Experiment 31: One Radio, No Solder, No Power
262 Chapter 5
Experiment 31: One Radio, No Solder,
No Power
Time now to go back one more time to inductance and capacitance, and dem-
onstrate an application which also makes use of the way that waveforms can
be added to each other. I want to show you how a simple circuit with no power
supply at all can receive AM radio signals and make them audible. This is of-
ten known as a crystal radio, because the circuit includes a germanium diode,
which has a crystal inside it. The idea dates back to the dawn of radio, but if
you’ve never tried it, you’ve missed an experience that is truly magical.
You will need:- Rigid cylindrical object, such as a vitamin bottle. Quantity: 1.
- 22-gauge hookup wire, solid-core. Quantity: 60 feet.
- 16-gauge wire, stranded. Quantity: 100 feet.
- Polypropylene rope (“poly rope”) or nylon rope. Quantity: 10 feet.
- Germanium diode. Quantity: 1.
- High-impedance headphone. Quantity: 1.
The diode and headphone can be ordered from http://www.scitoyscatalog.com.
You cannot use a modern headphone of the type you wear with an MP3 player.
Some of these items are shown in Figure 5-59.
First, you need to make a coil. It should be about 3 inches in diameter, and you
can wind it around any empty glass or plastic container of that size, so long
as it’s rigid. A soda bottle or water bottle isn’t suitable, because the cumula-
tive squeezing force of the turns of wire can deform the bottle so that it isn’t
circular anymore.
I chose a vitamin bottle that just happened to be exactly the right size. To re-
move the label, I softened its adhesive with a heat gun (lightly, to avoid melt-
ing the bottle) and then just peeled it off. The adhesive left a residue, which
I removed with Xylol (also known as Xylene). This is a handy solvent to have
around, as it can remove “permanent” marker stains as well as sticky residues,
but you should always use latex or nitrile gloves to avoid getting it on your
skin, and minimize your exposure the fumes. Because Xylol will dissolve some
plastics, clearly it’s not good for your lungs.
After you prepare a clean, rigid bottle, drill two pairs of holes in it, as shown in
Figure 5-60. You’ll use them to anchor the ends of the coil.
Now you need about 60 feet of 22-gauge solid-core wire. If you use magnet
wire, its thin insulation will allow the turns of the coil to be more closely spaced,
and the coil may be slightly more efficient. But everyday vinyl-insulated wire
will do the job, and is much easier to work with.
Begin by stripping the first 6 inches of insulation from the end of the wire.
Now measure 50 inches along the insulated remainder and apply your wire
strippers at that point, just enough to cut the insulation without cutting the
Figure 5-59. Just add wire and a coil, and
this is all you need to receive AM radio
signals. The black disc becomes the
tuning dial, after it is screwed onto the
variable capacitor (right). This is actually
an optional extra. A germanium diode
(center) rectifies the radio signal. The
high-impedance earphone (top) creates a
barely audible sound.
Figure 5-60. A large, 3-inch diameter empty
vitamin bottle makes an ideal core for a
crystal radio coil. The drilled holes will
anchor wire wrapped around the bottle.