Abstract: A metal detector circuit includes a transmit coil (12) and a receive coil (56) arranged in a balanced induction configuration in an electromagnetic field. The receive signal from the receive coil (56) is input to electronic switches (146, 148) which receive quadrature reference inputs from a phase shift circuit (112). The phase demodulated outputs of the switches (146, 148) are passed through amplifiers and input to an analog-to-digital converter (324) to produce digital signal samples which are transmitted through a bus (326). The bus (326) is connected to random access memory (414, 416) and a read only memory (418) which includes a stored signal processing program. A microprocessor (374) is connected to the bus (376) for receiving the digital signal samples and the stored program from memory (418). The microprocessor (374) executes the stored signal processing program to produce a digital output signal which is transmitted through the bus (326) to a digital-to-analog converter (360).

Abstract: A metal detector circuit of the balanced induction type is utilized to detect hidden metal objects. A receive signal is provided with two fixed concurrent phase shifts relative to the transmit signals. Each of the phase shifted receive signals is synchronously demodulated to produce first and second vector signals. The vector signals are combined in a summation circuit to produce a third vector signal. The first and third vector signals are then selectively combined by an operator to produce a fourth vector signal having a desired vector range between minimum and maximum vector angles. The metal detector circuit of the present invention provides a response for objects having a phase response signal outside the selected phase range. The first and fourth vector signals are each differentiated twice then level detected. The outputs from the level detection process are logic signals which are input to an AND gate.

Abstract: A metal detector is disclosed which has a transmit coil, a receive coil, and circuit means for detecting metal objects due to a change in the magnetic coupling between the coils. Circuit board arrangement techniques are disclosed which enable the close placement of the transmit and receive coils and the circuit board, thereby allowing packaging of the metal detector in a small package. A low battery voltage indicator circuit is provided to warble an audio indicator signal to indicate sufficient battery voltage. In the event the battery voltage drops below a certain level, the warbling of the indicator signal ceases and a steady tone is generated thereby indicating a low battery voltage condition.

Abstract: A metal detector is disclosed which has a transmit coil, a receive coil, and circuit means for detecting metal object due to a change in the magnetic coupling between the coils. Circuit board arrangement techniques are disclosed which enable the close placement of the transmit and receive coils and the circuit board, thereby allowing packaging of the metal detector in a small package. A low battery voltage indicator circuit is provided to warble an audio indicator signal to indicate sufficient battery voltage. In the event the battery voltage drops below a certain level, the warbling of the indicator signal ceases and a steady tone is generated thereby indicating a low battery voltage condition.

Abstract: A low drift rate tuning circuit (10) is operated in conjunction with a feedback network (36). An operational amplifier (12) is connected to first and second power supply buses (18, 20). The negative input terminal (16) of the amplifier is connected to the lower voltage supply bus (20). The output of the amplifier (12) is connected to an inverting amplifier (51). The output from the circuit (10) is connected as the input of the feedback network (36) and the input to the circuit (10) is connected to the output of the feedback network (36). In the seek mode the circuit (10) balances the voltages at the input terminals of the operational amplifier (12). In the balance condition the common mode input voltage to the operational amplifier (12) is essentially zero at which voltage the input leakage current is at a minimum. With the leakage current reduced to a minimum the circuit is made more stable by reason of reduced drift over time.

Abstract: A metal detector (10) is disclosed which has a transmit coil (48) and a receive coil (150) for detecting a metal object due to a change in the magnetic coupling between the coils (48, 150). The metal detector (10) has a plurality of operating modes and for each mode an operating parameter is selected and stored. Each operating parameter is used to generate a demodulation signal. A multiposition switch (36) is manipulated by an operator to select one of the operating modes. A logic circuit (191) is provided to route the demodulation signals to a demodulation circuit (110) to operate on a receive signal from the receive coil (150). An audible signal produced by the metal detector (10) is inhibited when operating modes are changed. An automatic tuning circuit (228) has selectable slewing rates for nulling the output from the metal detector (10).

Abstract: A metal detector (10) is disclosed which has a transmit coil (48) and a receive coil (150) for detecting a metal object due to a change in the magnetic coupling between the coils (48, 150). The metal detector (10) has a plurality of operating modes and for each mode an operating parameter is selected and stored. Each operating parameter is used to generate a demodulation signal. A multiposition switch (36) is manipulated by an operator to select one of the operating modes. A logic circuit (191) is provided to route the demodulation signals to a demodulation circuit (110) to operate on a receive signal from the receive coil (150). An audible signal produced by the metal detector (10) is inhibited when operating modes are changed. An automatic tuning circuit (228) has selectable slewing rates for nulling the output from the metal detector (10).

Abstract: A metal detector (10) is disclosed which has a transmit coil (48) and a receive coil (150) for detecting a metal object due to a change in the magnetic coupling between the coils (48, 150). The metal detector (10) has a plurality of operating modes and for each mode an operating parameter is selected and stored. Each operating parameter is used to generate a demodulation signal. A multiposition switch (36) is manipulated by an operator to select one of the operating modes. A logic circuit (191) is provided to route the demodulation signals to a demodulation circuit (110) to operate on a receive signal from the receive coil (150). An audible signal produced by the metal detector (10) is inhibited when operating modes are changed. An automatic tuning circuit (228) has selectable slewing rates for nulling the output from the metal detector (10).