Figure 43.2: A galena crystal with cat’s whisker. (©GNU-FDL,Wikimedia Commons.)kind of one-way valve that allows current to travel in one direction, but not another. By connecting a diode to
the tuned LC circuit (Fig. 43.1(c)), we get just one-half of the resonating signal. That is, the incoming signal
will cause the LC circuit to oscillate back and forth with an alternating current, where the current sloshes
back and forth, going clockwise, then counterclockwise, then clockwise again, etc. The diode allows only
the current going in one direction to pass, which allows us to pick up the audio signal that is modulated on
the carrier wave.
If we were to connect headphones directly to the LC circuit with no diode, we would hear nothing. The
LC current sloshing back and forth would average out to zero, so we would hear no audio. Adding the diode
leaves a net non-zero signal coming out of the diode, which has the audio signal in it.
In a traditional crystal radio of the 1920s, a simple diode was constructed from a crystal of the mineral
galena, which is a heavy silvery metallic mineral consisting of crystalline lead sulfide (PbS). The galena
crystal was touched with a fine wire called acat’s whisker. The cat’s whisker was attached to a movable arm
so that it could be placed in contact with different areas of the galena crystal surface (Fig. 43.2). At some
point you would find a “sensitive” area of the crystal that would allow the whole assembly to act as a diode,
and conduct current in only one direction.
In building a “foxhole radio,” soldiers found that galena crystals were very difficult to come by. Instead,
they would substitute a razor blade, and used a safety pin or pencil lead as the cat’s whisker. This was
somewhat less satisfactory than a galena crystal, but was often adequate for picking up a station or two.
In more modern crystal radios, we often replace the galena crystal and cat’s whisker with a germanium
diode (called a1N34 germanium diode). This kind of diode contains a tiny crystal of germanium metal and
tiny cat’s whisker wire already placed so that the device will always conduct current in just one direction.
Headphones
Finally, we connect a set of headphones or crystal earpiece to the circuit (43.1(d)). This takes the signal
coming from the diode and uses it to drive the vibration of a diaphragm that produces sound waves that can
be heard by the ear.
Variable Tuning
The radio built so far can tune only one station, whose frequency is at the resonant frequency of the LC
circuit. By making either the inductor or the capacitor variable (Fig. 43.1(e)), the circuit can be made to tune
different stations. Typically we choose an inductor withLD250 H and a capacitor withC D 365 pF,
which gives a resonant frequency (Eq. 43.2) off D 527 kHz, which is at the lower end of the AM radio
band. If the inductor can vary between 0 and 250H or the capacitor can vary between 0 and 365 pF, then the