THE PROJECT CRYSTAL
(updated 4 Feb 01)
Crystal Radio involves the construction, testing and operation of a
crystal radios, starting with very basic set and ending up with a medium performance dx set, complete with antenna tuner. While it uses a smallish gage enameled wire (#24) and the cores of toilet paper or towel rolls, it works rather well throughout, and the coils are easy to wind. In addition to the suggested parts used here, you will also need a decent antenna and ground, preferably with alligator clips attached so you can fix them to your sets wherever needed. Throughout, I selected parts based on availability and cost. Using standard catalog parts from Mouser and Radio shack, you should be able to complete the project for about $25, less if you shop around, more if you decide to go with more expensive components. Here are the electronic parts I used:
Rf tuner capacitors- 2 - ElectronixExpress p/n
14VCRF10-280-P @ $1.35 ea.
Screws for tuner caps - 2 - Mouser p/n 48ss003 @ $1.20/100
Capacitors with knobs and mounting screws are also available at kitsrus
Capacitor with knob also available at Ocean State Electronics
Ceramic earphone- 1 - Mouser p/n 25CR060 @ 1.85
Magnet wire #24- 1 lb - Mouser p/n 501MW24H-1LB @ 10.42 (this gets you about 750 feet, you will only need about 200)
1N34A diode -Xtal set society, Radio shack, and others
47 kohm resistor- Radio Shack - @ .49 for 5 - either 1/4 or 1/2
watt will do
Parts from Mouser can be gotten from their web site; search using the part numbers I have given here.
Other stuff I used:
clips - 3 - Mouser p/n 524-11-4034C @ .21 ea.
Alligator clips - 8 - Radio Shack - p/n 270-356 or something similar -about 2.29.
Double sided sticky tape - couple of inches will do - rtv or hot glue will do as well, but is more permanent. Use for mounting the capacitors
Antenna, ground and hookup wire - a 90 foot roll of #22 stranded hookup wire from Radio Shack at 3.99 will do.
1 foot of 1 x 4 or 1 x 6 pine
1 foot of 1/4 inch lath (see photo)
1/2 inch wood screw - 3 - to hold fahnestock clips to base.
1 inch wood screw - 2 - to hold uprights onto base
Two plastic bottle caps for tuning knobs
Two small washers from hardware store (Ace), about 1/3 inch diameter, to fit under screws for capacitors and hold "knobs" in place.
1/8 to 1/4 inch dowel, 13 inches
Toilet paper cores - 3. Paper towel rolls cores also are ok. Diameter should be about 1 3/4 inch.
The two larger coils we will use are about 400 uH each, and the capacitors will go from about 20 to 220 pF -enough to cover the AM broadcast band. If you want to play around with different size coils, wire, capacitors, etc., go ahead if you know how to adjust. If not, I recommend you keep to the directions.
Construction: First, let's look at a picture of the completed base of the set and the two main coils you'll wind.
The coil on the left is coil#1, and has 17 taps. The other coil is coil #3. The vertical strips are the lath material, screwed to the 1 foot long base. Holes drilled in the ends of the lath hold the dowel rod; it can be removed to change coils. The bottle caps are the knobs of the two polyfilm rf tuner capacitors, which are stuck to the base with sticky tape. On the right of the base are screwed 3 fahnestock clips. The detector is connected between the rear clip and the right front clip. Leads with alligator clips are connected to the right rear and front left fahnestock clip, and leads and alligator clips are also connected to each capacitor. The two front fahnestock clips are for phones. Antenna and ground leads should be connected to the coils using alligator clips. You can use the picture to figure out how to put the base and parts together. I will show a "ready to listen" picture later.
Before you wind coils, I recommend you put your base together as shown above. You can be much less "elegant" than this, such as putting the whole mess on a shoe box top. I used the wooden base for stability, however, and the elevated coils to give better access to the coil connections and to get the coils a bit away from surrounding metal, which can adversely affect coil Q.
Preparation of the capacitors:
The capacitors I used have three connections. The center one is the base, and the outside ones are for the rotors of the two sections. You need to connect the two outside ones, and connect them to one lead, about 8 inches long, and connect the center connection to another lead. If you look on the back of the capacitor, you will notice two small trimmer capacitors with a screwdriver adjustment. Adjust both these to minimum capacitance ( you will see a full circle instead of a half-moon shape); this will give you full BC band coverage when used with the coils you will wind next. Cut holes in the center of the two plastic bottle caps ,and shape them with a knife to fit snugly over the tuning shaft of the capacitors, and then hold them in place with a small washer and screw. Use the double sided sticky tape to mount the capacitors on the base, and fasten alligator clips to the ends of the leads. To avoid working the delicate capacitor leads, I used an ordinary stapler to staple the leads to the base close to the caps.
Construction of coil #1: This takes about 72 feet of #24 wire. Using a pin, punch two holes about 1/4 inch apart close to and parallel to one end of your coil form. Thread about an inch of wire into one hole and out the other to anchor your coil. Now wind about 152 turns of wire in a single, closely wound layer along the coil; at every 10 turns (yes 10), make a small loop that sticks above the coil, each loop being about 1/4 inch in diameter - you will get the hang of this quickly. To avoid crowding these loops, which you will use as taps, you might stagger them so every other tap is offset on the coil by about 1 inch; you will end up with two rows of taps. After the last turn, make a final tap and, use the pin to make two more small holes as you did in the beginning, and thread about an inch of wire in and out to anchor the other end of the coil. You can use either cement or white glue to hold your finished turns onto the coil form. Tape is ok too; just don't cover the taps. Now comes the really tedious part; getting the enamel off the taps. I usually take a small flame, as from a lighter or candle, and burn the enamel off the tap, then lightly sandpaper them clean. If you like, do as I do and tin the taps with solder. This coil will eventually be used for your antenna tuner, hence all the taps. I don't draw well, so here is a link to what a tap should look like:
After you have completed coil #1, assembled the base, and put up your antenna and ground, you are ready to build the first couple or 3 radios in the project.
of coil #2: For this coil I recommend 30 turns on the same
diameter core, with taps for connections only at either end. Takes
about 14 feet of wire. If you want to, you can actually
skip making this coil, and use coil 1 or 3 in its place, using
the clips to select a different number of "active" turns.
Construction of coil #3: This coil can be identical to coil #1, but, since I didn't think I needed all the taps, I just put them at 0, 15, 25 50, 75, and 152.
make a radio: The first radio is a simple conductively
coupled set, using only the antenna/ground, coil #1 and the detector
and phones (earplug). It is commonly built as the "oatmeal box"
set. Here is the schematic for it:
things to try:
The set is "tuned" by using the inductance of the coil and the capacitance between the coil turns by attaching the antenna to various taps. Put the detector tap on different taps for the best signal. You will notice that this set tunes rather broadly, but may receive some shortwave as well as broadcast band stations. You can try disconnecting the ground connection from the coil and connecting it and the detector tap to each other, putting the detector and earphone in series with the coil. Try reversing the diode, and try it without the 47 kohm resistor.
Radios number two and three. These use the same components as above, but with the addition of one of the variable capacitors. Leads from the capacitor go to both ends of the coil for radio number two, creating a parallel tank circuit. For starters, tap the detector about midway on the coil, and connect the antenna to a tap close to the ground end. Adjust both taps for best reception. At this point, you should be wondering "what should I be able to hear?" An educated guess can be obtained by using the scan function on your car radio. You should expect to hear with your xtal set the stations that the radio stops on every time it tunes through the AM band. Strong stations may dominate parts of the band on your xtal set, but we'll work on that later. For some reason, most renditions of this set put the detector and the antenna right at the top of L1/C1; this never works very well for me, but putting the detector between 30 and 50 percent up the coil and the antenna tap somewhere near the bottom seems about right. For a simple radio, just connect the antenna to the top of the tank circuit, and forget the ground; this is the basic kid radio, and you can go around touching the antenna connection to various metal objects around the house to see what you can get. I get a couple of locals this way. Here is a schematic of this radio.
An alternative to this arrangement is to put the coil and capacitor in series instead of in parallel, thereby making radio number three. Your choice as to which is closest to the antenna. Give them both a try. Tap the detector and phones at several points and see which is best. By this time you should be getting a feel for the best taps for selectivity and sensitivity (read loudness) for your detector and phone taps. Feel free to reverse them, to reverse the diode, and to try different diodes or add diodes in series or parallel with each other to see the results. Oh yeah, by tapping the antenna way down the coil, you may pick up some shortwave stations, particularly at night. The series xtal set is shown here:
Radio number four. By now you may be wanting a little more selectivity, particularly if you have two strong locals close together on the dial. Here is one way to do it. First, return your radio to the parallel tank circuit of radio number 2. Put coil #2 on the rod near the ground end of coil #1 (the end connected to the earphone lead). Now connect the antenna and ground to coil #2. By separating coils 1 and 2 you vary the coupling between the antenna system and the set, and can pick up some selectivity. This should allow you to keep the detector on the best all round tap, and not lose sensitivity by having to go to a lower tap to gain selectivity. Feel free to experiment. Here is the arrangement for this set:
Radio number five. This one uses coil #3, which has been waiting patiently in the wings; if not, then wind it now. For this set, you will take coil #2 off the dowel, and put on coil #3 in its place. Now connect the detector/phone leads to coil #3. This gives you a separate antenna tuner using coil#1, inductively coupled to the set's tank circuit which now uses coil #3. Adjust for the best taps. Use coil #1 in the series or parallel tank circuit configuration of radios two and three, or just use as an antenna loading coil, without the capacitor. A schematic of this arrangement withcoil #1 as a series tuned antenna tuner is here: (note: as built, the antenna tuner configured as a series tuner shown below didn't tune belowabout 650 kHz; to go lower I had to shift to a parallel aarrangement, and connect the antenna to a tap on the coil)
From here on out, we will use the configuration of radio number five, and try out different ways to tune the antenna. With the set you now have, you can try out any of the configurations on my antenna page, linked below, with the single exception of the one with two capacitors on the antenna tuning coil. For that, you will need to get either a third capacitor or a ganged dual capacitor of sufficient capacity - I recommend the latter. From here on out you're on your own, but you now have a set that will pick up ssome good dx, and give you some experience in preparing to build your own ultimate rig Have fun.
Oh yeah, here's a picture of the set in a "ready to listen
configuration" - just add antenna and ground.
One final after note: This set has a number of compromises which were made on the basis of parts availability and cost. In fact, I scaled it up to incorporate some more advanced (and costly) features for my own use. The basic design is sound however, and it is my new "gold standard". It also has the flexibility to allow me to try out new ideas with little difficulty. The only claim I make is that you cannot buy a better set, either ready-made or in a kit. During the latest (2000) dx contest, I used this set as shown, but with the earphone under a pair of sound isolation muffs, and with one of my in-line wave traps. Got 52 stations just listening in my spare time, which wasn't bad at all. The rig I sent in my logs for was a scaled up version of this with some detector and headphone mods, and logged 98.
For a nicer looking version of this set, see what Dennis Foster, KK5PY sent me: