CA1171908A - Method and apparatus for detecting a conductive mineral - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

The invention is concerned with a method for detecting a conductive mineral or metal in a soil and for discriminating the magnetized minerals from the so detected conductive mineral. According to this method, a high-frequency signal is continuously transmitted with an oscillator comprising a loop whose inductance varies in the presence of a conductive mineral at the vicinity thereof while the high-frequency signal is continuously received, the loop is moved away from the soil and the frequency of the received signal is counted to give a reference value. Then, the frequency of the received signal is counted again while moving the loop over the soil to obtain an updated value. This updated value is continuously compared with the reference value; and an output signal is gi-ven whenever the updated value is higher than the reference value. The invention is also concerned with an apparatus for carrying out the above method.

Description

i-~7~ 8 The present invention relates to a ~ineral detector for locali~ing conductive minerals and for discriminating the magnetized minerals from the so localized conductive minerals.
~ umerous metal and/or mineral detectors of different structure are known and presently available on the market.
Most of these detectors make use of a " searching" coil or loop connected to an electrical circuit comprising a first oscillator having a frequency varying as a function of the signal received by a loop and a second oscillator having a fixed frequency. At rest, the varying and fixed frequencies are similar and, when mixed, glve a low signal, sometimes not-audible. When the loop encounters a conductive mineral, its inductance varies and, the difference of frequencies between the frequencies of the first and second oscillators, when mixed, produces an audible higher signal.
Examples of such metal detectors are illustrated by way of example in United States patent no. 3,492,564 issued to Leslie H. Baker jr. on January 27, 1970, U.S. patent no. 3,528,004 issued to Hayao Katabami on September 8, 1970 ~20 and U.S. patent no. 3,769,575 issued to Bruno A. Rist and James L. Tanner on October 30, 1973. These metal detectors all produce an audible tone indicative of the absolute variation of the frequency shift due to the presence of a metal or indicate the absolute variation with a dial.
United States patent no. 4,130,792 issued to John W. Sullivan on December 19, 1978 shows anothRr circuit com-prising a tuned sending loop which changes its resonance and Q when located near a metal object. Again, no provision is ~ made to detect the direction of a frequency shift.
United States patent no. 4,030,026 issued to George Calvin Payne on June 14, 1977 describes a circuit which comprises three coils. An electronical circuit permits to 1~71908 extract only the acti~e component ~rom the signal WhiCh is sampled. The described circuit comprises processing circuits connected to each coil.
In the northern countries that have or had a glacial cover~it is known that morainic deposits are commonly found.
These deposits include economically valuable ore such as gold, si:Lver, copper, etc. and magnetite which is without interest for the prospectors.
In accordance with the present invention, it has been found that when a magnetic mineral is " detected" by a search coil, the variation of frequency occurs in one direction while when any conductive mineral is detected, the variation occurs in the other direction. Thus, a mineral detector that could discriminate between a magnetized mineral and a conductive mineral would be of a great interest.
The main drawback of most of the known detectors is that they detect only the absolute variation of the frequency shift. Therefore, they cannot make the above-mentioned helpful discrimination.
It is also known that some prospection is made in hard climate areas where it is difficult to calibrate ins-truments or to protect the prospection material against bad or rough climatic conditions.
It is thus an object of the present inventlon to provide a prospection tool, namely a metal detector, which can be used without necessitating any critical calibration during prospection.
It is still another object to provide a metal detec-tor which is easy to use in rough soil areas and which even permits someone to prospect while doing other acti~ities.
In accordance with the invention, these objects are achieved with a metal and mineral detector for detecting a 11719~8 conductive mineral, comprising:
- means for transmitting a high-frequency signal, sald transmltting means compr.ising means for varying the frequency of the radio signal in the presence of a conduc-tive mineral at a vicinity thereof;
- means for receiving said high-frequency signal and providing square-wave signals to be processed, the square-wave signals havlng a frequency proportional to the frequency of the radio signal; and - processing means comprising resetting means, timing means, counting means, memory means, comparing means and output means operatively connected to each other to provide an output signal depending on the frequency of the received high frequency signal.
The resetting means are operatively connected to the timing means, counting means and`memory means so that the counting means count during a first period the square-wave signal frequency to give a reference value that the memory means keep.
T.he timing means are operatively connected to the counting means, memory means, comparing means and output means so that during a second and the consecutive periods called updating periods, the updating periods being similar to the first period, the counting means count the square-wave signals frequency giving an updated value;
the comparing means.compare the re~erence.value with the updated value and give a detection signal whenever the updated value is higher than the reference value;
and the output means actuate the output signal upon receipt of the detection signal.
The invention also provide a related method for 11719C~8 detecting a conductive mineral in a soil. This method comprises the steps of:
- continuously transmittinq a hi~h-frequency signal with an oscillator comprising a loop, this loop having an incluctance varying in the presence of a conductive mineral at the vicinity thereof;
- continuously receiving said high-frequency signal;
- moving the loop away from the soil and counting the frequency of the received signal, the counted frequency acting as reference value;
- then counting the frequency of the received signal while moving the loop over the soil to obtain an updated value;
- comparing the updated value with the reference value; and - giving a output signal whenever the updated value is higher than the reference value.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of a plurality of preferred embodiments thereof taken in connection with the accompanying drawings wherein:
Figure 1 shows an embodiment of a prospective cane according to the invention;
Figure 2 illustrates a bottom view of a sole com-prising an emitter;
Figure 3 shows a cut-view of the sole of figure 2;
Figure 4 shows the box located in the sole of Figs 1 and 2 and comprising the emitter;
Figure 5 is an electrical diagram of the emitter used for prospection;
Figure 6 is a block diagram of the receiver and li7~9Q8 processing circuit embodying the invention, Figure 7 is a diagram of the pre-amplifier circuit of the receiver of figure 6; and Figure 8 is a detailled diagram of a processing circuit embodying the invention.
The principle of operation of a metal detector with a searching coil is already known in the art.
Basically, a first circuit with a searching coil is located in a searching heaa which is used to scan the soil.
The coil is a part of a free-running oscillator circuit. ~
transmits a signal whose frequency depends on its inductance.
- When a metal or a mineral is detected, the inductance of the coil varies, as well as the oscillation frequency of the oscillator circuit. The frequency of the oscillator is compared with a fixed frequency and if a difference is detected, a signal is given to indicate that a metal core has been detected.
In the present invention, the comparison is made with a different method which gives improved results. Basically, the method according to the invention comprises two steps.
In a first step, the frequency of the osaillator is measured to give a reference value. To avoid that this reference value be too high or too low because of the presence of conductive or magnetic minerals in the neighbor-hood, the loop is moved away from the soil, at a distance suffisant to avoid any influence from the minerals or metals barried in the ground. This distance is preferably selected so as to correspond to the limit of sensibility of the detec-tor.~
Then, the frequency of the oscillator is sampled regularly and compared with the reference value. If the frequency increases, then a signal indicating the presence of 11719~3 metal is given and a sound alarm is tu~ned on. If i~ decreases, no signal is given.
The metal detector according to the invention basically comprises a first counter to count the reference frequency and a second counter to count the frequency at each moment. A clrcult compares together the outputs of both counters in order to detect any frequency variation and the direction of such variation and subsequently monitorlzes output circuit such as a loud-speaker, a dial or any suitable display.
Preferred embodiments of the various elements that form together the discriminating metal detector according to the invention will now be described.
A prospecting cane whose general aspect is widely known in the art of metal detection, is shown as first embodiment on figure 1. This cane includes a stlck 10, a foldable coil 12 connected to a process-sing module 16 and a handle 20. The processing module 16 includes a display 18 and a push bu~ton 22.
A box 24 containing the emitter is located near the coil. The box 24 can be designed to form a single unit with another enclosure containlng the coil. However, it has been diagrammatically shownlseparated from the coil for con-venience.
Opening and closing of the power supply of the emitter is controlled by a magnet 26 preferably hiden) but also shown in the drawings for the purpose of clarification.
When the coil 12 is folded, a magnetic switch located in the box 24 is opened by the magnet 26.
For prospecting purposes, the searching head of the cane, that is its coil 12, 1s scanned over the soil.
Another em~odiment of the discriminating metal 1~71~8 detector according to the invention, is shown in figure 2, 3 and 4. According to this embodiment, the emitter of the det:ector is carried in a felt sole 30 forming part of a working boot or in the heel of the boot while the proces-sing unit is carried by a belt.
The sole 30 carries a box 34 enclosing the emitter. A plastic housing 32 is also provided in the sole to physically protect the circuit. The position of the searching coil 36 around the general circuit is re~
presented in a diagrammatic manner in fig. 4. For pros-pecting purposes, the user wears a pair of boots, comprising the sole 30. The processing module is enclosed in a small box fastened to the belt of the prospector which can do something else during the prospection as both of this hands are free; the prospector will be advised by an audible sound of the presence of metals and will then pay attention to the indications of the processing module.
The box 34 gives a protection against dust, moisture and other dirty elements. It also provides an electrical shielding to the maynetic components it contains.
The electronic circuits used in the embodiments disclosed hereinabove will now be described.
The emitter circuit as such is shown on figure 5.
This circuit comprises a power supply including a battery 40, a magnetic switch 42, a voltage regulator 44 and a regulation capacitor 46. The regulator permits to stabilize the frequency of the emitted signal with res-pect to the power supply, so that the single source of substantial variation be the presence or the absence of minerals only.

~his power supply circuit furnishes a regulated working voltage to an oscillator comprising a digital li719~8 inverting gate ~8, a pair of capacito~s 52 a~d 54 and ~ coil 50. The oscillation frequency is about 1 MHz. The coil iS
preferably located in a shield 56 and the whole circuit is located inside another shield 58. This external shield 58 can be the box 24 or 34 protecting the whole circuit against the moisture and the dust, as explained hereinabove.
The magnetic switch 42 permits to open and close the power supply of the emitter without degrading the tightness of the box. For opening the circuit and thus closing the apparatus, after a prospection journey, for example, the switch is put near a magnet.
Obviously, other kinds of switches can be provided like manual switch, but the described magnetic switch is a very reliable switching system in hard climate. The other switches would be used under better conditions.
A functional diagram of the receiver and processing circuit is shown in figure 6.
The receiver comprises an antenna 60 that receives the radio wave transmitted by the emitter and furnlshes it to a receiver 62. This receiver converts the radio signal to a square-wave signal having digital levels compatible with the processing circuit.
At reset, a counter 64 is connected through a - multiplexer 66 to the receiver and counts the frequency of the square-wave signals to obtain a reference value related to the frequency of the emitter. This count must be made after having raised the searching coil from the soil so that it be far from any close metal. Such resetting is made automatically when the processing circuit ls powered and should be made reguIarly to avoid any frequency shift due to weat~er con-ditions and also to re-adjust the apparatus depending on the general electro-magnetic surrounding.

1~719~8 After this reset step, the c~cles of detection may begin.
At the beginning of each cycle, a~other counter 68 is resetted and connected to the receiver to count the frequency of the square-wave signals to give a corresponding value for comparison. ~t the end of each counting period, the value indicated by each counters 64 and 68 is compared by a comparator 70. If the value given by the counter 68 is higher than the reference value, a signal is given to an output unit 72 which then transmits an audible signal to a loud speaker 74. It also gives a visual indication, through a dial 76, of the intensity of the frequency shift.
A reset circuit 78 is also provided to enable the counter 64 to count only during the first period after the reset switch 80 has been depressed. At this moment, the counter 64 counts the reference value. ~ tlming circuit 82 is initialized by the reset circuit and provides control pulses for each counting period to control the duration of these periods and the proper resetting of the counters and other sequential digital circuits. A switch 84 is opened after the first period and only the counter 64 is resetted for each period to count the frequency for purpose of comparison.
The timing circuit is also connected to a 15 minutes delay which produces an audible signal about 15 minutes after reset, to indicate to the prospector that it is time to reset again. This timing circuit 86 is also connected to the output unit 72 and the loud-speaker 74. It conttains to re-adjust regulaxly the reference value.
A power supply 88 provides the necessary voltage for operation of the circuit, such as a 5V regulated voltage, and a general 9V voltage. This power supply is also con-nected to a battery state monitor 90 which commands the 11719~8 output unit to give an audible signal when the voltage of the battery is under a reference voltage.
The output unit 72 selects the proper tone cor-responding to the end of the 15 minutes delay and ~he low state of the battery, as well as the presence of metal and is therefore connected to the relevant circuits.
The various electronics circuits of the apparatus will now be described in detail.
The receiver section is shown in figure 7.
It comprises the antenna 60 and a four-sections amplifier-filter. The first section 621 is made of a low power video amplifier Ul. The second section 622 is made of two C-MOS inverting gate U2 and U3. The third section 623 also comprises a C-MOS inverting gate U4. These three sec-tions 621, 622 and 623 are shielded and the four active amplification circuits Ul, U2, U3 and U4 are taken from separated circuits to reduce the noise. The fourth section comprlses a C-MOS inverting gate U5A and furnishes a square-wave signal of the frequency of the received signal.
In this circuit, the values of the passive components are calculatedto give a clean 1 MHz square-wave signal.
The circuit shown in Fig.8 is an example of practical embodiment of the clrcuit generally shown in Fig.6.
In this much more detailed circuit, the multiplexer 66 of Fig 6 corresponds to a NOR-gate UlOa forming a first output and a NAND-gate U9a forming a second output; the counter 64 corresponds the 24-bit up-counters Ull, U16; and the counter 68 corresponds the 24-bit programmable down-counter U7, U15, U17. The comparator 70 is not a dedicated circuit, and its function is done as follows: the down counter is loaded with the value available at the output of the up-counter and when the output of the down-counter is zero, then 117~9~8 a zero detection signal is gi~en to indicate th~t ~he two counted value are equal. A latch U14a is also provided to keep the zero detection signal after the ze~o value has been detected because the down-counter will not stop counting after this zero detection.
The functions of the output unlt 72 are shared between a plurality of circuits including mainly the latch U14a, the counter U20, a monostable multivibrator U13b, a multiplexer U12 and transistors Q4 connected to a needle dial Ml and Q3 connected to a loud-speaker LSl.
The reset circuit 78 comrpises the switch Sl together witha pair of resistors R12 and R20 and capacitors C8 and C10. It co-operates also with the timing circuit.
The timing circuit 82 comprises mainly the 14-bit-counter U18 and the monostable multivibrator U13a. The 15 minute delay 86 is given by a 12-bit-counter U8.
The power supply 88 is built around a 9-volt bat~
tery Bl and a series of capacitors like C6 is also provided on the printed circuit board. - -The battery state monitor 90, comprises a comparator ~6 receiving on one input the voltage given by the battery and on another input a regulated voltage, suitable resistors being also provided for the purpose of the comparison.
The frequency chosen in this embodiment for the emitter is l-MHz, which is chose to the operating frequency of many C-MOS devices. The propagation delay of the various gates should thus be taken in account. It is also necessary to take into account the one count variation usually encounter-ed in frequency counters. For these reasons, programmable down-counters U20 and U21 are provided; they permit to add a variable offset value to the value counted in the first or the other counting period.

117190~
The operation of the circuit will ~ow be explained in details.
When the circuit is powered, the ~esistox R20 and the capacitor C8 gives a dealy so that a reset step begins at thLs moment. During the consecutive operation, the switch Sl is depressed and the capacitor C8 is discharged through the resistor R12. A protection diode CR7 is also provided.
As long as the switch 51 is depressed and for a short period after reset, the line R remains low and its inverted line R is high. Due to the connection of the capa-citor C10 and resistor R21, a second reset line R' also goes highbut for a shorter period. Thus, two reset signals R and R' occurs. During the reset R', the clock U18 and th~ 15-mlnute delay clock U8 are reset to zero while the latch U14c is reset and the latch U14d is set.
After the end of the reset R', the counter U18 is enabled and begins to count counting periods of 100 ms.
During the reset ~, the!counter U11, U16 and the latch U14b are reset while the counter U21 is loaded. The reference counting period, however, does not begin immediately.
When the switch Sl is again opened, the ring-counter U19 which was up to now reset and held to reset, is enabled to count.
It will count a first positive pulse and activates successively its outputs Ql, Q2, Q3 and Q4. As the output Q4 is connected to an enable input E, the counter Ul9 is then inhibited.
The output Q2 is active during a window of 100 mS
duration during which the multiplexer U9a, UlOa selects its second output U9a connected to the reference value counter Ull, U16. The square-wave signal is not counte~ immediately by the counter Ull, U16 which ls blocked by the NAND-gate U9b and the latch U14h. Instead, it will be counte* by the counter U21 which has been loaded with an offset value of about 255 -~1719~)8 (250 for example), depending on the ~arious dela~s in ~he circuit, as explained before. When this count of 250 pulses has been reached, the latch U14b is set and the NAND-gate U9b enables now the counter Ull, Ul6 to count the pulses frorn F ln for the remalning of the perlod. Thls perlod is 100 mS minus 250 ~S, as the period between each/pulse to be counted is about 1 ~S. At the end of the period, the mul-tiplexer second output U9a is deselected and no more pulses are counted by the counter Ul~ Ul6 which remains with a refer-ence value corresponding to the frequency at which the emitter oscillates when,there is no metal in its detection range.
It is also to be noted that during the reset period, proper inputs of the multiplexer U12 are selected so that its output bestable and no tone be audible.
After this first counting period after reset, the counter Ull, U16 indicates a reference value. The flrst output UlOa of the multiplexer UlOa, U9a is selected and is kept in this state until the next reset.
At the beginning of each counting period, pulse signals are given by th,e monostable multivibrator U13a at the beginning of each counting period, so that the counter U7, Ul5, Ul7 is loaded with the reference value; the latch Ul4a is set and its output goes high; and the counter U20 is loaded.
The pulses of the received square-wave signal F in are then counted by the down-counter U7, U15, U17 for a period of 100 mS. This period is longer than the reference counting period of about 255 ~S. Thus, about 255 ~S before the end of the period, the zero detection output ZD of the last section of counter Ul7 goes low.
When the signal from the multiplexer ls also low, ~ 7196;~3 the output 11 of the ~OR-gate UlOb goes high, reset~i~g the latch U14a. From this moment and for the remal~ing inter~al of the counting period, the output Q remains low. The input 6 of the NOR-gate U10c being connected to the reset line which ls low, the output of the NOR-gate U10c remains high.
The signal at output of NOR-gate U10c, rectified by the diode CR3 together with the RC circuit Rl-C9 is then amplified and averaged by the amplifier filter built around the transistor Q4 and operates a microammeter Ml. This microam-meter is calibrated with resistors R28 and R29 to indicatea value of zero when there is no variati~n of the fre~uency.
Protection diodes CRl and CR2 are also provided with other resistors R2, R3, R4 to complete the microammeter circuit.
Assuming that the remaining interval is of 254 pulses in this case, i the frequency increase, the remaining interval is longer than 254 pulses and the mean output is higher. This moves the needle of the dial in one direction. If the frequency decreases, the means output is lower and the needle of the dial is moved in the other direction. The position of the needle is then proportionnal to the variation of frequency.
The output of the latch U14a is also connected to the enable input 3 of the down-counter U20 programmed with an offset value of 253. At the zero-detection, this counter U20 begins to count until it reaches zero itself. This zero detection occurs only if there is an increase in the frequency, which causes an increase of the number of pulses which is then over 253. Thus, when metal is located near the emitter, the frequency thereof increases and a zero detection output signal occurs. On another hand, if magnetite is located near the emitter, the frequency thereof decreases and the zero detection output signal is not sent as the counting period ends ~efore that the counter U20 reaches the 7~908 zero value and the relevant counters and latches a~e ~esetted.
The zero-detection output 14 of counter U20 is connected to a pulse-shaping circuit comprising the NAND-gate U9c and the inverting gate U5c and the RC network R18-Cl.
The output of this buffering and shaping circuit is connected to a monostable multivibrator U13b which is connected to a RC
network R8-C4 having a RC constant of about .8 seconds.
The counting period is about .1 seconds; as long as a metal is detected and for about .7 seconds after, the output Q of the monostable multivibrator U13b is continuously high.
This output Q is connected to the multiplexer U12.
It commands the emission of a low frequency signal through the loud-speaker LSl to indicate that metal has been detected.
The dial indicates visually the same thing, adding an evalua-tion of the proximity and/or the size of the metal detected.
It also indicates the presence of magnetite, which does not influence the loud-speaker.
The 15 minute delay circuit comprises a twelve bit counter U8 connected to an output of the clock circuit U18.
One of a series of output Q9, Q10, Qll, Q12 is selected by a jumper so that different periods may be chosen depending on the duration required. With a connection in Ql0 as shown, the delay is about 13.65 minutes. Selecting other outputs the durection would be approximately 6.8, 27.3 or 54.6 minutes.
The exact duration of the 15 minutes delay is arbltrary and depends primarily of the clock frequency which is 10.24 kHz in this example and of the dividers employed.
Preferably, it has more than about 5 minutes.
This counter is reset at the reset and from this moment, begins to count.
The circuit as shown sends a 12-second signal at li719~8 the end of the delay. If the apparatus is not reSet, the electronic swltch Q5 is opened and the apparatu~ stops.
The circuit also comprises gates USd, U9d, UlOd, U5e and latches Ul~c and Ul4d diodes CR4 and CR5 forming a OR-gate with reslstor R31, transistor Q2, resistors R9 and Rl0 and capacitor C5.
The circuit, finally, may indicate that the battery is in a low power state. This function is made by a comparator U6 which will give an indication when the battery voltage will be under a given reference value. For this purpose, the 9V
voltage from the battery is compared through the resistor scaler Rl4, Rl5, Rl6 to a 5V regulated voltage obtained from the voltage regulator Ql. Rl5 is selected when the apparatus is tested so that the comparator U6 indicates a low-power state when the battery voltage is below a given value. Other cir-cuits for the proper operation of the comparator include resistors Rl3, Rl7 and R30 and capacitor C27.
In the above-described circuit, many variations not yet discussed can be made: for example, the clock frequency of the timer U18 can be varied. The output lS in this counter Ul8 can also be used instead of outputl so that the counting period be 400 mS and not l~0 mS. Obviously, the counting period must be selected in function of the capacity of the 24 bit counters and of the operating frequency of the emitter so that there is no overflow in the counters.
If necessary, the range of the counters can be modified.
Preferably, the counting period is sufficiently long to - avoid small disturbances and to use the counters near thén fuIl capacity without overflow.
In another embodiment, the offset values of counters U20 and U2l may be selected with DIP-switch or other similar switches selectively closed when the circuit is manufactured and tested.
The counting and comparating means can be ~a~ied.
For instance, the comparator circuit can comprise an adder or a subtractor which will add or substract an offset, to correct for the variation of delay in the circuits.
The exact arrangement of the circuits can be varied in function of the digital components available and of the physical arrangement of the printed circuit board carrying the clrcuits.
Finally, some elements of the circuit, such as the 15-minutes delay and the battery low power detector, are not necessary for the proper working of the circuit itself, but improve its performance. The delay permits to readjust constantly the reference value to follow deviations due to local surrounding including general magnetism. The low power detector avoids the incertainty on the proper working of the circuit in a area where metal is rare.
Concerning the general working of the C-MOS circuit, it has been noted that by using a 9V power supply instead of a 5V power supply, the operating frequency is increased, which is a pertinent factor when the emitter frequency is close to the maximum C-MOS operating frequency. It would be also possible to reduce the voltage and the frequency of the emitter or to use another type of digital circuits for components whose operating speed is critical.
The ground and power terminal connections of the various gates and latches integrated circuits are not illustrated. These connections are however standard.
While several embodiments of the invention have been described, it will be understood that it is capable of still further modi~ications and this application is intended to cover any variations, uses, or adaptations of the invention, li719~J18 following in general the principles of the in~ention and including such departures from the presen~ disclosure as to come within knowledge or customary practice in the art to whi.ch the invention pertain, andas maybe applied to the essential features hereinbefore set forth and falling within the scope of the invention or the limits of the appended claims.

Claims (24)

We claim.

1. A method for detecting a conductive mineral in a soil, said method comprising the steps of:
- continuously transmitting a high-frequency signal with an oscillator comprising a loop, said loop having an inductance varying in the presence of a conductive mineral at the vicinity thereof;
- continuously receiving said high-frequency signal;
- moving the loop away from the soil and counting the frequency of the received signal, said counted frequency acting as reference value;
- then counting the frequency of the received signal while moving the loop over the soil to obtain an updated value;
- comparing the updated value with the reference value; and - giving a output signal whenever the updated value is higher than the reference value.

2. Method as claimed in claim 1, wherein the com-paraison step is carried out after counting the updated value.

3. Method as claimed in claim 1, wherein the com-parison step is carried out simultaneously with the counting of the updated value and the output signal is given as soon as the updated value is found to be higher than the reference value.

4. A mineral detector for detecting a conductive mineral, comprising:
- means for transmitting a high-frequency or radio signal, said transmitting means comprising means for varying the frequency of the radio signal in the presence of a conduc-tive mineral at a vicinity thereof;

- means for receiving said high-frequency signal and providing square-wave signals to be processed, said square-wave signals having a frequency proportional to the frequency of the radio signal; and - processing means comprising resetting means, timing means, counting means, memory means, comparing means and output means operatively connected to each other to provide an output signal depending on the frequency of the received high frequency signal;
said resetting means being operatively connected to the timing means, counting means and memory means so that the counting means count during a first period the square-wave signal frequency to give a reference value that the memory means keep ;
said timing means being operatively connected to the counting means, memory means, comparing means and output means so that during a second and the consecutive periods called updating periods, said updating periods being similar to said first period, the counting means count the square-wave signals frequency giving an updated value;
said comparing means comparing the reference value with the updated value and giving a detection signal whenever said updated value is higher than said reference value; and said output means actuating the output signal upon receipt of the detection signal.

5. A mineral detector as claimed in claim 4, wherein said transmitting means comprises a free-running oscillator circuit including a loop which changes its im-pedance when located at the vicinity a conductive mineral and thus changes the frequency of said free-running oscil-lator.

6. A mineral detector as claimed in claim 5, wherein said loop is an air-core coil having an electrostatic shield.

7. A mineral detector as claimed in claim 5 or 6, wherein said free-running oscillator comprises an inverter having an input and an output, and a first and a second capacitors connected respectively between the input and a common terminal and between the common terminal and the output, said loop being connected between the input and the output of said inverter.

8. A mineral detector as claimed in claim 5, further comprising a battery for powering said transmitting means and a magnetic switch mounted between the battery and the transmitting means, said switch opening in the presence of a magnet and disconnecting the transmitting means.

9. A mineral detector as claimed in claim 8, wherein the transmitting means, the switch and the battery are located in a hermetically closed sole.

10. A mineral detector as claimed in claim 8, wherein the transmitting means, the switch and the battery are located in a removable hermetically closed block, said block being located in a recess provided in a sole.

11. A mineral detector as claimed in claim 5, wherein the loop is foldably mounted at one extremity of a cane, the other components of said transmitting means being carried by said loop and comprising a magnetic switch controlling the operation of said transmitting means and a magnet located on said cane to act on said magnetic switch when the loop is folded, to disconnect said transmitting means.

12. A mineral detector as claimed in claim 11, wherein said receiving and processing means are mounted near the other extremity of the cane, without any electronic material connection with the transmitting means.

13. A mineral detector as claimed in claim 4, wherein said receiving means comprise a resonnant antenna circuit to pick up the radio signal, a circuit to amplify to a compatible level said radio signal and to filter said radio signal and a pulse-shaping circuit to convert said radio signal to said square-wave signals having logical levels.

14. A mineral detector as claimed in claim 4, wherein said processing means further comprise a demultiplexer having a data input for receiving said square wave signals, a first and a second outputs and a selection input for select-ing one of said outputs and inhibiting the other output, said counting, memory and comparing means comprising together a first up-counter having an input connected to the first output of said demultiplexer and a second programmable down-counter connected to the second output of said demulti-plexer, said multiplexer, up-counter and down-counter being operatively connected to the timing means, whereby a) an initialization step is provided during which step:
- before the first period, the up-counter is preset-ted and the first output of the multiplexer is selected, - during the first period, the up-counter is enabled to count said square wave signals, - after the first period, the reference value is available at an output of the up-counter, and b) a repeated detecting step is provided during which step:
- before each updating period, the reference value is loaded in the down-counter and the down-counter output of the multiplexer is selected, - during each updating period, the down-counter is enabled to count said square-wave signals, a zero detection output being activated when the count of the down-counter is equal to zero, - said zero detection output being connected to a memory device providing on an output said detection signal, said output being kept active for a duration at least equal to the updating period when the zero detection output is ac-tivated to give a continuous detection signal, an offset value being provided in said counting means so that said zero detec-tion output is given only if the frequency of the square-wave signals at this instant is higher than the frequency of the square-wave signal during the first period, added to said offset value.

15. A mineral detector as claimed in claim 14, wherein the first and second period are equal, the up-counter is programmable and presetted to said offset value and all the counters have the same range.

16. A mineral detector as claimed in claim 14, wherein said timing means are controlled by a crystal elec-tronic clock and comprises a series of frequency dividers and logical gates to generate signals for controlling said multiplexer and counters.

17. A mineral detector as claimed in claim 14, wherein the resetting and timing means comprise a reset switch which starts said initialization step and said repeated detection step at a regular rate until the reset switch is depressed again or the detector is stopped.

18. A mineral detector as claimed in claim 14, further comprising a third and a fourth programmable counter and a second latch, said second latch being resetted and said third counter being loaded before the first period, the third counter having a counting input connected to the up-counter output of the multiplexer and giving a signal when it reaches zero, said second latch receiving the signal from said third counter to be setted, said latch inhibiting said first counter when it is resetted, the fourth counter being loaded and the first latch being setted at the beginning of each updating period, the first latch being resetted when the second counter reaches zero, said first latch inhibiting said fourth counter when it is setted, said fourth counter when enabled counting the square wave signals, the third and fourth counters having programming inputs connected to offset values choosen so that said fourth counter reaches zero only if the mean frequency of the square-wave signals increases.

19. A mineral detector as claimed in claim 4, wherein said counting and memory means comprises a first and a second counters and said comparing means comprises a comparator, said first counter being enabled to count said square-wave signals during said first period to give the reference value and keeping it, said second counter being enabled to count the square-wave signals during each updating period and giving at the end of each updating period the updated value, the comparator comparing the reference value and updated value and giving a detection output if the updated value is higher than the value.

20. A mineral detector as claimed in claim 19, wherein a offset is added to said first value so that the second value must be greater than the first value plus the offset value to give a detection output.

21. A mineral detector as claimed in claim 20, wherein one of the counter is a programmable counter and said offset is given by the preprogrammed value of said programmable counter.

22. A mineral detector as claimed in claim 4, wherein said output means comprises an audible frequency oscillator connected to a loud-speaker.

23. A mineral detector as claimed in claim 4, wherein said output means comprise means to produce a mean voltage proportional to the difference between the first and the second value, and a meter indicating a reading pro-portionnal to said difference.

24. A mineral detector as claimed in claim 4, wherein the timing means comprises a long period counter resetted when a reset switch is depressed, said long period counter actuating an audible signal when said long period is expired to indicate that the reset switch has not been depressed since the beginning of said long period, and that recalibration is necessary, said long period being at least 5 minutes, and closing the detector if said reset switch is not depressed before the expiration of another fixed period.