Photo and Text information
Entry in the Crystal Radio Building Contest
By Kent Stevens
entry in the Crystal Radio Building Contest is actually only the second
crystal radio that I have ever built. My first attempt was to build
the first project in K. E. Edwards
Radios that Work for Free. My entry in the contest started out
as an attempt to build a rock and catwhisker detector. As I was in
the process, I learned of the crystal radio building contest. Once
this was successfully completed. I then had to come up with a set to
go with it. After much trial and error, and many answers to many questions
I came up with a design that works for me.
have never had a cat whisker detector, and for the life of me never
quite knew what they were all about. When I learned of this contest
I decided to build a radio to enter. I had received the book
Voice of the Crystal for Christmas. When I saw the boom stand
detector I decided that I could build one for myself. I scrounged around
where I work, got some brass shim stock, and a piece of 3/4 inch brass
rod. I was on my way. I sliced of a thin piece of the rod for the
pivot point, and with my tin snips a fashioned the cat whisker stand.
The rock holder was made from the nut of a * in tube fitting. I use
a couple of washer to help fill the hole that the tube fits in and used
a 10-24 brass screw to mount it to the base. I also drilled and tapped
three hole around the side so that I could use three 6-32 brass screws
to hold the rock in place. For the whisker I used a spring from a cheap
ball point pen. And for the handle I had a small wooden spindle, but
then I found a couple of old glass drawer pulls in my junk drawer, so
I decided that they would look fancier. I threaded the end of the rod
that holds the catwhisker, and used two 6-32 nuts to hold it in place.
After finishing the detector, I then attacked the problem of building
my own variable capacitor.
The Variable Capacitor;
my own variable capacitor proved to be an education. My first thought
was to build a pop can capacitor, I thought that this would work, but
I really wanted something that looked a little more finished. If you
know what I mean. Then one night I decided that maybe I could build
my on pop can, as it were. I set to work, first I liberated some .015
inch thick brass shim stock for work. I then came up with the mistaken
idea that bigger is better. So I wrapped a piece of shim stock around
an empty Chunky Clam Chowder soup can. When I got the seam soldered
up I wrapped several layers of aluminum foil around this and then put
a second layer of brass around this. Once this was soldered, I could
slip the brass cylinder off, remove the aluminum foil, and then I had
two brass cylinders that would slide one inside the other. The space
between them allowed me to use a sheet of plastic to separate them.
After all of this work, I assembled the detector, capacitor, and the
coil from my other set. (Project one from
Radios that Work for Free). And guess what it didn*t work.
After asking a few questions, I found out that bigger is not necessarily
better. So, back to the workbench. This time I started with a six
inch piece of 1 * inch plastic pipe. This is about 1 * inch inside
diameter or about 1 3/4 inch outside diameter. After soldering a piece
of .010 inch brass stock around this, I then wrapped several layers
of aluminum foil and then wrapped a piece of .010 brass about 3 inches
long around this and soldered the seam. For the dielectric I tried
the plastic from a 2 liter pop bottle. After trying this I decided
that I had too much capacitance and replaced it with pieces of typing
paper. Now back to the test bench. When I hooked this all together
I was surprised to find that it did indeed work. This led me to rethink
my whole project. If I could get this to work, why not make another
3 inch long cylinder and with the two of them build a double tuned circuit.
After all I had seen in Radios that Work for Free, there was
set number two that had two capacitors, and that they were connected
at the frames. Again my limited knowledge of just how things work led
me to the idea that I could slip one small cylinder over each end of
the longer cylinder and have the same effect as two separate capacitors.
This then led me to the prospect of winding another coil.
part of the project was pretty much straight forward. I got a toilet
tissue roll and sprayed it with a couple of coats of clear spray. Punched
the necessary holes and wrapped the coil using 20 ga. enamel coated
wire that I scrounged from an old electrical motor starter. (That is
the 440 volt disconnect that they use in factory electrical work.)
The wire is from the transformer that cuts the 440 down to 110 volts
for the starter circuit. Anyway after winding the coil with the needed
taps as described in the good old Radios that Work for Free,
and clipping the parts together I strung my antenna through the kitchen
and with my fingers crossed, and a diode in place so that I knew that
I wouldn*t have to rely on my homemade detector. I put the ear plug
in my ear and tuned. Was I surprised when I heard a real radio station.
Next, was to unhook the diode and see if I could make it work with my
detector. Afer just a little tickle of the catwhisker, I got the station
and it was actually louder than with the diode. I let out a yell, and
my wife came in to see if I was having another heart attack. I had
her listen to the earplug and she got a smile on her face that went
from ear to ear. For a short moment she actually thought that maybe
I wasn*t completely nuts. Then she remembered who she was dealing with.
all that was left to do was to put it all together so that it looked
like something worth showing off. After a little thought and some looking
around I decided to mount it on and in a cigar box. As a kid I remember
making pretend radios in cigar boxes. They never really worked but
we had fun with them anyway. So off to the local tobacco store to get
a box. The lady there was glad to give me several boxes. Once I found
one that fit my needs I mounted the capacitors (one for tuning, one
for the ground) and the detector on the lid. I also put the switch
for changing the coil taps on the lid. The switch points were made
by taking nine, 10-32 brass screws, and grinding, and filing the slot
off of the top, so that they were smooth. The switch arm was made from
a piece of brass stock and the knob is another glass drawer pull that
matches the one on the detector. And then I mounted the coil on the
inside. I thought about putting a switch on it so that I could use
a diode, but I decided that I wanted this radio to be totally homebrew.
So, no diode. Once all of this was assembled, I removed everything,
stripped the paper off of the cigar box and gave it several coats of
black paint. I used black so that the bass would show off, and I figured
that it would give it an old time look. My next project is to try to
build a homebrew headphone as described in
The Voice of the Crystal. With the piezo crystal, but for now
I will enter this radio with a store bought earplug. All of my connections
are either 6/32 or 10/32 brass screws, and the wire was scrounged from
an old wiring harness.
should also add that without the help from the guys at the crystal radio
club, and the very informative books that I got from the Crystal Set
Society, I would never have been able to start, let alone complete a
project like this. This has been a very educational project for me.
It has led me to the decision to build another set that I hope will
be better and more sophisticated. As for now even if I do not win,
I will still have the bragging rights to the fact that I built a radio
completely from scratch with my own hands and my own ideas. Maybe someday
I too will be one of the old timers of crystal radio.