This metal detector is a low sensitive type can be powered by a battery
3 to 15V, and may be constructed employing four exclusive OR gates from
a CMOS integrated circuit 4030. The gates have been connected as two
oscillators and the detection coil metal's inductive element works
together as one.
When the detection coil is brought near to a metal body, the net
effective inductance gets transformed in the same way and so does the
frequency of the concerned oscillator.
Gates A1 and A2 are the two oscillators whose frequency needs to be
predetermined to around 160kHz and 161kHz, respectively, without a metal
body near its vicinity and by using a frequency counter meter.
The pulses generated by each of the oscillators are mixed by the gate A3
and across its output a frequency ranging from 1 to 321kHz is created,
which is actually the result of the sum or difference between the two
321kHz signal is filtered by low pass filter having a bandwidth of about
10kHz, which may be seen built with the A4 gate and enables 1kHz signal
to be amplified by crystal based headphones connected to the output.
To construct the search coil 140 turns of super enameled 18 SWG copper
wire must be wrapped over a mold or rigid tubular support having a
diameter of 15cm.