Kenton's set

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My Entry in the Crystal Radio Building Contest 

By Kent Stevens 

This 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. 

The Detector;

I 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;

Building 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. 

The Inductance;

This 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. 


Now 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.

I 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.