Elenco Electronics EP-130 - Audio Metal Detector

-145
-
This experiment demonstrates how a metal detecto
r
works. When the coil
g
ets close to somethin
g
that i
s
made of metal, the oscillator chan
g
es in frequency.
This t
y
pe of metal detector has been used to locate
l
ost treasures,
b
ur
i
e
d
p
i
pes,
hidd
en
l
an
d
m
i
nes, an
d
so on. These have been used to save many lives b
y
locatin
g
mines and booby traps set out by the enem
y
d
ur
i
n
g
wart
i
me
.
Thi
s c
i
rcu
i
t
i
s a
l
ow
di
stort
i
on osc
ill
ator t
h
at
d
raw
s
only one milliamp
f
rom the 9V supply. Usin
g
lo
w
power allows the nearby metal to have maximu
m
e
ff
ect on oscillation
f
requency
.
You need a small transistor radio to use as the
detector; tune it to a weak AM broadcast station
.
Adj
ust t
h
e tun
i
n
g
capac
i
tor unt
il
you
h
ear a
l
ow-
f
requency beat note; this beat note is the di
ff
erence
between the si
g
nal of a broadcast station and this
osc
ill
ator.
D
o not
b
r
i
ng t
h
e ra
di
o any c
l
oser t
h
an
necessar
y
. The best position is where the levels of th
e
two signals are about equal, because this give
s
max
i
mum sens
i
t
i
v
i
ty
.
Try using keys, plastic objects, coins, and so on, a
s
sample objects. Of course, a real metal detector doe
s
not have a small
f
errite coil like this. It usuall
y
uses a
Farada
y
electrostatic shield, which is an air-core coil
s
hi
e
ld
e
d
w
i
t
h
an a
l
um
i
num e
l
ectrostat
i
c s
hi
e
ld
.
T
ry revers
i
n
g
t
h
e w
i
re connect
i
ons on term
i
na
l
s 9 an
d
1
0 i
f
the oscillator does not oscillate no matter wha
t
you do. If this fixes the problem, reverse the wir
e
connect
i
ons un
d
erneat
h
t
h
e
b
oar
d
so
y
ou can use
the proper terminals
f
or this and other simila
r
exper
i
ments
.
N
o
t
es:
EXPERIMENT #12
3
: A
U
DI
O
METAL DETE
C
T
OR
Schematic
Wiring
S
equence
:

6
-11-85-47

8
-12-119

9
-
109

10
-7
9
-
86
-
46

4
8-72

71
-
80
-
110
-
124

121
-
122