Chapter 2
Building a Crystal Radio
In This Chapter
Understanding how crystal radios work
Building your own crystal radio
Listening to stations on your radio
In this chapter, we guide you step by step through how to build one of the simplest of all useful electronic circuits: a crystal radio. On this radio receiver you can pick up AM broadcasts.
Now, this radio is pretty basic: it isn’t a particularly good receiver, only one person can listen to it at a time (it uses an earphone instead of a speaker) and its tuning isn’t very sensitive. You’re lucky if you can receive two or three different stations even if dozens are broadcasting in your area. But this crystal radio is unique in that, unlike every other electronic circuit we describe in this book, it has no obvious source of power: no batteries or other power supply. The only source of power a crystal radio uses is from the radio waves themselves.
If you prefer, you can buy a kit to build your own crystal radio. Amazon sells some pretty good ones priced from £8 to 20. Although you can probably round up the parts separately for less than the cost of a kit, a few of the parts are relatively hard to find otherwise. (One option to consider is buying a kit to obtain these few hard-to-get parts, but build the radio itself using the instructions we provide in this chapter.)
Looking at a Simple Crystal Radio Circuit
Figure 2-1 shows a basic crystal radio receiver circuit. As you can see, this circuit consists of just a few basic components: an antenna and a ground connection, a coil, a variable capacitor, a diode and an earphone.
Figure 2-1: Schematic diagram for a crystal radio.
The antenna, of course, captures the radio waves travelling through the air and converts them into alternating current (AC). In order for current to flow, a complete circuit is required. The ground connection is what completes the circuit, allowing current to flow.
The combination of the coil and the capacitor form the tuning circuit. The inductance of the coil combines with the capacitance of the variable capacitor to create a circuit that resonates at a particular frequency, allowing that frequency to pass but blocking others. In a basic crystal radio such as the one shown in Figure 2-1, the tuning circuit isn’t very precise. As a result, you may well hear several stations simultaneously. You can, however, build more sensitive tuning circuits that can hone in on individual stations.
The diode forms the detector part of the circuit. It simply converts the amplitude modulated RF signal that comes from the antenna and tuning circuit to an audio frequency AC signal combined with the RF carrier wave. The characteristics of the sensitive piezoelectric earphone eliminate the RF component and any DC offset without the need for a low pass filter or AC decoupling capacitor (see Chapter 1 of this book). The remaining audio frequency AC signal is extremely small, but it’s enough for the earphone to convert the current to sound.
So that, in a nutshell, is how a crystal radio works. The rest of this chapter shows you how to build a crystal radio of your own.
Figure 2-2 shows the finished crystal radio that you build. Note that you can construct a crystal radio in many different ways, and so the instructions in this chapter are by no means definitive. Use your imagination when looking for materials to build your radio. You could use an old tin, the case of an old gadget or a really beautiful wooden box, for example.
Figure 2-2: A finished crystal radio.
Gathering Together Your Parts
You need a handful of parts to build your crystal radio. The following is a recommended list:
At least 15 m of antenna wire: You can use almost any wire for the antenna. A roll of 18-AWG or 1 mm diameter solid core wire does just fine.
A few metres of hook-up wire: To connect the radio to a ground connection.
At least 15 m of 30-AWG or 0.3 mm diameter, enamel-coated transformer wire for the coil: Plus, don’t forget something to wrap the coil on – we use an empty drinks bottle.
Variable capacitor (also called a tuning capacitor): These are becoming hard to obtain, though you can purchase them online. Alternatively, you can easily harvest one out of an old radio that no longer works.
The variable capacitor is an optional component. If you can’t find one, you can still build your crystal set; you just can’t tune out competing stations.
Germanium diode: You can order one over the Internet. Just use your favourite search engine to search for ‘1N34A’ to find several suppliers.
Piezoelectric earphone: Regular earphones such as the kind you use with an MP3 player or mobile phone don’t work. Search online for ‘piezoelectric earphone’ and you can pick one up for a few pounds.
Board to mount the radio on: About the size of a paperback book (15 by 23 cm) is sufficient.
Something to make your electrical connections: We like to use a four-pole terminal block.
The germanium diode, variable capacitor and piezoelectric earphone are the three parts that can be a bit tricky to track down. You may want to purchase a crystal radio kit from a hobby or school-supply shop and harvest those three parts from the kit.
Creating the Coil
When you look at a crystal radio, the first thing you’re likely to notice is the large coil. This coil usually consists of 100 turns or more of small-gauge transformer wire wrapped around a non-conductive tube anywhere from 3 to 13 cm in diameter. The coil is an essential part of the radio’s tuning circuit.
You can use many different types of materials to wrap the coil around. Here are a few ideas:
A small empty drinks bottle.
An empty toilet-paper roll.
An empty bottle of contact-lens fluid or another similarly sized plastic bottle.
A short length of PVC sprinkler pipe.
A short length of broom handle.
A short length of a cardboard tube.
In short, you can use any sturdy cylindrical object made of an insulating material as the core of your coil. As long as it’s cylindrical and not made of metal, go right ahead.
We use an empty drinks bottle (see Figure 2-3). To make the coil look better, you can spray-paint the bottle with black paint (but don’t use metallic paint!).
Figure 2-3: A coil wound on an empty drinks bottle.
As for the choice of wire for the coil, the most common is transformer wire, which is coated with thin enamel insulation rather than encased in plastic insulation. Wire wrapped with plastic insulation works, but enamel insulation is thinner and so allows the turns to be spaced closer together.
The number of turns in the coil and the diameter of the cylinder you wrap the coil around determine how much wire you need, but you want to wrap at least 100 turns.
To determine how much wire you need for each turn, multiply the diameter of the cylinder by 3.14. Then, multiply the result by the number of turns to determine how much wire you need.
For example, suppose you’re wrapping the coil around a 5 cm wide cylinder and you want to wrap 100 turns. In this case, each turn requires 15.7 cm of wire (5 × 3.14) and so you need around 16 m of wire (15.7 × 100). Remember to allow another 50 cm or so of extra wire at each end of the coil to connect the coil to the radio circuit, meaning that you need about 17 m of wire in total for the coil.
Here’s the best way we’ve found of winding the coil:
1. Place the tube you’re winding the coil around on a screwdriver blade or other long narrow object so that the tube spins freely.
That way, you can turn the tube and slowly feed wire from its spool onto the tube. This method keeps the wire from becoming twisted as you wind the coil. If you have a vice on your workbench, you can clamp the screwdriver horizontally in the vice and then slide the tube onto the screwdriver so that the tube spins easily.
2. Attach one end of the transformer wire to one end of the tube.
You can do so with a dab of strong glue or you can punch a hole through the tube and feed the wire through. Either way, be sure to leave 15 cm or more of wire free. This gives you plenty of wire to connect the coil to the circuit when you finish winding the coil.
3. Turn the tube slowly while feeding wire from the spool onto the tube.
Each half turn or so, use your fingers to carefully scoot the wire you’ve just fed up against the turns you’ve already wound. The goal is for each turn of wire to be adjacent to the previous turn, with no gaps between the turns.
You need to keep a bit of tension on the wire as you feed it onto the tube in order to keep the windings nice and tight. If you slip and let go of the tension, several turns may unravel and you have to untangle them to restore the coil’s tightness.
4. Wrap the coil in sections of about ten turns each.
When you finish each section, dab a little hot glue on it to hold it in place.
5. Use a little hot glue to secure the last turn of the coil, or cut a slit in the tube and slide the wire through it, when you reach the end of the tube (or run out of wire).
Be sure to leave about 15 cm of free wire after the last turn.
The coil needs to have a nice, tight appearance with no major gaps between the turns and you require about 15 cm of free wire on each end of the coil.
Assembling the Circuit
When you’ve prepared your coil (as we describe in the preceding section), the next step is to assemble the various parts of the radio on your wooden base measuring around 15 by 23 cm. To make your radio look good, consider painting or staining the wood before you assemble the circuit.
Here’s a list of the parts you need to assemble the circuit:
Your coil
A four-position terminal block
A germanium diode (1N34A or similar)
A tuning capacitor
One length of hook-up wire, approximately 4 cm long
Two lengths of hook-up wire, approximately 8 cm long
You need the following tools to build the crystal radio circuit:
Phillips-head screwdriver
Soldering iron and some solder
Strong glue, such as a hot glue gun and some glue sticks
Wire cutters
Wire strippers
Figure 2-4 shows the layout for the assembled crystal radio circuit.
Figure 2-4: Layout for the crystal radio circuit.
We refer to the individual terminals of the terminal block by numbering them from left to right: 1 to 4. Terminal connectors in the top row are given the letter A and those in the bottom row are given the letter B. Thus, the terminal at the top left of the terminal block is terminal 1A, and the terminal at the bottom right is 4B.
To assemble the crystal radio circuit, follow these steps:
1. Glue the terminal block, tuning capacitor and coil to the board.
Use Figure 2-4 to judge the placement of each of these parts. Be sure to give the glue enough time to cool and harden before you continue.
2. Connect the diode between terminals 1A and 3A.
Interestingly enough, the direction in which you connect the diode doesn’t matter in a crystal radio circuit.
3. Strip about 1 cm of insulation from both ends of all three lengths of hook-up wire.
4. Connect one end of the 4 cm length of hook-up wire to terminal 2A on the terminal block, and then connect the other end to terminal 4A.
5. Use some sandpaper to scrape gently the enamel insulation off the ends of the wire.
6. Connect the two wires from the coil to terminals 1A and 4A.
7. Solder one end of one of the 8 cm hook-up wires to the centre lead of the capacitor and one end of the other 4 cm wire to either one of the other leads.
Which of the two outside leads you use doesn’t matter.
8. Connect the free ends of the wires you soldered in Step 7 to terminals 1A and 1D of the terminal block.
You’re done!
When the radio circuit is assembled, look it over to make sure that you’ve connected all the pieces as shown in Figure 2-4.
Stringing up an Antenna
A long antenna is vital to the successful operation of a crystal radio. In general, the longer the antenna, the better. If possible, try to make your antenna at least 15 m.
You can make your antenna from just about any type of wire, insulated or not. A large roll of 18-AWG, solid hook-up wire is perfect.
The best configuration for a crystal radio antenna is to run the wire horizontally between two supports as high off the ground as you can get them, as shown in Figure 2-5. You probably won’t find convenient poles as we show in the figure, but if you look around you should be able to locate two suitable points to which you can connect the ends of your antenna. Fence posts, trees, washing lines, a swing or almost any other tall structure does the trick.
Figure 2-5: Setting up your antenna.
Notice that one end of the antenna wire has to run to the ground to a convenient place for connecting it to your crystal radio. You need to run this wire to the location where you intend to operate your radio.
Ensuring that your antenna is well insulated from the ground is vitally important. Remember that wood isn’t a great insulator and most metals, of course, are excellent conductors. Therefore, be very careful about how you support the ends of the antenna wire to make sure that you don’t inadvertently ground the antenna.
If you use insulated wire for the antenna, you can secure the ends to wood by using small eye screws available from any hardware store. Screw the eye screw into the wood and then simply tie the end of the antenna wire to it.
If the wire is uninsulated, you need to support it with something that doesn’t conduct electricity. We suggest browsing the sprinkler parts department of your local hardware store to find a PVC pipe fitting. You can screw this fitting into wood or use duct tape or zip ties to secure it to metal, and then loop your antenna wire through the fitting and tie it.
A crystal radio is a relatively safe electronics project, but a few dangers are associated with the antenna. Here are some things to be careful of:
Don’t string up your antenna in a thunderstorm! Lightning loves wires, and you don’t want to tempt Mother Nature by providing her with a convenient path to discharge her fury through.
Don’t operate your crystal radio in a thunderstorm!
Don’t under any circumstances run your antenna wire anywhere near a power cable or other utility line. That’s a sure way to end up in hospital.
Connecting to Ground
An effective ground connection is every bit as important as a good antenna (see the preceding section). The best way to create a good ground connection is to use a metal cold-water pipe. Assuming that you’ve placed your antenna outdoors, you may be lucky enough to find an outdoor water tap near the end of the antenna. Then, you can connect one end of a length of hook-up wire to the water pipe and the other end to your crystal radio. (Note that this doesn’t work with plastic pipe, only metal pipe.)
If you can’t find a water pipe, hammer a length of metal bar into the ground. Fifteen centimetres will probably do it but the deeper you go, the better the ground connection.
The easiest way to connect a wire to a water pipe (or whatever you’re connecting it to) is to use a pipe clamp, which you can find in the plumbing section of any hardware store. Use some coarse sandpaper to sand the pipe where you attach the clamp to improve the electrical connection, especially if the pipe has been painted or varnished. Strip 3–5 cm of insulation from the end of your ground wire and wrap it around the clamp. Then slip the clamp around the water pipe and tighten it up as shown in Figure 2-6.
Figure 2-6: A good ground connection.
Using Your Crystal Radio
When your crystal radio circuit is built, your antenna is up and your ground wire is connected, you’re ready to put your crystal radio to the test. Follow these steps:
1. Connect the two leads of the piezoelectric earphone to terminals 3B and 4B of the barrier strip.
2. Connect the antenna lead to terminal 1B of the terminal block.
3. Connect the ground lead to terminal 2B of the terminal block.
4. Put the earphone in your ear – you probably hear a radio station immediately.
5. Turn the knob on the tuning capacitor to hear other stations.
The tuner on this crystal radio circuit isn’t very sensitive, and so you can probably distinguish only two or three different stations.
If you haven’t added a tuning capacitor, you can’t tune to a specific station. Instead, you probably hear several stations at once. Even with a tuning capacitor, you may still hear several stations at the same time, because the tuning circuit for a simple crystal radio like this one isn’t very discriminating.