Metal detector coil resistance testing

Apparatus for testing the resistance of a detection coil in a magnetic metal detector, the magnetic metal detector having a balanced resistor network including first and second resistors and a bias voltage source connected to a first end of the first and second resistors, and differential amplifier means for producing an output signal, a second end of said first resistor being coupled to a first end of said detection coil and a first input of said differential amplifier means and a second end of said second resistor being coupled to a second end of said detection coil and a second input of said differential amplifier means, said apparatus further comprising:

• means for injecting a test signal of varying magnitude into said metal detector at a point between a second end of one of said first and second resistors and an end of said detection coil to thereby develop across said detection coil a potential difference having a magnitude related to the resistance of the detection coil whereby said differential amplifier means produces an output signal having a magnitude that varies as the magnitude of said test signal varies and is related to the resistance of said detection coil;
• means, operative while said test signal is varying, for comparing the magnitude of the output signal from said differential amplifier means with first and second threshold magnitudes representing the maximum and minimum magnitudes, respectively, that said output signal should have when the resistance of said detection coil is within a normal range;
• and,
• means for producing an indication that said detection coil is defective when the magnitude of said output signal is greater than said first threshold magnitude or less than said second threshold magnitude at any time during injection of said test signal.

Apparatus for testing the resistance of a detection coil in a magnetic metal detector wherein the detection coil has first and second ends connected to first and second inputs of a detector circuit which produces an output signal having a magnitude dependent on a difference in potential at said first and second inputs, said apparatus comprising:

• means for injecting a test signal of varying magnitude into the detection coil to produce across said detection coil and said first and second inputs a potential difference that is dependent on the resistance of the detection coil;
• comparing means, operative while said test signal is varying in magnitude, for repetitively comparing the magnitude of the output signal produced by said detector circuit with first and second reference signal values representing a maximum and a minimum magnitude, respectively, that said output signal may have when the resistance of said detection coil is within a normal range;
• and
• means for producing an indication that the resistance is outside said normal range if the magnitude of said output signal becomes greater than said first reference signal value or less than said second reference signal value at any time while said test signal is being injected.