The crystal radio
A
crystal radio set is able to detect radio signals without a power
supply. It works best if there is a transmitter within 25 miles (40 km)
of the set. The antenna, a very long wire, picks up the waves and passes
them through the set as electronic current, and then down to the
ground. The set itself is a tuned circuit that can select a desired
frequency from the many that are picked up by the antenna. The electric
signals cannot be directly converted into sound because they vibrate
back and forth too rapidly. The crystal (or diode) between the tuned
circuit and the earphones allows the current to pass through in one
direction only. The earphone contains a small solenoid and a thin metal
plate. The current passing through the circuit and then through the
diode causes the solenoid to move, which in turn moves the metal plate,
whose vibrations create (faint) sound waves in the air.
A
very simple crystal radio is not very selective, and if there are more
than one nearby stations broadcasting near the same frequency, there
will more than likely be some overlap, and you will hear two or more
broadcasts at once. The solution is to add a tuner and a capacitor to
the circuit. The coil length is what determines the frequency the
circuit is tuned to. A simple tuner effectively changes the length of
the coil by selecting how much of it is in the circuit. A capacitor
(condenser) helps refine the tuning further.
A note about the crystal detector
It
is a fairly simple process to make a working radio using a modern diode
for a detector. However, the reason these are called crystal sets is
because the detector was originally a piece of crystal. You can still
make your radio the authentic way with a crystal and cat's whisker.
Detector stands are still manufactured, one good source being Antique
Electronics Supply, 6221 S. Maple, Tempe, AZ 85283-2856 USA (phone
602-820-5411). A crystal detector includes a crystal, a cat's whisker,
which is a special thin wire that contacts the crystal, and the stand
that holds the components in place. The most common crystal used is a
small piece of galena, which is fairly common, and can be found in many
rock and hobby shops.
The
cat's whisker is most often composed of phosphor bronze. Once in
circuit, the whisker can be moved about on the crystal's surface to find
the most "sensitive" spot. The pressure of the whisker on the crystal
is also adjustable. There are some other crystals that will work, so
there is much room for experimentation with crystal fragments that you
may already have. Also, it is not absolutely necessary to use a detector
stand, and the cat's whisker can be improvised with a safety pin.
Although it will be less selective and more difficult to adjust, it can
be made to work quite satisfactorily. A small piece of rubber pencil
eraser impaled on the safety pin helps to insulate it from your fingers
while adjusting.
The coil, antenna, ground, and phones
The
coils for these sets are typically wound around a 1 1/2 to 2 1/2 inch
(38 - 64 mm) diameter core, using 75 to 150 turns of 24 to 20 gauge
wire. These are typical numbers, not critical. What is critical is that
the individual loops of wire around the coil are wrapped touching the
next one over, but that they do not ever overlap. It is also important
that whatever attaches the coil to the base can not touch the coil's
wire, especially if it is a metal tack or nail. A coat of shellac or
varnish helps to keep the coil together. Let it dry thoroughly before
using. If a wiper type switch is used, the varnish will need to be
scraped away along its path.
Ideally,
the antenna should be 100 feet ( 30 m) or so long, and strung as high
as possible. Insulated or non-insulated wire can be used. Either way,
the un-insulated ends should not touch anything that will ground them.
It is best if they are tied off to ceramic or plastic insulators, which
can in turn be tied off between two high points outdoors, such as a tree
limb and your house. Never string an antenna anywhere where it has even
the slightest chance of coming into contact with a power line, or in a
place where you will need to go near a power line to hook it up. Always
take the antenna down if a storm or lightning is predicted. It is safe
practice to add a lightning arrestor to you lead wire. You can purchase
in many radio and electronic hobby shops antenna kits which include the
antenna, insulators, lead wire, and lightning arrestor.
The
ground wire can be attached to a metal cold water pipe, or to a metal
rod stuck a couple of feet into the earth. Do not attach it to a line
carrying gas or electricity.
The
headphones (or earphone) need to be high the impedance type designed
for crystal sets. They are still available through electronics suppliers
and some hobby shops.
The capacitor
The
capacitor, or condenser, though not essential for operation of these
sets, does help to refine their use when it is added. More complicated
sets have a variable capacitor. For the simplest sets, however, a fixed
capacitor of around .002 mF or so is sufficient. A capacitor is also
very simple to build. The Cub Scouts, being the great caretakers of
crystal radio lore that they are, included this picture in the 1954
edition of the Wolf Cub Book. It lacked annotation of any kind other
than what is here. The most important thing to know is that all of the
tinfoil pieces need to be completely insulated from one another. They
cannot touch each other in the least. The whole thing should be bundled
tightly with cellophane tape. Also, aluminum foil is more common these
days than is tin foil. It will work just as well.
Set one
What
is presented here in crude ascii schematic is the simple basis of many
simple crystal sets. This is not intended to tell you how to construct a
particular set, but it should be enough to get a design started.
The
diode D1, a germanium diode (1N34A or eq. usually recommended), is the
detector, and in old sets this would have been the crystal and cat
whisker assembly
(see
"A note about the crystal detector", above). The capacitor can either
be a standard mica type of around 0.002 mfd, or a simple variable type,
which you can easily build yourself based on a simple design of two
metal or metal clad plates, which can be slid apart or together, and
which are separated by an insulating material (kraft paper). L1 is the
coil. Volumes could be and have been written on coils. A very simple one
can be made with a 5 or 6 inch (127 - 152 mm) long, 1 inch (25 mm)
diameter plastic pipe, wood dowel, or any fairly sturdy non metallic
cylinder that can be easily worked with. Wind using magnet wire (#16
will work). Secure the free ends to the former somehow. The enamel
should be sanded off the ends before including them in the circuit. You
will have to mount the coil so that it does not contact the base. Sand
the side lightly that switch S1 contacts. S1 is a simple piece of metal
shaped and fastened so that it may slide across the coil. It is usual
practice to attach it to the circuit with a screw or rivet loosely
enough so that it can be pivoted to contact most of the coil. The ground
can go to a water pipe (not a gas or electric pipe). A steel rod
hammered 2 feet into the ground will also work. You can be as elaborate
as you wish with the antenna. Don't use during an electrical storm! If
you can't get it to work, flip the diode around. This might get things
going. If not, check all your connections and make certain there is a
good ground connection. You may also be using the wrong type of
headphone. You will need a very high impedence phone. There are phones
made specifically for crystal sets.
Set two
This design is a bit different, but mostly in the way the tuner is set up.
The
coil for this set should be wound with app. 22-gauge wire, but it isn't
critical. The form it is wound around is 1 to 2 inches (25-50 mm) in
diameter and 4 inches (10 cm) long.
It
is wound in pretty much the same manner as the coils in the other
projects listed here. A few inches are left loose at one end, the wire
is taped down, and winding begins. Every so often, a loop of wire is
left out of the winding and twisted together. This loop should be 2
inches (50 mm) or so when stretched out. Try to keep their lengths
uniform. They should be closer together near the end of the coil that
will connect to the ground connection. The more of these "tails" that
are made, the greater the selectivity of the set. Also, as with any of
these sets, the more turns in the coil, the greater the range of
frequencies that can be received. When the coil is mounted to the base,
one end wire is joined to the circuit. The tails are thumb tacked to the
base, sticking straight out in front of the coil. The coil's other end
wire is tacked to the board along with the tails.
The
capacitor is 300 picofarad. The enamel will need to be stripped from
the coil's free ends as well as from the tails. The wire that is shown
as a "free wire" in the schematic is actually the tuner. It attaches to
the coil along it's tails to select frequency.
Set three: bare bones set
A
crystal set can be very simple to very complex. If you want to go the
complex route, which will produce a much more sensitive and tunable
device, I suggest checking out the
Xtal Set Society,
which produces several fine books on the subject. If you want to
construct a very simple one, then the schematic below is about as simple
as you can get. It won't produce the easiest set to tune, but it is a
great first set, and you will, if you are patient, be able to pick up
several stations using it.
Using a toilet paper roll, poke two small holes about a half-inch from each end.
Pass
about one foot of the free end of a roll of magnet wire (#16 or so)
through one of the holes, from the outside of the tube to the inside.
Tape it in place. Wind the coil, making sure the adjacent windings touch
but do not overlap. It should be wound firmly. When you get near the
second hole, roll off another foot or so of the wire and cut it off from
the roll. Pass the end through the second hole, and tape in place.
Mount the coil on the base, which is a 5 by 7-inch or so piece of wood,
corrugated cardboard, or foam core. The coil should be mounted with
spacers at each end, under the points of attachment, so that there is
open air all around the coil (this is more critical in more sensitive
sets, but it is good construction practice always). It can be attached
with push pins or small brads.
The
tuning switch (S1) is a piece of copper strip, which is mounted to the
base at one end with a screw or rivet so that it may turn freely. The
metal is lifted (not sharply bent) so that it sits on top of the coil,
making good but gentle contact across the entire range. It should be
positioned so that the mounted end is 2 or three inches from the coil,
and centered with the coil (long ways). It is 1/4 to 1/2 inch or so
wide, and long enough to cover the entire range of the coil. There
should be no sharp edges, as this will wear out the coil.
You
will need to add 3 terminals, two on one end of the base, and one on
the other (at opposite ends of the coil). Small nails or pins work well.
Sand off the enamel from the free ends of the coil's wire. Attach the
wires to one of the terminals on each side. Lightly sand off the enamel
under the path where the switch swipes the coil. A diode (1N34A or eq.)
is attached between the attachment point of the switch and whichever of
the 3 terminals is not attached to the coil. The banded side of the
diode faces away from the switch.
The
antenna wire attaches from the side of the coil that is opposite the
diode (the side with only one terminal). It should be as long as is
practically possible. Attach the ground to the terminal on the opposite
side that is attached to the coil (not to the diode). The ground wire is
then attached to a cold water (not gas!) pipe. High ohm headphones (the
type designed for crystal sets) or an appropriate earphone is attached
to the two terminals between the diode and the coil. A .001 - .002 uF
capacitor can be added across the earphone terminals (not necessarily
essential).
The
switch needs to make good contact with the coil. If necessary, bend the
end somewhat in its middle, long ways, to refine contact. Or else,
solder a short piece of heavy wire to the bottom of the contact point.
Set four: Cub Scout set
This
is a set which appeared in the 1943 Wolf Cub Book, part 1, pp. 75-77.
The text is verbatim. It may be public domain by now, but assume for now
that the Boy Scouts of America hold the copyright. There is also a nice
gif image of the set from the manual. If you have trouble getting this, let me know and I will see what I can do.
How to make a simple CRYSTAL RECEIVER
A-Dry
a cardboard mailing tube in the oven, then paint with a coat of
shellac. The tube should be about 6 or 7 inches long and 2 1/2 inches in
diameter. Buy one pound of 26-gage (B. & S.) single cotton covered
wire. Start and finish winding one half inch from ends of tube. Then
shellac all over again. When dry, fasten down to a baseboard at each
end, with a screw and washer to prevent it from touching at any point.
B-Cut
a strip of cardboard as a test place for selector. When length has been
determined, cut a brass strip the same size. Screw this down at a
slight angle and mark on coil just where it touches. Fold a piece of
sandpaper and with the folded edge remove the shellac and covering from
wire on coil. This will leave an arc the bras strip will touch. In
fastening on brass selector, use two fibre washers.
C-One
end of wire from coil should go to "ground" clamp. Wire from selector
should go to "antenna" clamp. Other end of coil wire should go to
crystal detector. Run a wire from "ground" clamp back to one phone
binding post. From other binding post, run wire to crystal. Between the
phone posts, connect an .001 or .002 microfarad receiving-type fixed
condenser, which costs very little.
This
type of receiver works best when within 25 miles of a broadcast
transmitter. Antenna and ground connections should always be tight.
- end of article
By the 1950's, the basic set changed only slightly, and has remained pretty much the same up to current Cub Scout manuals.
Final notes
This
page is intended to get you started on a simple set, as well as to
introduce a bit of theory to assist in constructing more complex sets.
Crystal radios can be refined and precise pieces of equipment with
careful design and construction. There is room for experimentation in
every aspect: basic design, the coil, rheostats, the antenna, traps for
the antenna (which help to filter the signal before it reaches the set),
and so on. Changes in these areas can effect the sets sensitivity,
range, accuracy, and even volume. Most of this is beyond the scope of
this page. If you want to pursue crystal sets further, a good place to
start is the
Xtal Set Society. Also, you will find out a great deal on your own by experimentation.
Crystal radio lore
Gordon Johnstone writes:
"My grandfather was
one of the first electric torpedo men on the Royal Navy ships during the
first World War. He built one of the early crystal radios for my
grandmother, but was annoyed at only one person being able to hear at
any time. So he took a large pudding basin, and mounted the headset a
little up from the base inside. Voila, early parabolic loudspeaker
system."
Thanks, Gordon!
Construction of a diode
If you want to try your hand at making your own diode, Allan Charlton, of Sydney, Australia, adds:
"When I was a kid in a small town in Tasmania, Australia, our school
was at the base of a hill, and the local radio transmitter was on top of
the hill. We had lots of fun with crystal radios.
This is how we made our diodes:
Take
a small length of glass or plastic tubing--an inch of the case of a
plastic pen works well. Close one end with wax, sealing a wire through
the wax. Pour a little copper oxide into the tube: enough to cover the
end of the wire. Fill the rest of the tube with copper filings or
turnings. Poke a wire into the copper filings or turnings (but don't
let it go down to the oxide) and seal the end of the tube with wax.
Can't find copper oxide?
Throw
some copper wire into a fire. When it's cool, scrape the oxide off the
wire. Yes, there are two oxides of copper, a red oxide and a black
oxide, and they both work well. We preferred the red, but I have no
idea why."