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Making
a metal
detector
was always
my
dream.
I was
curious
listening
to those
beeps at
airport
for which dad
explained
that they
were metal
detectors. Which
did not
mean
anything
to me.
Last
summer I
had opportunity
to find
secrets
behind
this
little
stuff from
my dad
which
turnout to
be simple
project
one can
make at
home.
Recently
I decided
to make
one for my
school
anyway. I
spent some
time with
my dad
&
found that
things
were not
very
tough!!.
And then,
we found
that we
had
everything
in those
component
boxes that
I have
been
arranging
for dad
for last
few years.
Finally, I
understood
that these
boxes were
"Useful"
!!
Basic
Principle: Metal
detectors
have two
oscillators.
One is
tunable
and the
other is
"Fix"
with a
loop. Both
are
"Tuned"
at same
frequency.
Outputs
from this
two
oscillators
are then
mixed in a
mixer
which
produces
only the
difference.
Means, if
both
oscillators
are at the
same
frequency,
there is
nothing
heard in
the
speaker. To
start
with, you
need to
"Zero
beat"
both
oscillators
or say
"Null"
them while
keeping
your
detector
in the
free air,
away from
metal
objects.
Use the
tuning
control
provided
for this
purpose. Once
this is
done,
start
placing
detector
around
metal 
objects
and you
will
notice
that
"Zero
beating"
is getting
disturbed
and an
audio tone
from the
speaker is
heard.
Wow, the
detector
has found
a metal
& its
working!! The
Circuit: Both
oscillators
are built
using
MPF102 or
J310 FETs.
The Mixer
is BF998
dual gate
MOSFET.
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Audio IC
amplifier
is LM386
which
drives a
speaker. I
have used a
stabilized
supply for
two
oscillators
and mixer.
U1, a
78L05 is
doing this
job
nicely. |
Circuit
is being designed
on a
Single
Sided
Glass
Epoxy
Printed
Circuit
Board. It
will be
using PCB
mounted
volume
control
and a
variable
capacitor
to tune
the
oscillator.
(or
varicaps
& a
pot)
However,
final
design may
change. You
will have
to wind
the Loop
Coil
yourself. Contraction
details
are given
in the
project
document
given
below for
download. I
am happy
that now I
have a
metal
detector
constructed
by me and
also
learnt how
it works,
how to
design
simple
oscillators
and make
PCB
drawing
for my projects. I
hope you
will enjoy
this small
kit
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