Quite a few years ago I found a book in the library called "Radios That Work for Free" by K E Edwards and found a set of plans for a radio that I just had to make, unfortunately I did not have the resources to build it at the time so I made a photo copy of the pages from the book for future reference. Well I was poking around on the net the other day and found a site that sells copies of that old book, along with various other electronics books and parts related to crystal radios and decided to order a copy for myself along with two 365 pf variable capacitors needed for the circuit I copied from the book.
The radio that I am going to build is the second one shown in the book but I am going to make some changes to the original set. The three changes I am going to make to the set are that I am going to build the set into a box instead of just having the control panel attached to a base board. I am also going to make an antique style "Cats Whisker" style detector instead of using a Germanium diode and use a matching transformer so I can use a set of modern headphones instead of trying to find an antique high impedance set or a crappy little crystal earplug.
I have written this instructable assuming the reader has basic wood working and electronics skills like making boxes and soldering, explaining the more involved and specific tasks in detail, as in winding the coil.
Step 1: Circuit and Theory
Without getting into the rather intensive math involved, I have taken classes on this stuff, here is in a nutshell how it all works.
Crystal radios mainly work on AM or Amplitude Modulation radio waves, there are some that well work on FM as well. AM radio waves are actually composed of two waves combined together, one is a high frequency wave called the carrier wave and the other is the lower frequency sound waves being transmitted. The carrier wave is on the order of 750 to 1500 kHz. and is used to transmit the sound wave. The way it works is that the two waves are added together, or modulated, producing a signal that is of the same frequency as the carrier wave but its amplitude is determined by the sound wave(see second picture).
The way a crystal radio actually works is pretty ingenious, it uses the coil L and variable tuning capacitor C1 as a low pass filter to only allow frequencies at the desired frequency of the carrier wave to be passed to the detector D. The detector then demodulates the signal by only letting the low frequency part of the signal to pass, ie the sound wave, to the headphones where they are then turned back into sound. There are no batteries, no transistors or amplifiers of any kind, the sound is produced solely by the energy of the filtered radio wave.
Step 2: Materials
3 x 6-32 2" long screws
11 x 6-32 3/8" long screws
2 x 6-32 3/4" long screws
6 x 6-32 1/4" long screws
4 x #6 1/2" long wood screws
22 x 6-32 hex nuts
2 x 6-32 thumb nuts
8 x #6 washers
- sheet of 1/8" thick Plexiglas for panel
1 x 1/2"x6"x4' board, I used poplar as it is cheaper and looks nice you could also use pine or oak or what ever you like.
1 x 3/8"x6"x2' board, again I used poplar
-some thin copper sheet, or brass (Scrap metal dealer or hobby shop) $4 for a large piece
1 x 3/8" dowel (All above items are from the hardware store. I used brass but steel would be a significantly cheaper) $9
1 x 2" diameter 5" long cardboard tube, you could also use pvc (mailing tube or hardware store)
1 x binding post (AES S-H226B or RS, AES has single binding posts instead of multi packs) $2
14 x solder lugs (AES S-H112) Package of 10 from AES $1
2 x 3/4" Fahnstock (AES)
1 x 1N34 Germanium diode
1 x 1/4 lb roll of 20 awg enameled copper wire (AES S-WL3-610 has 21 gauge wire which should be close enough)
1 x short piece of 12 gauge bare copper wire (got mine from a scrap if Romex)
2 x 365 pf air gap variable capacitors (AES C-V365) $12 each
1 x chunk of iron pyrite (Rock shop or sometimes gift shops have things of different minerals)
1 x 1K to 8 ohm matching transformer (RS# 273-1380) $3
1 x 1/4" stereo phone jack (RS# 274-249) $3
2 x large knobs (RS) $2
- 22 gauge hookup wire (RS or AES has the old fashioned style fabric covered wire which would look real good but it is expensive) $6 for three 25 foot rolls from radio shack
- Stain for staining the box
- Lacquer or other clear coat to protect the box
- Black paint for the panel and tap switch knob
AES: Antique Electronic Supply, has all kinds of antique style parts, vacuum tubes and stuff you generally can not get any where else. Antique Electronic Supply
RS: Radio Shack, they still have one or two things in stock that are useful.
Substitutes: The copper, iron pyrite, 10 3/8" screws, one of the 2" long screws and 12 of the nuts are used to make a tap switch and detector for the set and can be substituted with commercially made parts if you do not wish to make them by hand. The tap switch can be replaced with a 10 position rotary switch and the detector can be replaced with a 1N34 Germanium diode, both of which are available from Radio Shack. I made them on mine because I wanted to make the set as authentic looking as possible, mind you the modern substitutes would be more reliable and quicker.
- Hobby Knife or utility knife
- Drill and bits 1/16" and 5/32"
- Pliers, needle and regular
- Tin Snips
- Metal File
- Wood Saw
- Hammer and 1" long finish nails
- Nail set
- Wood putty
- Wood glue
- Super Glue
- Soldering iron and solder
- Wire cutter and striper
Step 3: Winding the Coil
- 2" diameter tube
- 1/4 lb of 20 gauge enameled copper wire
- small drill
- wire cutters
1) Mark and cut the tube to length:
-With a ruler mark the tube at 5 inches from on end and wrap tape around the tube at that mark.
-With a sharp knife cut along the edge of the tape to make a nice even cut.
2) Using a ruler draw a strait line across the length of the tube.
3) Using the ruler make a mark on the line 1/2 inch from one end of the tube, this is were the coil well start.
4) Using a small drill, make a hole on the previously made mark, also make another hole about 1/4 down from that hole, the two holes well be used to secure the end of the wire.
5) Unwrap a few inches of wire from the end of your spool of wire and feed the end through the first hole drilled in the tube.
6) Pull the end of the wire through the first hole a couple of inches and then feed it back out through the other hole, this well lock the end of the wire onto the tube.
7) Start carefully wrapping the wire around the tube away from the starting hole. The hardest part is to keep the windings tight and even.
8) At the fifth winding use the pliers to make a loop in the wire by holding the wire in the pliers and folding the two ends down the twisting the wire around once.
9) Continue to wind the coil and make another loop at wraps 10, 16, 24, 32, 40, 52 and 65 after the loop at wrap 65 there are no more loops but you have to continue to wind the coil until you have 80 wraps. These loops well become the taps in the coil in the finished circuit.
10) Once you have made 80 wraps drill two more holes, one on the line and one about 1/4" above it, right next to the previous wrap.
11) Cut the wire from the spool leaving a few extra inches of wire to feed through the holes.
12) Feed the wire through the first hole and back through the second, just like at the start and pull the wire tight to insure the windings are tight.
13) Measure the tube 1/2 inch from the end of the windings and cut off the excess tube so that the tube is even.
Step 4: Making the Tapswitch
To make the tap switch you well need:
- strip of thin copper or brass 3/8" wide
- one 2" long screw
- one 1" long piece of dowel
- one hex nut
- one washer
- super glue
- paint or stain and lacquer
- tin snips
- metal file
- paint brush
1) Using a ruler and utility knife score a line 3/8" of an inch from one end of you sheet of copper.
2) Using tin snips cut the strip out along the line and re-flatten the strip with a hammer on a hard surface.
3) Cut the strip to 1-9/16" long.
4) Mark the strip 3/8" from one end all the way across. At the other end of the strip opposite the end you just marked measure in from each side 1/16" along the top. scribe a line from the marks on the top to the line 3/8" from the bottom and cut along the line with tin snips, this tappers the pointer down to 1/4" at the top. See forth picture.
5) Use a file to clean up the edges and round the ends of the pointer.
6) Cut a piece of dowel 1/2" long and drill a hole 5/32" down the center.
7) Holding a the dowel in position on the pointer use a nail in the center hole of the dowel to make a mark and drill a 5/32" hole through the pointer at that mark.
8) Slide a washer over the 2" long screw and then slip the dowel and pointer on after. Using a small drop of super glue, glue the pointer to the dowel and thread the nut onto the screw and tighten the nut.
9) You can either paint the knob, stain it like I am doing or just clear coat it and keep it the natural color of the wood.
Step 5: Making the Cats Whisker Detector
I actually got the plans for this detector from the book, "The Boys first book of radio and electronics" by Alfred P. Morgan. This is a really great old book with loads of information on this kind of stuff with numerous plans for crystal radios, a tube radio and tube amplifiers, I would highly recommend this and any other book by this author if you can find them.
To make the detector you well need:
1 - 3/8" wide strip of copper 1-7/8" long
1 - piece of heavy gauge copper wire about 2-1/2" long
1 - spring from a worn out ball point pen
1 - binding post
- tin snips
- strait edge and utility knife
- drill and 5/32" drill bit
- soldering iron and solder
1) Start by cutting a strip of 3/8" wide copper 1-7/8" long to make the crystal holder.
2) Mark and drill a 5/32" hole in the middle of the strip of copper.
3) Using a file on edge cut teeth into the ends of the strip of copper.
4) Using a large pair of pliers with jaws that are about 1/2" wide, hold the hold the strip of copper in the middle so that the strip is perpendicular to the jaws. Fold the two ends up.
5) Bend the serrated ends over to make a set of jaws to hold the raw iron pyrite.(see photos)
6) Take the spring from the pen and straiten out one end so that it is about 3/4" long and then solder the spring to the end of the piece of bare copper wire.
Step 6: Making the Case
To make the case you well need:
1 x 1/2"x6"x4' long board
1 x 3/8"x6"x2' long board
- finishing supplies, paint, stain, whatever
- Saw, I am using a power miter box because I am lazy
- Hammer, again I am lazy so I am using a pneumatic brad nailer with 1" long brads
- Sanding block and sand paper, I use 220 because it well wear down to finer and finer grades as you use it so it leaves a nice surface for finishing.
1) The first thing that has to be done is to cut the boards to length to make the sides and front and back. You well need to cut 2 1/2"x6"x9" long boards for the front and back and 2 1/2"x6"x7" long boards for the two sides, also cut 2 3/8"x6"x7" long boards to be glued together for the bottom.
2) Next you need to glue the two sides and front and back together to make a box, the two side pieces go between the front and back not on the ends of the front and back.
3) Now would be a good time to use some wood putty to fill in the holes left by the nail set, or in my case the hammer of the pneumatic brad nailer.
4) To make the bottom you well need to glue the two 3/8" pieces that are cut to 7" long along the sides that are 7" long. Start by sticking a wide piece of masking tape, 1" is probably good enough, along the 7" long edge of one of the boards leaving it sticking out half way, see photo.
5) Butt the other piece of wood up to the first one and align the ends so that they are flush and stick the tape down to the other board while holding the two firmly together.
6) Once the tape is stuck to the other board flip the two over and fold them together so you can apply glue to the edges that well be stuck together.
7) Spread some glue to the edge of one of the boards where the seam well be and lay the two boards out flat again of a flat hard surface.
8) Push down along the seam to ensure that the two boards and even and all the excess glue squeezes out and wipe up the excess.
9) Put another piece of tape alone the seam on this side making sure that the seam is flat and even then place another board on top of the two being glued together and place a heavy weight on top of that to hold it down while it is drying.
10) After the glued has dried on the bottom, pull the tape off and trim it to length so that it fits inside of the bottom of the box and glue it in.
11) I also decided to make a lid for my prototype, I glued two boards together the same way as I did on the bottom and cut it to size so that it sets over the top of the box.
12) Next add some slats to the inside of the box to screw the panel to, if you are making a lid set the slats down 1" from the top, it not set them 1/8" from the top so that the panel well be flush with the top of the box.
13) Finish the box with what ever finish you desire, I chose to staining mine and then coating it with two coats if polyurethane, you could paint it instead which would be simpler.
14) Once the finish has had adequate time to dry, tape the top down to the top of the box in the orientation that is well be in when finished on the three sides that well not be hinged.
15) Using pieces of tape to hold the hinges in place, drill pilot holes for the hinge screws and install the lower screws. After the bottom half of the hinge is mounted pull the tape off and attach the upper half.
16) Remove the tape holding the top on and make sure the hinges do not bind.
Step 7: Making the Front Panel
- some 1/8" Plexiglas
- graph paper
- pencil and ruler
- drill and 5/32" drill
- metal strait edge
- utility knife
1) Start by cutting the Plexiglas to size, the easiest way to do this it to use a strait edge and knife to score the Plexiglas and then to break it by firmly holding it over a sharp edge and snapping off the excess. If you are mounting the panel on top of the box make the panel 8" by 9", if you are mounting it inside of the box like I am cut the panel to 7" by 8".
2) Next you well need to figure out who you well want to have the panel laid out. To do this you well need to collect all of your parts so that you know who big things are and how they well have to be in relationship to everything else.
3) Next take your graph paper and draw a full size representation of the panel on the graph paper and use your parts as a guide to figure out where all of the holes well need to be drilled to accommodate everything. The only two measurements that are already determined are that the holes for mounting the coil have to be 3-1/2" apart and that for the tap switch you well need 9 evenly spaced holes around a center hole with a radius of 1-1/4". It also helps to have the coil and tap switch close together making it easier to wire the two together and keep the wires short.
4) Now make a photocopy of your plan and tape it to the front of the panel and use it as a guide to drill the holes in the panel. Note: Plexiglas can be tricky to drill, don't run your drill at too high of speed as it well melt the plastic to much and also don't use too much downward force when drilling as that well cause cracking. It is also a good idea to practice on a small scrap before hand so you can get an idea of what works the best.
5) After all the holes have been drilled you can paint the panel if you want, you can also leave it clear if you want to see the insides of the radio which is cool. Start by taking some fine sandpaper or a Scotchbrite pad and scuffing up the surface to be painted. Once the surface is evenly scuffed up clean it with dish soap and dry it thoroughly.
6) Paint the panel, I am using black spray enamel.
Step 8: Assembly
- all the screws, washers, nuts, solder lugs and other miscellaneous hardware
- the case, detector parts, panel, tap switch wiper ect
- flat head screwdriver
- 5/16" open ended wrench
- soldering iron and solder
- wire cutters and strippers
1) First we well start by assembling the tap switch.
a) Start by adding the screws for the tap switch, place a screw in the hole and add a solder lug to the back side and tighten it all down with a nut. Make sure to position the solder lug pointing to the center of the the circle.
b) Slide a washer over the end of the tap switch wiper and install the wiper in the center hole.
c) Slide another washer over the screw and add a nut tightening it down so that the wiper turns with a some resistance so that it well not lose its setting when in use. Slide a solder lug onto the screw and add another nut tightening it down to the other nut locking the two together.
d) Cut the excess screw off.
2) Installing the coil.
a) Install the 2" coil mounting screws through the front of the panel and secure them with nuts.
b) Solder short, about 3" to 4" long, pieces of wire to each of the 8 coil taps.
c) Solder the tap wires to the tap switch contacts starting with the first tap a the fifth winding in the first contact and progressing all the way around the tap switch until all 8 taps are wired.
d) The ninth contact of the tap switch is connected to the end of the coil along with a piece of wire that well be used later.
e) Solder a length of wire to the tap switch wiper.
f) Install the coil onto the coil mounting screws with a washer and nut on each side.
3) Installing the remaining components.
a) Mount the stereo headphone jack to the panel.
b) Next mount the crystal holder with a screw, solder terminal and nut. If you are using a germanium diode instead of the crystal holder install a Fahnstock clip instead, still using a screw and solder lug.
c) Mount the binding post for the cats whisker tightening down only lightly so that the binding post can be swiveled back and forth to help aid in setting the detector. Again if using a diode install a Fahnstock clip using a screw and solder lug.
d) Now using 6 6-32 1/4" long screws, mount the variable capacitors and turn the knob to make sure that the stator does not run into the ends of the screws, if it does you well need to place washers between the panel and the capacitor frames so that they clear.
a) First solder the wire coming from the 9th tap switch contact/ end of coil to the bottom of the binding post.
b) Glue the impedance matching transformer to the back of the panel between detector and the headphone jack and solder one side of the primary winding to the detector crystal holder.
c) Solder one side of the matching transformer secondary to both the ring and tip terminals of the stereo headphone jack. Solder the other end to the sleeve terminal.
d) Solder a wire to connect the frames of the two variable capacitors together. Also solder the unused end of the matching transformer primary to the one of the two capacitor frames. Solder the wire coming from the tap switch wiper to one of the capacitor frames as well.(see photo)
e) Solder a wire from the stator of the tuning capacitor C1, the one closest to the antenna and ground binding posts, to the antenna binding post. Solder another wire from the ground capacitor C2 to the ground binding post.
5) Final Assembly
a) Set the panel into the box onto the mounts and install the mounting screws.
b) Use glue stick to glue the dial faces to the front panel and install the knobs.
c) See next step on how to use the set.
Step 9: How to Use the Radio
2) Connect up the headphones.
3) Mount a small piece, about 1/4", of iron pyrite in the jaws of the detector and position the cats whisker so that it is touching the surface of the iron pyrite. If you are using a germanium diode install it in the Fahnstock clips with the band facing the antenna and ground binding posts.
4) Turn both knobs all the way to the left and select the 1st tap contact. Start turning the left tuning knob slowly to the right and see if you can find a station, if you find a station but it is towards the high end of the dial move the tap selector up one and turn the tuning knob down until you find it again.
5) If you are using the antique style detector and the signal is coming in very weakly you can try moving the cats whisker around on the surface of the iron pyrite until it comes in more strongly, this well take some trial and error until you find were it works best.
6) If you are receiving two stations at once and one is drowning the other out you can use the ground coupling knob to help single out one or the other.
7) Make a logbook of what settings brought in what stations, this well help finding the station again later.
8) Fiddle with it, it takes some practice to tune these kinds of radios and remember the only thing powering the radio is the radio waves received by the antenna so the signals well not always be that loud but a nice long antenna well collect more energy producing louder signals.