Elenco 130-in-1 Electronics Playground - Manual (Page 145)

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This experiment demonstrates how a metal detector
works. When the coil gets close to something that is
made of metal, the oscillator changes in frequency.
This type of metal detector has been used to locate
lost treasures, buried pipes, hidden land mines, and
so on. These have been used to save many lives by
locating mines and booby traps set out by the enemy
during wartime.
This circuit is a low distortion oscillator that draws
only one milliamp from the 9V supply. Using low
power allows the nearby metal to have maximum
effect on oscillation frequency.
You need a small transistor radio to use as the
detector; tune it to a weak AM broadcast station.
Adjust the tuning capacitor until you hear a low-
frequency beat note; this beat note is the difference
between the signal of a broadcast station and this
oscillator. Do not bring the radio any closer than
necessary. The best position is where the levels of the
two signals are about equal, because this gives
maximum sensitivity.
Try using keys, plastic objects, coins, and so on, as
sample objects. Of course, a real metal detector does
not have a small ferrite coil like this. It usually uses a
Faraday electrostatic shield, which is an air-core coil
shielded with an aluminum electrostatic shield.
Try reversing the wire connections on terminals 9 and
10 if the oscillator does not oscillate no matter what
you do. If this fixes the problem, reverse the wire
connections underneath the board so you can use
the proper terminals for this and other similar
experiments.
Notes:
EXPERIMENT #123: AUDIO METAL DETECTOR
Schematic
Wiring Sequence:
6-11-85-47
8-12-119
9-109
10-79-86-46
48-72
71-80-110-124
121-122