Introduction, The basic oscillator, The metal detector operation – Elenco Metal Detector User Manual

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There are many buried treasures waiting to be
found. While everyone dreams of finding a fortune in
gold coins, few do anything to look for them. Now
with the K-26 Metal Detector, you have a chance.
Although you may not find a fortune in gold, you
should find a few silver coins if you work the
beaches or parks. If nothing else, you’re sure to
have fun with this metal detector.

The Metal Detector basically is an oscillator that
transmits in the AM radio band. When a radio is
placed near the oscillator, it will emit an audible
tone. The frequency transmitted by the oscillator will
vary when brought close to a metal object. Thus, the
tone produced by the radio will vary in pitch,
indicating that a metal is present. We shall study the
theory of how the oscillator works and why its
frequency changes when brought near a metal


Figure 1 shows the basic circuit of the oscillator. When the switch SW1 is
closed, a current will flow in the P1-R1 ciricuit. This will turn on transistor Q1
and send a current into the inductor-capacitor circuit (LC circuit). The inductor
is made by winding a 5” diameter coil with 30 turns of copper wire. The LC
circuit is the heart of the oscillator. Every LC circuit has its own resonance
frequency. The resonance frequency of this circuit is in the AM radio band.

If we take part of the energy in the LC circuit and feed it to the emitter of Q1,
the transistor will amplify this signal and cause the circuit to oscillate at the LC
resonance frequency. By varying P1, the current through transistor Q1 will
change, thus forcing the frequency of the oscillator to change slightly.

Figure 1











Obtain a small portable AM radio and place it near
the Metal Detector. Tune the radio around the
midband and away from radio stations, you should
hear only static. If there are AM stations nearby you
will get whistling. This is because the metal detector
oscillator is mixing with the frequency of the AM
station. Now turn control P1 until the static gets
quiet. The metal detector is now tuned to the AM
radio frequency. Now take a piece of iron metal and
bring it close to the detector. Note the change in the
static in the radio or a change in the pitch of the
whistle. This is because you have increased the
inductance of the coil and thus changed the
resonance frequency of the circuit. The radio is now
tuned to the new oscillator frequency.

Magnetic fields move easier in the presence of iron,
nickel and other materials. Thus the inductance of
the coil will increase when these metals are present.
This increase in inductance will cause the LC circuit
to oscillate at a lower frequency. These magnetic
fields are disturbed when in the presence of silver,
aluminum, copper and other highly conductive

metals. Thus, when these metals are brought near
the coil, the frequency transmitted by the oscillator
increases. The radio responds to the oscillator

Now you have some understanding of how the
Metal Detector works. Go out and look for your
fortune. Wish you lots of luck.

AM Radio

Attach the circuit
board to the radio

Non-metal Support


Figure 2