 and buffer 116 of FIG. 4 is connected to terminal 119. Terminal 120 is 
connected to the output of amplifier 113 (this is the same arrangement 
that was used in the schematic of FIG. 6). The base of transistor 122 is 
directly connected to terminal 120. The emitter of transistor 122 is 
connected to circuit ground by resistor 123 and the collector is connected 
to a +6 volt source by resistor 124. The purpose of transistor 122 is to 
provide equal output signals of opposite polarity, in response to input 
signals applied to terminal 120. The cathode of diode 125 is connected to 
circuit ground. Diode 125 is held in conduction by resistor 126, connected 
from the anode of diode 125 to the +6 volt source. Thus connected, there 
is provided at the junction of the anode of diode 125 and resistor 126 a 
voltage with the voltage being positive with respect to circuit ground. 
The anode of diode 127 is connected to circuit ground. Diode 127 is held 
in conduction by resistor 128, connected from the cathode of diode 127 to 
a -6 volt source. Thus, there is provided at the junction of the cathode 
of diode 127 and resistor 128, a voltage, with the voltage being negative 
with respect to circuit ground. The collector of a transistor 129 is 
connected to a +6 volt source, and the base is connected to the junction 
of diode 125 and resistor 126. The collector of a transistor 130 is 
connected to the -6 volt source, and the base is connected to the junction 
of diode 127 and resistor 128. The emitter of transistor 129 is connected 
to the emitter of transistor 130. With the base of transistor 129 being 
held positive with respect to circuit ground by the voltage across diode 
125, and with the base of transistor 130 being held negative with respect 
to circuit ground by the voltage across diode 127, the voltage at the 
junction of the emitters of transistor 129 and transistor 130 is 
established at the level of circuit ground. It should be noted that 
transistor 129 must be of the NPN type to conduct on the positive voltage 
supplied by diode 125, and that transistor 130 must be of the PNP type to 
conduct on the negative voltage supplied by diode 127. 
The junction of the emitters of transistor 129 and transistor 130 is 
connected to terminal 119. Capacitor 131 is used to couple the junction of 
transistor 122 and resistor 124 to the junction of diode 125 and resistor 
126. Capacitor 132 is used to couple the junction of transistor 122 and 
resistor 123 to the junction of diode 127 and resistor 128. 
Proper operation of this circuit requires that a positive going signal be 
applied to terminal 120 to cause unclamping. For this reason, the design 
of amplifier 113 should be such that a signal of positive polarity is 
applied to terminal 120 when a metal object passes through the field of 
the search head of the detector. In addition, the DC level at terminal 120 
should be positive with respect to circuit ground in order to hold 
transistor 122 in conduction. 
The operation of the circuit connected as described will now be explained. 
While the voltage at terminal 120 remains constant, current through 
resistor 126 will hold diode 125 in conduction, and current through 
resistor 128 will hold diode 127 in conduction. The voltage appearing 
across diode 125 applied to the base of transistor 129, and the voltage 
appearing across diode 127, applied to the base of transistor 130 will 
establish the voltage at the junction of the emitter of transistor 129 and 
the emitter of transistor 130 at the level of circuit ground. 
While in this state, signals applied to terminal 119 will be clamped to 
circuit ground. However, if a positive going signal is applied to terminal 
120, transistor 122 will cause a positive going signal to be applied to 
capacitor 132, and an equal negative going signal to be applied to 
capacitor 131. The negative signal applied by capacitor 131 to the base of 
transistor 129 will permit a negative going signal applied to terminal 119 
to drive the emitter of transistor 129 negative with respect to circuit 
ground by an amount equal to, but not more than, the signal applied to the 
base of transistor 129. At the same time, the positive going signal 
applied by capacitor 132 to the base of transistor 130 will permit a 
positive going signal applied to terminal 119 to drive the emitter of 
transistor 130 positive with respect to circuit ground, by an amount equal 
to, but not more than, the signal applied to the base of transistor 130. 
Suitable values for the components of FIG. 7 are as follows: 
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122 transistor 2N5828A 
123 resistor 2.7K 
124 resistor 2.7K 
125 diode 1N 4148 
126 resistor 100K 
127 diode 1N 4148 
128 resistor 100K 
129 transistor 2N5828A 
130 transistor 2N5087 
131 capacitor 100.mu.f 
132 capacitor 100.mu.f 
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The circuit of FIG. 7 operating as described, will therefore prevent the 
signals of the selective detector 106 from appearing at terminal 119, 
unless the ground neutralizing detector 111 causes a positive going signal 
to be applied to terminal 120; in the latter case, the change in signal 
level which may occur at terminal 119 will be limited by the amplitude of 
the signal applied to terminal 120. 
It will be apparent that the clamping and limiting circuit of FIG. 7 could 
be replaced with a more complex circuit to provide more vigorous clamping 
action and reduced power consumption. 
Although the system of the invention is described in connection with a 
search head of the transmitter receiver type, a search head containing a 
single coil used as both transmitter and receiver could be employed. In 
this case, a selective detector circuit of the present invention, and a 
ground neutralizing detector circuit of the present invention, would 
provide response to changes in the sine wave signal appearing at the 
terminals of the single coil. Then, in the manner previously described, a 
clamping circuit of the present invention, being controlled by the ground 
neutralized detector circuit, would remove the effects of ferromagnetic 
minerals from the output of the selective detector circuit.