Popular Posts

Sunday, September 8, 2013

The crystal radio

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

Galena is the most commonly used crystal.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. BIG safety pinThe 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."

No comments:

Related Posts Plugin for WordPress, Blogger...