Electricity and Magnetism > AC Circuits DCS# 5L20.xx

METAL DETECTOR



APPARATUS
wavetek function generator 107-01-A
large coil 101-07-D
3 ft coaxial cable
015
audio amplifier 202-09-B
speaker
202-09-A
mixer / low pass filter circuit
202-20-B3
decade isolation amplifier
202-10-A
oscilloscope with camera 202
metal samples:  nonferromagnetic plate, iron
202-20-B3


DESCRIPTION
The signals from an LC oscillator and a function generator are mixed and the resulting difference frequency is amplified and sent to a speaker.  Any changes to L or C will shift the resonance frequency, the difference frequency, and the pitch heard from the speaker.

The circuit can be used to demonstrate (qualitatively):
the dependence of the LC resonance frequency on L and C
dependence of inductance on core material,
eddy currents
dependence of capacitance on distance between capacitor plates
how airport metal detectors work
how theremins work.
Circuit details:
The LC circuit has resonance frequency f1.  Its output is amplified and fed back to the circuit to maintain the oscillations.  The function generator is set to a frequency f2 just below f1 and the two signals are fed to a diode which acts as a mixer.  Because the diode is nonlinear, its output includes sum and difference frequencies.  The function generator is adjusted so that f1 - f2 is in the audio range.  A low pass filter reduces the high frequency components, and the remaining signal is amplified and sent to the speaker.

A ferromagnetic material placed in the core of the coil will increase L, decrease f1, decrease f1-f2 and the audible pitch drops.  A non-ferromagnetic material decreases L and the pitch is heard to rise.  If the non-ferromagnetic plate is placed perpendicular to the plane of the coil, little change in pitch is heard - the eddy currents are restricted by the thickness of the plate, illustrating the reason transformer cores are in the form of stacks of laminated sheets.

Connect an aluminum plate antenna to the LC circuit, and the circuit becomes a theremin.

NOTES

The coil inductance is about 6 mH.  The capacitance is provided by the cable and connections is ~ 700 pF, so the resonance frequency is around 75 kHz.
Set the gain of the decade amplifier to 10 x.
Set up the scope to display the LC oscillator signal and the output of the audio amplifier.
REFERENCE

Physics Education, Vol. 40, Issue: 2, February 01, 2005. pp. 167-171