BY PAUL FLORIAN
■ FIGURE 2. Search coil form.
inch diameter coil form (see Reference
1). However, I wound the coil with 21
turns, resulting in an inductance of 88
μH. The inductance measured into the
phono cable with the search coil
connected is 126 μH. In order to work
properly, you must use the audio cable
specified in the Parts List. The resonant
frequency is F = 1 / (2π*SQR(L1*C)).
C= 1 nF in series with 10 nF + 56 pF.
Solving yields C = 965 pF. Calculating
for F gives 456 kHz. Therefore, the
oscillation frequency of Q1 is 456 kHz
when no metal object is near the
The oscillator formed by Q2 is the
reference oscillator. This frequency
can be changed by adjusting C6. The
output must be set within 2 kHz of
Q1’s output frequency. To obtain this
result you may have to use a slightly
different capacitor for C8. The part I
used has a tolerance of ±20%. Note
that the oscillation frequency of Q1 is
very sensitive to stray capacitance.
The output of Q1 will vary about
70 Hz per each picofarad change of
stray capacitance. Transistor Q4 forms
a mixer. Q1 and Q2 both feed signals
into this circuit. When a metallic
object is close to the search coil, the
inductance of L1 decreases. This
causes Q1’s oscillation frequency to
increase. The output of the mixer is
the sum and magnitude of the
difference of the two signals: Fosc +
Fref and |Fosc - Fref| (where “|x|”
means “absolute value of x”).
Suppose Fref = 500 kHz and Fosc
= 501 kHz. The output of the mixer
will be 1 kHz and 1,001 kHz. If Fref =
500 kHz and Fosc = 499 kHz, the
output of Q4 is 1 kHz and 999 kHz.
The frequencies output by the mixer
feed a high input impedance BJT
amplifier formed by Q3. The output
of Q3 is capacitively coupled to the
volume control R16.
The earphone used in this application must have a high impedance.
The ceramic earphone specified in the
Parts List has a 20 megohm imped-
ance and works well. Note that both
the high land low mixer frequencies
are sent to the earphone. The
earpiece shunts the higher frequency
to ground, acting as a low-pass filter.
■ FIGURE 3. Search coil RCA
cable strain relief.
The circuit was built on a 2. 6” x
3. 5” piece of perfboard cut from PC
board specified in the Parts List. Make
a copy of the schematic and
cross off the components as they
are installed. An adapter was
used to mount C6 (refer to Photo
1). To construct the board, cut a
1/2” by 3/4” piece of double or
single sided PCB material. Use a
permanent marker to mask the areas
where the traces should go. Then,
etch the board and clean the ink off
with rubbing alcohol. Finally, solder
the capacitor and SIP pins to the PCB
in surface-mount fashion.
This adapter plugs into a two
position SIP socket on the PCB. The
■ PHOTO 2.
■ FIGURE 4.
■ PHOTO 1. C6
February 2008 51