GB1585485A - Metal detectors - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO METAL DETECTORS (71) We, ALERT ENTERPRISES (H.K.) LTD., a Hong Kong Company, of Hip Sun Industrial Building, 21-23 Chi Kiang Street, 5th Floor, Block "B", Kowloon, Hong Kong, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a metal detector.

Metal detectors are known which operate on the principle that when a search coil is incorporated into an oscillator, the approach of a conductor to the search coil results in a change of the frequency of oscillation of the oscillator.

This change in frequency may be detected by comparison with a reference frequency the resulting beat frequency being indicative of the presence of a conductor near the search coil.

Such known metal detectors suffer from the disadvantage of requiring a stable oscillator and amplifier as well as shielded cables and the present invention seeks to mitigate at least one of these disadvantages.

In accordance with the present invention there is provided a metal detector comprising a handle connected by way of a tube to a search coil assembly section, the search coil assembly section containing an oscillator which incorporates a search coil assembly and the amplitude of the oscillations of which depends upon the proximity of a metal object, and a preamplifier and detector for amplifying and detecting the output signal of the oscillator and the handle containing circuitry responsive to the output signal of the preamplifier and detector crossing a threshold level for generating an audible signal indicative of the proximity of a metal object, a power supply being further incorporated in the handle and the circuitry in the handle being connected to the circuitry in the search coil assembly section by way of a cable passing through the tube.

The oscillator circuit may suitably comprise a tank circuit including a primary winding of the search coil assembly and an unshielded feedback winding. The omission of a Faraday shield from the windings of the search coil assembly simplifies construction and reduces costs.

Advantageously, a control is provided on the handle to vary the sensitivity of the metal detector.

The invention will now be further described, by way of example, with reference to the drawings accompanying the Provisional Specification in which: Figure I is a side elevation of a metal detector in accordance with the invention; and Figure 2 shows the circuit of the metal detector.

In Figure 1, there is shown a metal detector which comprises a telescopically collapsible tube 10, a search coil assembly 12 at one end of the tube and a handle 14 at the other end. The tube is provided with a resilient sleeve 16 which fractionally resists expansion and collapse of the tube. The handle 14 incorporates a grip portion, coarse and fine sensitivity control knobs 8 (arranged one on each side of the handle), a loudspeaker 20 and a compartment (not shown) for containing circuitry and a power supply, which may suitably be a 9V battery.

In Figure 2, the search coil assembly 12 and the handle 14 have been represented schematically by dotted lines which have been allocated the same respective reference numerals in order to indicate the location of the elements of the circuit within the metal detector.

In the search coil assembly section, there are provided inductively coupled windings Ll and L2 constituting the search coil assembly which are connected together with a capacitor C1 and a transistor Ql to form an oscillator. The principle of operation of the oscillator is well known and the amplitude of oscillations depends on the supply voltage. The supply voltage to the oscillator is stablized by a Zener diode Dl in the search coil assembly section and a resistor Rl 1 and a capacitor C6 in the handle so that the amplitude of oscillations is essentially determined by the Q factor of the oscillator which is in turn dependent upon the presence of a metal within proximity of the search windings.The losses associated with eddy currents induced in the metal lower the Q factor of the oscillator and thus reduce the amplitude of oscillations.

During oscillation, a signal is taken from the coil L2 and applied to a pre-amplifier and detector circuit comprising a transistor Q2 and a resistor R4. The base bias voltage of transistor Q2 exceeds the emitter potential only by the forward bias across a diode D2 connected to the cathode of the Zener diode Dl by way of a resistor R3. Thus, transistor Q2 amplifies positive half cycles of oscillation and is off during negative half cycles. The provision of the diode D2 is the base bias circuit of transistors Ql and Q2 serves to stabilize the circuit against temperature fluctuations. The amplified and detected oscillations are applied to the circuitry in the handle by way of a wired cable. The handle also includes a pair of variable resistors Rl and R2 which constitute coarse and fine control knobs for varying the sensitivity of the metal detector.

The variable resistors Rl and R2 are arranged within the emitter circuit of the transistor Ql to vary the amplitude of oscillations and they additionally provide negative feed back to stabilize the amplitude of oscillations of the oscillator at any given Q factor. The signal derived from the collector of transistor Q2 is smoothed by a smoothing circuit comprising a capacitor C2 housed within the search coil section and by a circuit consisting of capacitors C3 and C4 and resistors R5 and R6 arranged within the handle section so that there is developed across the resistor R6 a voltage dependent upon the amplitude of oscillations.

When the voltage across R6 lies below a predetermined level, the transistor Q3 is switched off. However when the voltage rises above a predetermined level then there commences oscillation of an audio frequency oscillator which comprises the transistor Q3, a further transistor Q4, a feed back capacitor C5 and resistors R7, R8 and R9. The principle of oscillation of this oscillator is also well understood and the oscillator may be considered as an amplifier with positive feed back through the resistor R7 and the capacitor C5. The oscillatory signal derived from the collector of the transistor Q4 is applied by way of a resistor R10 to a power amplifier transistor Q5 in the collector circuit of which there is arranged a speaker LS which provides an audible tone when the audio frequency oscillator is in operation.

In use, the resistors are RI and R2 are set such that in the absence of a metal the level of oscillations is sufficiently high so that the voltage developed across the resistor R6 is insufficient to initiate oscillation of the audiofrequency oscillator. When a metal object comes within proximity of the search coil windings, the amplitude of oscillations of the oscillator circuit including the search coils the capacitor Cl and the transistor Ql drops and the mean potential at the collector of the transistor Q2 rises. The mean potential obtained by smoothing the output of the transistor Q2 initiates oscillation of the audio frequency oscillator and results in the production of an audible tone indicative of the proximity of a metal object.

WHAT WE CLAIM IS: 1. A metal detector comprising a handle connected by way of a tube to a search coil assembly section, the search coil assembly section containing an oscillator which incorporates a search coil assembly and the amplitude of the oscillations of which depends upon the proximity of a metal object, and a preamplifier and detector for amplifying and detecting the output signal of the oscillator and the handle containing circuitry responsibe to the output signal of the preamplifier and detector crossing a threshold level for generating an audible signal indicative of the proximity of a metal object, a power supply being further incorporated in the handle and the circuitry in the handle being connected to the circuitry in the search coil assembly section by way of a cable passing through the tube.

2. A metal detector as claimed in Claim 1, wherein the tube is telescopically collapsible.

3. A metal detector as claimed in Claim 1 or Claim 2, wherein the circuitry responsive to the output signal of the preamplifier and detector includes an audio frequency oscillator arranged to oscillate only when the said threshold level is crossed, the output of the audio frequency oscillator being connected to a loudspeaker to produce an audible tone.

4. A metal detector as claimed in any preceding Claim, in which the oscillator sensitive to the proximity of a metal object comprises a tank circuit which includes a primary winding of the search coil assembly and a feedback winding, the feedback winding being unshielded.

5. A metal detector as claimed in any preceding Claim, wherein a control is provided on the handle to vary the sensitivity of the metal detector.

6. A metal detector as claimed in any preceding Claim, wherein means are provided for stabilising the supply voltage to the oscillator.

7. A metal detector constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. During oscillation, a signal is taken from the coil L2 and applied to a pre-amplifier and detector circuit comprising a transistor Q2 and a resistor R4. The base bias voltage of transistor Q2 exceeds the emitter potential only by the forward bias across a diode D2 connected to the cathode of the Zener diode Dl by way of a resistor R3. Thus, transistor Q2 amplifies positive half cycles of oscillation and is off during negative half cycles. The provision of the diode D2 is the base bias circuit of transistors Ql and Q2 serves to stabilize the circuit against temperature fluctuations. The amplified and detected oscillations are applied to the circuitry in the handle by way of a wired cable. The handle also includes a pair of variable resistors Rl and R2 which constitute coarse and fine control knobs for varying the sensitivity of the metal detector. The variable resistors Rl and R2 are arranged within the emitter circuit of the transistor Ql to vary the amplitude of oscillations and they additionally provide negative feed back to stabilize the amplitude of oscillations of the oscillator at any given Q factor. The signal derived from the collector of transistor Q2 is smoothed by a smoothing circuit comprising a capacitor C2 housed within the search coil section and by a circuit consisting of capacitors C3 and C4 and resistors R5 and R6 arranged within the handle section so that there is developed across the resistor R6 a voltage dependent upon the amplitude of oscillations. When the voltage across R6 lies below a predetermined level, the transistor Q3 is switched off. However when the voltage rises above a predetermined level then there commences oscillation of an audio frequency oscillator which comprises the transistor Q3, a further transistor Q4, a feed back capacitor C5 and resistors R7, R8 and R9. The principle of oscillation of this oscillator is also well understood and the oscillator may be considered as an amplifier with positive feed back through the resistor R7 and the capacitor C5. The oscillatory signal derived from the collector of the transistor Q4 is applied by way of a resistor R10 to a power amplifier transistor Q5 in the collector circuit of which there is arranged a speaker LS which provides an audible tone when the audio frequency oscillator is in operation. In use, the resistors are RI and R2 are set such that in the absence of a metal the level of oscillations is sufficiently high so that the voltage developed across the resistor R6 is insufficient to initiate oscillation of the audiofrequency oscillator. When a metal object comes within proximity of the search coil windings, the amplitude of oscillations of the oscillator circuit including the search coils the capacitor Cl and the transistor Ql drops and the mean potential at the collector of the transistor Q2 rises. The mean potential obtained by smoothing the output of the transistor Q2 initiates oscillation of the audio frequency oscillator and results in the production of an audible tone indicative of the proximity of a metal object. WHAT WE CLAIM IS:

1. A metal detector comprising a handle connected by way of a tube to a search coil assembly section, the search coil assembly section containing an oscillator which incorporates a search coil assembly and the amplitude of the oscillations of which depends upon the proximity of a metal object, and a preamplifier and detector for amplifying and detecting the output signal of the oscillator and the handle containing circuitry responsibe to the output signal of the preamplifier and detector crossing a threshold level for generating an audible signal indicative of the proximity of a metal object, a power supply being further incorporated in the handle and the circuitry in the handle being connected to the circuitry in the search coil assembly section by way of a cable passing through the tube.

2. A metal detector as claimed in Claim 1, wherein the tube is telescopically collapsible.

3. A metal detector as claimed in Claim 1 or Claim 2, wherein the circuitry responsive to the output signal of the preamplifier and detector includes an audio frequency oscillator arranged to oscillate only when the said threshold level is crossed, the output of the audio frequency oscillator being connected to a loudspeaker to produce an audible tone.

4. A metal detector as claimed in any preceding Claim, in which the oscillator sensitive to the proximity of a metal object comprises a tank circuit which includes a primary winding of the search coil assembly and a feedback winding, the feedback winding being unshielded.

5. A metal detector as claimed in any preceding Claim, wherein a control is provided on the handle to vary the sensitivity of the metal detector.

6. A metal detector as claimed in any preceding Claim, wherein means are provided for stabilising the supply voltage to the oscillator.

7. A metal detector constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.