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■ FIGURE 2. The complete crystal radio.
The coil is on the left. Only one section of
the capacitor on the right is used. You can
see the diode between the coil and
capacitor. The black wire with the alligator
clip on the right goes to Earth ground. The
antenna connects to the right end of the
diode where it is soldered to the capacitor.
connected to the other end of the coil. By
tuning with the capacitor, you should pick
up one or more AM radio stations.
The coil and capacitor together create
a tuned or resonant circuit. You can
calculate the resonant frequency from the
F = 1/2π√(LC)
F is the frequency in Hertz (Hz) or
cycles per second; L is the inductance in
henries; and C is the capacitance in
farads. You select the values of L and C to
tune to the AM radio band which is from 530 to 1,700
kHz. A typical value of inductance is about 240
microhenries (µH). This matches up with a variable
capacitor with a capacitance tuning range of about 30 to
365 picofarads (pF).
The resonant circuit is connected to an antenna. The
antenna picks up the radio signal and converts it into a
voltage. The voltage appears between the antenna and
Earth ground. The tuned circuit responds to that signal.
When the circuit is tuned to a station, it becomes resonant
at that frequency and has maximum current flowing
thanks to the process of resonance. The result is a signal
to the diode.
The diode rectifies the AM
signal. The AM radio signal is an
alternating current (AC) voltage that
oscillates or regularly reverses
polarity at the station’s frequency.
The diode chops off one half of the
signal making it a DC variation. The
amplitude of the DC variation is the
AM signal being transmitted — either
voice or music. This rectified signal is
applied to an earphone. The
earphone cannot respond to the high
frequency AM signal variations, but it
does average their peaks into the
original audio voice or music signal.
BUILDING THE CIRCUIT
You don’t need many parts for this project. You can
buy them from several sources. I bought mine from Crystal
Radio Supply (
www.crystalradiosupply.com) but other
sources are available. Figure 2 shows the complete rat’s
The inductor is essentially just a coil of wire. The
inductance should be about 240 µH. I found two good
ones; a small coil wound on a ferrite core as shown in
Figure 2 and a second one that is a larger air core coil as
■ FIGURE 3. Two inductors. The one
on the left is the commercial model,
while the one on the left is obviously
homemade. Both are four inches in
diameter and have an inductance of
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