Crystal Radio to Go: a Portable, Battery-less Crystal Receiver in a Pizza Box

Greetings, all!

The original “Crystal Radio To Go” project first appeared about 2 ½ years ago, and I have been overwhelmed with the positive response since then. Thanks, everyone, for the encouragement to keep improving this Instructable! I’m happy to say that now there is a…

NEW & IMPROVED RECIPE

…for the pizza radio. The project is simpler to build and easier to use, and it’s even more reliable in picking up AM broadcast stations than before.

Even more good news…the entire step-by-step procedure for building the radio is now illustrated in a series of short videos on YouTube (as you can see)!

Part One

Experience the fun of building one of the earliest kinds of broadcast radio receivers! With it, you’ll be able to pick up stations without using any batteries. It relies entirely on the electrical energy already present in the airwaves. You’ll also be able to take your pizza box radio “to go,” because (unlike circuits of the past) this one does not depend upon a connection to a cold water pipe or a fixed antenna to work surprisingly well.

1. Begin by gluing two 12” cake rounds together, white sides facing out. Spray adhesive is faster and stronger, but ordinary household glue works fine if you put weight on top while they dry. Make a second set the same way.

2. On one side of both sets, draw a line down the middle and put an ‘X’ in the center. Punch a hole with a nail, a small screwdriver, or an awl. Make sure the hole is no bigger than 3/16” (the diameter of an 8-32 bolt).

3. Now is the time to decorate one side (opposite the side with the center line) of one pair of stuck-together cake rounds. Choose your favorite pizza-esque colors!

4. Find the center of the top lid and draw a mark, and then punch a hole with a nail, a small screwdriver, or an awl. Make sure this hole is also no bigger than 3/16” (the diameter of an 8-32 bolt).

5. With the box upside-down, put a nail all the way through, up from the lid and through one set of cake rounds. This way it is centered and stays in place. Next, trace the outside of the round with a pencil, so you’ll have a guide when it’s time to glue it. For now, remove the set of rounds after you’ve traced the circle.

6. Find and mark the center of the bottom inside the box. Next, mark a spot about 2” directly to the left, and another 2” directly to the right. Make holes in both of these in the same way you did with the lid. A bolt goes through these holes, with a washer, and two nuts for center connectors (this is so the bolts stay tight while the ring terminals are being connected).

Part Two

7. Cut two wires 15” long, strip the ends, and add ring terminals at one end of each. These will be the “input” leads to the breadboard.

8. Cut two more wires 30” long, strip the ends, and add ring terminals at one end of each. The other ends are soldered to pieces of copper foil tape (~2” long).

9. Cover one half of a disk with foil. Tape in a couple of places to hold it in place. Cut a V-shaped notch at the center by the hole. This is just to make sure the foil does not come into contact the bolt. Trim off the excess foil.

10. Attach the copper tape near the outer edge of the cake round in line with the center hole.

11. Cover the foil completely with packaging tape, and allow some extra just to be on the safe side. Follow the same procedure above with the other set of cake rounds.

12. Glue the “stator” (it’s “stationary” and it’s not the pizza-esque “rotor” that “rotates”) to the lid, with the foil side facing up. Use a nail through the center hole as a guide to position the stator as you glue it down.

Part Three

13. Note that this crystal radio operates with its pizza box “chassis” upside down, with the tuning capacitor located on the lid and the loop antenna located on the outside walls of the bottom of the box. Wires from the loop you are about to wind in this step will come through two holes and be attached to the terminals. One half of the variable capacitor (the stator) was glued down to the pizza box lid in a previous step. The ring terminal from this 30” wire will be attached to one of the bolts above it later in the build. The rotor fits on top, and the 30” wire from it will be attached to the other bolt.

14. The loop consists of about 65-70’ of magnet wire (this is 24 gauge but the thickness is not critical), wound 11-12 times around the four outer sides of the box, with each turn spaced 1/8” apart.

Using a razor blade or a utility knife, carefully scrape the insulation completely all the way around a 1” length of the magnet wire, on both ends.

It is very important that copper is visible anywhere the wire makes contact with the two terminals inside the box – without those connections, the radio will not work!

15. On the top (narrow) side of the pizza box, punch two holes for the loop wires to enter. Both holes are directly above the two bolts on the inside of the box, one of the holes near the edge next to the lid, the other hole near the edge of the bottom of the box.

16. Use either a ruler or a guide with marks every 1/8” to accurately space each of the 12 turns of the loop. Make sure the lines are clearly visible on the corners of the box.

17. Attach three strips of double-sided tape on each side of the box, the width of its side (about 1 ½”).

18. Thread a length of one end of the magnet wire down either one of the holes on the top side of the box. Then wind some of the bare copper wire around the bolt directly under the hole.

19. As you wind the loop, press the wire down firmly into the double-sided tape so it keeps the loops of wire secure. When the loop is completely wound, thread the remaining wire end through the other hole, and then wind the bare copper wire around the second bolt.

20. Wrap white tape (athletic tape works well) around all four sides. This will insure the loop stays wound and it also will maintain the appearance of the pizza box.

Part Four

21. Take a gander at the schematic diagram of the crystal radio. You’ve successfully completed the part of the circuit on the left (the coil and the variable capacitor or “pizza-tuner”). The rest of the circuit will be wired together on the small (170 tie point) solderless breadboard.

22. You can now attach the top half of the variable capacitor. Begin with the 1¼ ” bolt and a washer, and put it up from the bottom through the stator (which is now glued to the bottom of the pizza box and is indeed “stationary”). Place a washer on top of the stator before putting the rotor on top. This one acts as a sort of spacer between the two halves. Finally, put the top half on, add the third washer, then the nut, and tighten it all up (but don’t over-tighten!). Attach the ring terminals to the bolts above.

23. Note how breadboard holes are identified, with very tiny letters (A-J) and numbers (1-17) that are hard to see! You’ll find that the diode, resistor, and capacitor are easier to manage if you first trim their leads to about ½”.

24. Start by first loosening both screws of a gangable connector, inserting the fragile earphone leads into it and then tightening the screws. It doesn’t matter which lead goes on which side, but make sure that bare wire is making contact with both of them.

25. Component placement

· Gangable connector from earphone: B14, B16

· Diode: H1, H5 (striped end)

· 50K Resistor: G5, G8

· .001 mfd Capacitor: I5, I8

· Jumper wire: F5, E14

· Jumper wire: F8, G16

26. The ring terminals from the 15” wires are now attached to the bolts above, and the nuts are tightened down. The end of one of the leads is placed at J1, and the other at J8 (these are the “input” to the rest of the circuit).

Your crystal radio is now complete! Congratulations on a job well done!

Part Five

As mentioned in the previous video, if the radio isn’t working, first check your work and make sure all the connections are tight and secure.

Look over the schematic and see that you have put all the parts in the right order. Double check to make sure that the diode is not installed backwards.

It’s easy to get the wires wrapped around and yanked out, so take your time while tuning in stations.

Wing nuts on the bolts might help with keeping connections tighter between the magnet wire and the ring terminals.

Sometimes the copper tape has a tendency to come loose, so make sure there’s a solid connection between those two points and the aluminum foil.

Tightening or loosening the top nut on the tuning capacitor has a dramatic effect on what stations can be heard and at what point they appear on the dial.

High-impedance earphones are notorious for poor performance, so test that component if possible. The same is often true of germanium diodes, as well.

Try listening in a quiet room. Keep in mind that signals will usually be very faint like a whisper. Also, avoid fluorescent lights because they wreak havoc with reception.

There are several ways to experiment with the tuner. Pressure from your hands on the top of the tuner will also vary the frequency. Even side to side movement of the rotor will do the same.

Try adjusting the loop antenna. Stations that are “broadside” to it (facing the top or bottom side of the pizza box) are usually stronger.

Keep in mind that there are no “stops” built into the variable capacitor. What this means is that as it goes through the entire 360-degree rotation, it will tune through the AM broadcast band twice. In other words, the 12 o’clock position of the rotor wire is where the tuner is at maximum capacitance, because that’s where most of the aluminum foil on both sides is in closest proximity. As you rotate the capacitor wire to the 6 o’clock position, the tuner will be at the minimum capacitance, because at that point the least aluminum will be in proximity. As you continue to rotate the capacitor clockwise to the 9 o’clock position, you will tune through the band again, until you are back where you started at 12 o’clock.

It is possible to disconnect both wires from the tuning capacitor and still hear a radio station without it. The germanium diode by itself is fairly sensitive to electromagnetic signals in the atmosphere. Simply put, sensitivity is a measure of how many signals a receiver (in this case, the simplest receiver of all) can pick up. Selectivity, on the other hand, is a measure of the receiver’s ability to distinguish between signals at different frequencies.

While the crystal radio is highly sensitive in that it picks up many stations, it is not very selective, which is why you may actually be hearing several stations all at once. The tuning capacitor increases selectivity by “favoring” certain frequencies and rejecting others. In other words, the coil and the variable capacitor work together to resonate with one particular frequency so that (ideally) you hear only one station at a time. “Variable,” in this case, indicates that you are able to “vary” the frequency with a spin of the (pizza) dial.

This crystal radio circuit does not require either an external antenna or a connection to ground, but there is lots of room for experimenting with both! Absolutely do not try to make an antenna out of any wires that carry electrical current! But see what happens when you attach an antenna wire from the stator terminal bolt to a chain link fence or the backstop at a baseball field. Look for long metal structures, the higher the better. But please use common sense: never put up an antenna when there is a chance of an electrical storm.

If you are using this radio for a science fair project, or you are demonstrating it before a larger group, you should be able to slip a funnel over the earphone to make it act like a megaphone. It’s a simple way to make the signal audible to several listeners at once.

If you decide you’d like to add some amplification, so you don’t have to rely on the crystal earphone for listening, you might try a simple LM386 board with a battery and a small speaker. These boards can be sometimes be obtained online for less than a dollar or two.

Amateur radio operators (or “hams” as they are affectionately known) have been experimenting with radio ever since the early days of “wireless.” Knowledge of Morse code is no longer a requirement to become a licensed amateur, and even elementary school students have learned enough of the necessary theory and regulations to successfully pass the FCC test and get on the air. This crystal set might be your first step into the exciting and rewarding hobby of ham radio. For more information, please take a look at the American Radio Relay League’s website at www.arrl.org.

Happy listening & 73!

The plans for the original "Crystal Radio To Go" as they first appeared on Instructables are included below.

Believe it or not, by the time a radio signal makes its way from a local AM broadcast station (which may be transmitting at 50 kilowatts or more), down to your antenna, and into your receiver, and finally into your earphone, only an extremely tiny amount of the signal’s power will be flowing through it, as little as a billionth of a watt. And yet, even this is enough to make a high-impedance earphone respond with clear, intelligible sounds.

One version I built actually tuned below the bottom end of the broadcast band (550 kHz) and I was able to pick up our local airport beacon with it, You can hear it on the following video. Your mileage may vary, of course...

Airport Beacon

This circuit is basically identical to the designs used in the earliest days of broadcast (minus the pizza box, of course!). You are embarking on an adventure that fellow radio enthusiasts enjoyed nearly a century ago. You may discover, as they did, that the electromagnetic spectrum holds a fascination all its own. So let’s get started!

Step 1: Getting the Kit Ready

If you are making kits for yourself, or for a friend, or you are making several for a group at a summer camp or in a ham radio class, the following will show you how to get them ready. Note that the complete kit requires no soldering, so young people without soldering skills can easily build their own radio in a short time. The instructions may look lengthy and intimidating, but not to worry. Each part of the process is actually very simple, and the details are there just to walk you through each step. If you already have a complete kit and are ready to build it, skip this section and go right to the “Putting the Kit Together” section next.

Complete Parts List

· Large (16”) pizza box

· Six 1” wood squares

· Two Fahnestock clips

· One 1¼” 8-32 bolt, 3 washers, and a nut

· Four ¼” nylon machine screws, 2” long

· Four each: ¼” nylon washers, ¼” steel washers, ¼” steel nuts

· Four 12” diameter cardboard cake rounds

· Heavy duty genuine aluminum foil

· Adhesive-backed copper foil tape, (4” long and 1/2” wide)

· 22-gauge insulated hook-up wire (two 30” lengths, and two 12” lengths = 7’ total)

· 100’ 24-gauge magnet wire on a plastic spool

· High-impedance earphone

· .001 mfd ceramic capacitor

· Resistor, in the 50K ohm range (value is not critical)

· Germanium [not silicon!] diode (1N34, OA47, OA90, or similar)

· Small (170 tie point) solderless breadboard

· Two 2” breadboard jumper wires

· Two small alligator clips

· 50’ of insulated 22-gauge wire (for the antenna)

A few words of advice about parts before we begin: the three most essential components also tend to be the hardest to find. As you gather up the required pieces, the high-impedance earphone, the germanium diode, and the Fahnestock clips ought to be first on your shopping list.

Ordinary binding posts would make an acceptable substitute for the Fahnestock clips and are a little easier to find. These do require drilling two extra holes for mounting, however.

Unfortunately, the earphones are not only somewhat difficult to find, they are also notorious for poor performance. That, and the very faint volume of the signals, can make for a frustrating experience. You might have to try several earphones before you get a good one. Keep in mind that this radio will not work with typical low-impedance headphones or ear buds (not without an additional amplifier circuit, anyway).

A new (as in, non-greasy) pizza box is recommended for this project! I asked for one at the local pizzeria and they were glad to oblige. A little bigger or smaller won’t make much difference; you may just end up with a few more or less coil windings. As for all the other parts, most of them can be found at a hardware store, a hobby shop, an arts and crafts store, and around the house.

Step 2: The Pizza Box "Chassis"

Start by cutting off the two small flaps in the top corners of the box. This will allow the completed radio to close more easily.

Attach six 1” wood squares to the box, using hot glue or strong contact cement. Place four in the corners inside the top lid. Two are evenly spaced apart in the center. Placement of these two squares is not critical, but you can put them about 6” apart and 6” above the center of the unfolded box.

A Fahnestock clip is glued on each of the two wooden squares in the center, with the spring connector pointed down toward the bottom of the box. [Fahnestock, by the way, is German for “flag staff,” and the Fahnestock Electric Company was assigned a patent for the clip that bears its name over a century ago.] Use either hot glue or strong contact cement for attaching the two clips, and be sure not to let the glue interfere with the spring connector.

Drill a 5/16” hole in each of the four corner squares. Drill a 3/16” hole in the center of the bottom half of the box. You can find the center by drawing an “X” from the four corners.

From the top of the pizza box lid, insert one nylon machine screw, with a nylon washer, through each of the four holes. [The metal in steel bolts would interfere with the coil, so nylon ones are used instead, and nylon washers are used on the top of the lid to help maintain the appearance of the white box.]

On the inside of the lid, first place a metal washer on each screw, and then a nut. Tighten all four. A 7/16” wrench fits a ¼” nut, by the way. Then fold the box up and make sure everything fits the way it’s supposed to.

Step 3: A Tuning Capacitor From Cake Rounds

Four 12” “cake rounds” are used to make the two halves of the tuning capacitor. If you would like, now is the time to attach a pizza illustration onto one of the cake rounds. Otherwise, you can always use your imagination and create a pizza decoration of your own later.

Next, draw a pencil line through the center, exactly across the middle of all four disks. Drill a 3/16” hole at the center of each of the four.

Do the following steps for two of the four cardboard disks (but make sure to not use the one decorated as a pizza for this part!). Directly above the center holes in both, and perpendicular to the center line, drill another 3/16” hole, about 1” in from the edge.

Now you will make the two foil “plates” that actually create capacitance within the tuner. These two plates will each have heavy-duty aluminum foil with a copper tape “strap” sandwiched between two adhesive shelf paper “insulators.” Make sure you are not using flimsy, lightweight foil. Non-conductive foil will simply not work for the tuning capacitor. Note that copper foil also works very well here, but it is much more expensive.

Cut a piece of heavy duty aluminum foil 6” by 12”. Use one of the cardboard disks as a guide to trace a half-circle onto the foil. Carefully cut out the foil along the line you have traced. In the center of the straight edge at the bottom of the half circle, cut out a small triangle of foil with about a ½” apex pointing up to the top of the arc. Removing this small section will make it so that no foil comes into contact with the center bolt. Otherwise, the variable capacitor could be shorted out.

At the top of the half-circle of foil, stick a short length (4” or so) of copper foil tape, with most of it extending vertically up beyond the edge of the foil arc. Make sure the foil tape is made with electrically-conductive adhesive. Most of them are, but if in doubt, check continuity with a volt-ohmmeter.

You will fold the tape over onto itself so that about 1/2” is attached on both front and back of the foil, and about 1½” or so extends above the top of the foil (after it is folded over itself). This tab of copper tape will be where the wire will be soldered. Remove the backing slowly and carefully, because copper foil tape has a tendency to curl up on itself and it can be very difficult to undo. It may help to fold it and put a crease at the halfway point before removing the backing.

Prior to encasing the foil in plastic, first “dry fit” the foil and make sure it will fit on exactly one-half of one of the cake rounds.

[There is a shortcut you can take at this point, if you are making just one radio for yourself rather than several for a group or a class. You can follow the directions below on soldering the wire to the copper tab first, and then attach the foil directly to the cardboard with spray adhesive, making sure the wire goes through the 3/16” hole and out the back. Then you can insulate the foil on top of the cardboard disk with packaging tape or even a light coat of shellac. In other words, using adhesive shelf paper as outlined below is not absolutely essential, but it does make for a much easier kit to handle and put together if you intend to have the kit built at a later time.]

If you don’t take the shortcut described in the previous paragraph, then your next step is to cut out two identical pieces of adhesive shelf paper a little larger than 6” by 12” (8” by 14” should do it). The half-circle of aluminum will be sandwiched between these two. Remove the backing on one and position the foil so that there is a little extra plastic all the way around the edges of the foil. Tacking it down to a board helps, as does removing just a little backing at a time.

Once the aluminum foil is securely fastened to the plastic, repeat the process with the second piece of shelf paper. Then smooth out both sides. Trim the shelf paper so that about 1/8” edge surrounds the foil inside, which is now situated between the two sheets of plastic. Be sure there is some copper foil tape at the top that is not covered with plastic, so that a place to solder remains.

On the end of the copper foil tape, solder a 30” length of 24-gauge hook-up wire. Solder the wire onto the front of the copper tab, so that when you go to fold it over, the wire will be facing the 3/16” hole near the edge of the cake round and that will make it easier when it comes time to join the parts together with glue.

Next, attach the soldered tab to the back of the foil capacitor plate with a small piece of ordinary adhesive tape to prevent the thin copper tape from ripping.

The copper foil tape is necessary because solder does not stick to aluminum. Note also that simply attaching the wire to the aluminum foil using copper foil tape to keep it in place does not create a solid and reliable connection for the radio to work.

If you are building kits to be assembled later, secure the 30” length of wire by coiling it and wrapping a rubber band around it to prevent the wire from pulling at the copper foil and tearing it.

The next step is to go back and make a second capacitor plate identical to the first one.

Now you have just built by hand the essential parts of a variable capacitor, which will act as a tuner for the crystal radio. There are two aluminum foil capacitor “plates” insulated with plastic. Two cardboard cake rounds will make up the “stator” (stationary) part of the variable capacitor, and the other two cake rounds will make up the “rotor” (adjustable) part.

Step 4: Gathering Up the Rest of the Parts

Wind 100’ of magnet wire onto a spool. Be sure to keep a rubber band around the spool until you are ready to wind the coil, because the wire comes unwound very easily. Leave a few inches sticking out from both the start and the end of the spool. Carefully scrape about 2” of these ends until you see bright copper all around. A razor blade or craft knife will work better for this step than sandpaper or steel wool, but be careful and use common sense so as not to cut yourself. Also, try not to nick the wire because magnet wire will easily bend and break. You can fix breaks by splicing and soldering, if necessary, but it is much better to avoid them altogether.

To make the antenna, strip a short length of insulation from both ends of the 50’ of hook-up wire and attach alligator clips to the bare copper on the ends.

Collect all the remaining parts and place them into a plastic bag, if desired.

Now your kit is ready to be put together!

Step 5: Putting the Kit Together

Let’s begin! Please read through all of the instructions first, and take a close look at the illustrations of the radio you will be building.

Start by taking a quick inventory and make sure everything is present and accounted for. A complete kit includes:

Crystal Radio Kit Parts List

· Large pizza box (16”), prepared according to the previous steps

· The parts to make the variable capacitor for tuning the radio:

o Two cardboard cake rounds to make up the “stator”

o Two cardboard cake rounds to make up the “rotor”

o Two heavy duty aluminum foil “plates,” insulated with plastic sheets

· One 1¼” long 8-32 bolt, 3 washers, and an 8-32 nut

· 22-gauge insulated hook-up wire (two 12” lengths)

· 100’ 24-gauge green magnet wire on a plastic spool

· High-impedance earphone

· .001 microfarad ceramic capacitor

· Resistor, in the 50K ohm range(blue or brown, with several colored bands)

· Germanium diode (glass, with a single band on one end)

· Small solderless breadboard

· Two 2” breadboard jumper cables

· 50’ of insulated 22-gauge wire with two alligator clips (for the antenna)

Step 6: Winding the Coil

The first step may be the most important of all: winding the coil. Gently fold the pizza box over in half to make it easier to reach all four nylon corner posts. The key to building a good coil is maintaining the right amount of tension on the wire. If the wire is too tight, it will easily break; if it is too loose, it will start to unwind itself right before your very eyes. Magnet wire is brittle and breaks easily. The last thing you want is one big tangled mess of wire, so the way to avoid that is to wind slowly and carefully, and to keep the wire taut. If at all possible, plan to wind the coil all in one sitting.

Find the end of the green magnet wire on the spool. Scrape the insulation away from an inch or two of it (if necessary). A razor blade or craft knife usually works better for this step than sandpaper or steel wool, but be very careful so as not to cut yourself. Also, try not to nick the wire because magnet wire will easily bend and break. You can fix breaks by splicing and soldering, if necessary, but it is much better to avoid them altogether.

Mounted near the center of the inside box lid are two Fahnestock clips. Fold over about an inch of the wire onto itself, and then while pressing down on the right Fahnestock clip, insert the wire into the hole. Lift up on your finger and presto! The bare copper end of the wire ought to fit snugly into the clip. Now you are ready to begin the actual winding. Starting with the Fahnestock clip on the right, you are winding the coil in a clockwise direction.

While holding the plastic spool, stretch out just enough wire to make a turn around the bottom of the
corner post in the bottom right corner. The wire should be just the right size to fit into one of the threads of the nylon bolt. The idea is to keep the windings fairly evenly spaced. Try to keep just enough tension on the wire to bring it from that point over to your left. Turn the corner at the bottom of that post on the left, and then set your sights on the next goal, which is the corner post in the upper left of the box lid. From there, you will slowly and carefully wind the coil to the next corner post (in the upper right). Next, stretch the wire out to the same spot where you began your coil, in the bottom right corner. And…repeat. Do that same process about 15-20 or so more times.

At long last, when you finally get to the end of the spool of wire, you’ll need to make one more complete loop around one of the top threads of the nearest corner post. This tiny loop will keep the coil from unwinding itself. Then you’ll stretch the remaining wire to reach the Fahnestock clip on the left.

· What if you’ve got too much wire? You can wind some of the excess around the wooden square that the Fahnestock clip is attached to.

· What if you have too little wire? While still keeping tension on it, you can back up to the next closest corner post, to allow for more wire to work with, and make a small loop around one of the top threads on that post instead.

Attach the wire to the clip the same way you did the first one, making sure that bare wire comes into contact with the metal in the clip.

Before you continue, take a close look and admire your handiwork. Ideally, each turn of wire will be evenly spaced by your using successive threads on the posts. Try to avoid kinks or knots in the wire, and do the best you can in keeping the turns tight on the four posts. Now unfold the box so you can work on the other half.

Step 7: Assembling the Tuning Capacitor

Follow these procedures to build two capacitor assemblies. The two cake rounds without the 3/16” holes near the edge will serve to “sandwich” the wire in between so that the capacitor will turn more smoothly, and they will also provide added strength to prevent the cardboard from warping.

This is the time to cut out the pizza picture and attach it to one of those two rounds (or else, decorate it however you like). Don’t worry if the picture is a little bit larger or smaller than the cardboard disk underneath it. If your pizza does not already have a center hole in it, punch one through with a pin or sharp pencil using the center hole on the opposite side as a guide. It just needs to be large enough for the bolt to go through from underneath, but not so big that the washer slips through.

Spray adhesive is recommended for this step (and throughout this project) because it provides a bond that is generally stronger and smoother, but ordinary household glue will still give satisfactory results.

In a well ventilated area, spray adhesive on the half of the disk with the 3/16” hole near the edge (make sure the overspray does not come into contact with anything it shouldn’t.) It might help to mask off the other half of the disk. You can also spread household glue over the cardboard, but try to cover it evenly with as thin a coat as possible so you end up with a minimum of “glue bumps.” That way the capacitor will be able to turn more smoothly.

While the spray adhesive (or glue) is still tacky, carefully poke the hook-up wire through the 3/16” hole, and then attach the plastic-encased foil to the disk, making sure to align the bottom edge of the plate with the center line on the disk. Spread the foil to minimize wrinkles. If you do use household glue instead of adhesive spray, you ought to evenly distribute weights on top (or use clamps) and allow it to dry overnight.

Do the following for both of the two remaining disks. Spray adhesive or spread glue over one side of the “plain” undrilled disk, and then attach it to the foil-covered disk. Clamps (as long as they are not too strong!) or weights can help create a tighter bond. Obviously, the pizza illustration (if desired) will be on the top of the “rotor” section of the variable capacitor.

When completed, the flip side of the “rotor” will have the aluminum foil plate facing the “stator’s” foil plate on the bottom half. Thus, when the capacitor is fully assembled, the two plates will be separated by a total of only two layers of adhesive shelf paper.

The next step is to install the bottom half of the tuning capacitor (the “stator”). It will be easier to keep it aligned if you first put the 1¼” bolt and washer (temporarily) up from the center hole in the bottom of the pizza box. Another method is to first drill a wood screw through a small block of wood, and then put the screw through the bottom of the pizza box to serve as a guide. Just make sure the wood screw is not too big; otherwise it could enlarge the 3/16” hole that is already there.

Take the disk assembly and spray the non-foil side with adhesive. This will be attached to the bottom of the pizza box with the foil side up and the wire in the 12 o’clock position. Carefully slide it onto the bolt and make sure it is centered. Weights help ensure a tight bond. When the adhesive is dry, remove the bolt and washer.

You can now attach the top half of the variable capacitor. Begin with the 1¼ ” bolt and a washer, and put it up from the bottom through the stator (which is now glued to the bottom of the pizza box and is indeed “stationary”). Place a washer on top of the stator before putting the rotor on top. This one acts as a sort of spacer between the two halves. Finally, put the top half on, add the third washer, then the nut, and tighten it all up (but don’t over-tighten!).

Step 8: Wiring the Circuit

Take a gander at the schematic diagram. What you have just completed is the part of the circuit on the left (the coil and the variable capacitor or “pizza-tuner”). The rest of the circuit will be wired together on the small breadboard.

You might think of building on the breadboard sort of like playing Battleship or Bingo. In other words, with the long end going left to right, think of the holes in the top row as A-Q. With the short end up and down, think of those holes in the first column as 1-10. Electrically, A-1 through A-5 are all connected together, as are A-6 through A-10, as illustrated.

Component leads that are too long can create a problem if they get too near each other. Before installing them, first snip the leads with wire cutters to only about ½” long or so. This way, they will be easier to bend and fit into the breadboard holes. Cut the leads down to size on the resistor, capacitor, and diode. If you don’t have wire cutters handy, nail clippers will work in a pinch (so to speak).

Attach the earphone wires to the gangable connector by first placing the stripped ends of the wires in the proper holes and then tightening them down with the screws. The connector is necessary because the earphone leads are too flimsy to place directly into the breadboard holes very easily, and it also ensures a solid connection to the earphone. Once that’s done, you can then mount the connector onto the breadboard. The leads themselves will go into holes N9 and P9, but the plastic case will actually cover up most of the bottom right corner of the breadboard, as shown in the diagram.

Next, place the remaining components onto the breadboard in the following order:

Diode, unmarked end: A3

Diode, striped end: E3

Resistor: E4, H4

Disc capacitor: E2, H2

Jumper: E5, N6

Jumper: H5, P6

12” input wires: A1, H1

Once these parts are all installed, you can go ahead and take the backing off of the adhesive strip under the board and place it in one of the upper corners (left or right). The idea is to make sure the box will still be able to close with the board in place, without disturbing the coil windings.

You’re almost done! The final steps are putting the wires from the variable capacitor in place. The bare wire end from the “rotor” is attached into the spring on the Fahnestock clip on the left. Make sure the magnet wire doesn’t come out in the process.

The bare wire end from the “stator” goes onto (you guessed it) the Fahnestock clip on the right. Again, make sure the magnet wire doesn’t come out in the process. The placement of the two wires is just for convenience – the radio will still work fine if the location of these two wires are reversed. If possible, though, attach the alligator clip from the antenna to the same connection as the stator wire for better performance.

Now take one of the 12” wires and attach it to the left Fahnestock clip. The other end of the wire is connected to the diode on the breadboard (the side of the diode without the stripe) in the A-1 hole.

The remaining 12” wire is attached to the Fahnestock clip on the right. Its other end is connected to the capacitor lead in hole H-1. Check your work against the placement diagram.

Step 9: The Smoke Test

Notice that you will be listening with what is called a “high-impedance” earphone – the “ear bud” type headphones you are probably already familiar with will not work with this circuit, because they are “low impedance.”

The antenna wire has an alligator clip on both ends. One of these clips should be attached to a Fahnestock clip in the center of the radio (whichever one is connected to the stator of the variable capacitor). Once you have stretched the wire out, it is not necessary to clip the other end anywhere, but you can experiment with attaching it to a chain link fence or a baseball backstop, if you wish.

In fact, you should be able to pick up at least a station or two without attaching the external antenna at all (but performance will be greatly improved if you do). Also, if you are unable to make room for the entire 50’ length, don’t worry – it will still work with however much or little you have stretched out. The beauty of this design is that (unlike most crystal radios of the past) there is no need to connect the other Fahnestock clip to a cold water pipe for a ground. But again, it may be fun to see what happens if you do…

If you are listening to the radio at a desk or a table, you may find it helpful to put a stack of books behind the lid to support it, so that the coil remains at a 90-degree angle from the tuner. Also, you may want to experiment with the direction that the coil is pointed. Signals are strongest “broadside” to the coil. In other words, point the center of the coil in the direction of the nearest (or most powerful) radio station.

Now it’s time for what amateur radio operators (hams) used to call the “smoke test”! Check your work and make sure all the connections are tight and secure. Anywhere the magnet wire comes into contact with metal, you ought to see bare copper rather than green insulation (otherwise, there is no electrical connection). Look over the schematic and see that you have put all of the parts in the right order. Double check to make sure that the diode is not installed backwards. Also, any parts in the breadboard need to be tight and secure in their holes. Now, put the earphone in your ear and listen! Keep in mind that signals will usually be very faint like a whisper; it won’t be loud like what you’re probably used to. This is the way they did it in days of yore. Try tuning around with the pizza and see what you can pick up.

You should be able to listen to at least one local station without the need to attach an external antenna. Stretching out the antenna wire as long and as high as possible, and then attaching the alligator clip to one of the Fahnestock clips will make a dramatic difference in both the number of stations you hear and the volume in the earphone.

As for troubleshooting, the most common problem is loose or poor connections. If you’re not hearing anything, check every connection very carefully and compare it to the schematic.

To avoid a “rat’s nest” of wires, you can shorten the length of some of them if desired, but only after you are sure that the radio actually works first. The wire from the stator might be a little long, for example. It is not necessary to shorten any of the wires, however. If you do decide to, just make sure that there is still enough slack left for the box to close properly.

Congratulations! You have made a working radio receiver just like they did a hundred years ago (minus the pizza, of course!). Your radio should last a good long time. Obviously, there are no batteries that need recharging! Every location in the country should be well within range of at least one radio station that you will be able to “copy.”

Step 10: Operating the Pizza Box Radio

First of all, you will probably notice that listening to this radio is nothing at all like listening to modern audio devices. It is much more like trying to pay attention to a faint whisper. If possible, try to listen to the radio in a quiet room.

There are several ways to experiment with the tuner. Tightening or loosening the nut at the top will affect the frequency to which you are tuned. Pressure from your hands on the top of the tuner will also vary the frequency. Even side to side movement of the rotor will do the same.

With 100’ of magnet wire wound into a coil, the radio might very well be capable of tuning below the bottom end of the broadcast band at 500 kHz, although many factors are involved in determining your radio’s tuning range. For example, here in the Gem State, this same radio has been able to successfully tune in 379 kHz and pick up the Boise airport beacon. Two letters (“B” and “O”) are slowly and constantly being sent in Morse code at that frequency. At your location, try listening very carefully just to the right and left of the 12 o’clock position. Check AirNav.com under “airports” to see if there might be a beacon near you. Keep in mind that there are no guarantees that you will be able to hear one, but a huge part of the fun of radio has always been experimentation.

There are also all sorts of possibilities with antennas! Look for long metal structures, the higher the better. Absolutely do not try to make an antenna out of any wires that carry electrical current! But see what happens when you attach the antenna wire to a chain link fence or the backstop at a baseball field. There’s lots of room here for trial and error. And please use common sense: never put up an antenna when there is a chance of an electrical storm.

Another experiment you can try is attaching a cold water pipe ground connection (although nowadays, with PVC plumbing, these are getting harder to find in the typical house). Just connect the antenna to the stator’s Fahnestock clip and a wire running to ground to the rotor’s clip. A ground connection is not a necessity with the pizza box radio but you might improve reception by adding one. If you happen to know of any ham radio operators, they will know how to find or make a good earth ground connection. For the ambitious, you can drive an 8’ ground rod into the dirt and use that!

Happy listening & 73!