| 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
