 Crystal
Radio Science Fair Projects
Information
for Teachers, Parents, and Beginning Radio Builders
The Xtal
Set Society and its members are happy to
provide the information in these pages to parents and teachers. Let’s
encourage yet another generation to build and experiment with the magic
of the crystal radio (and let’s include the girls this time!). We
have a free project on-line, the Quaker Oats box radio. If you are building
your own radio from some other plans, many of the hints on these pages
should still be of help.
What is a Science
Fair Project?
Most school districts have specific directions for students to follow
as they try to answer a question. There are several types of projects
including
Projects that are experiments where you try to answer questions about
how something happens
Projects that use a model to show how something works using more than
one condition to compare how this works in different situations
Projects that are collections of items perhaps found in a particular geographic
region
What should our hypothesis be for a crystal radio science project?
There are many questions we can ask about crystal radios. Here are just
a few ideas:
• How many stations will I hear?
• Will I hear more radio stations during the day or night?
• Do radios with bigger coils work better?
• Do different lengths or kinds of antennas affect the performance?
• Which circuits work best?
• Which kinds of minerals work best as a detector?
What exactly is a crystal radio?
Crystal
set radios pick up AM radio without batteries or electricity. In the simplest
terms, the broadcast station puts out enough power in the form of a radio
signal to be picked up by a crystal set. The crystal radio antenna captures
this electromagnetic energy, and the signal then passes through the crystal
detector. Through a process called "rectification," the detector
samples half of the radio wave and transforms the wave into a signal that
can produce audio in the earphones. This mysterious process first intrigued
great inventors such as Braun, Marconi, and Pickard, and it continues
to fascinate electronics buffs, amateur radio operators, and engineers
today.
The
hobby of building and listening with a crystal radio had its first and
biggest craze in the 1920's. Once radio stations began broadcasting all
over the country, people began buying and building crystal radio kits.
At that time a true mineral crystal was used as the detector. The most
popular crystal was galena, and a fine piece of wire called a "cat's
whisker" was used to touch the crystal and find the "hot spot"
on the rock where a station would come in. These days, many hobbyists
use the modern day diode instead of a crystal, but there are still experimenters
who strive for the thrill of getting Radio Japan on a rock.
In
the 1950's the hobby enjoyed a revival of sorts; fathers remembered building
the sets as kids in the 1920's, and they wanted their sons to build them
also. A surprising number of men who grew up in the 1950's built crystal
radio sets with their fathers and as Boy Scouts. During the 1950's all
sorts of crystal set kits became available, including some on the backs
of cereal boxes. Now the hobby is coming of age again as those boys from
the 50's have become fathers and want to pass on this radio magic.
The
crystal set is the basis for modern day radio and communications equipment;
the basics it embodies live on in a wide variety of radio systems. For
this reason, a study of crystal sets is a great place to start to build
a foundation in radio electronics. You also can learn about natural processes.
For example, if you build a short wave (high frequency) AM set, you'll
experience firsthand the constantly varying state of the ionosphere. The
signals from distant stations will ebb and flow, particularly at night.
Similarly, it is possible to build a radio that will detect "whistlers,"
which are brief radio signals created by lightning strikes around the
globe. Many of our members are interested in the crystal set because of
its simplicity and complexity; it contains few parts but exhibits many
concepts. Crystal sets are a great tool for teaching, and they are a superb
hobby.
Quaker
Oats Box Radio summary and hints
Our Quaker oats box radio project can be built in a few
hours using a round Quaker oats box and parts that you can purchase from
the society. A sharp junior high school student may be able to put the
radio together all by herself, but grade schoolers will need adult help
and supervision. The directions include soldering the parts together which
obviously needs to be done with adult supervision. However, you can use
the "alligator clips" included in the kit and just clip the
wires together instead of soldering. This is what we did recently with
a 3rd grade student (shown in the picture on the right) – she just
clipped the wires together and was involved with every aspect of the project.
If the child is in late grade school, have them hold the Oats box between
their knees to hold it steady while they wrap the wire around it. It is
fine to use lots of masking tape to hold the wires and parts in place.
By varying the length (inductance) of the coil, this radio can be used
to receive either AM stations or shortwave (this is true for any crystal
radio).
Getting the radio
to work after it is built!
Crystal
Radios require a special high impedance earplug;
these are available from the Society and other sources. Regular stereo
earphones will not work. Why? The first crystal radio headphones were
different from the modern stereo headphones of today, they usually had
input impedances of 2000 ohms or higher. (The input impedance for modern
stereo earphones is a small fraction of this value). If you are an electrical
techy type you can get a transformer and rig up newer phones to work,
but the cost is much more than the inexpensive $3.50 crystal earplug.
(Of course, experimentation with input impedances could be the basis for
a more advanced science fair project).
The Quaker
Oats box requires that you have a good ground. Clamping or tightly wrapping
the ground wire onto a cold water pipe works well. The antenna can just
be some hook-up wire from a parts store (or the society), strung around
the room. If you want the best reception, string up a 75 foot horizontal
antenna.
 Setting
up your Science Fair Project in the school gym
Many
of the school science fairs take place in the gym or in another big meeting
room at the school. This isn’t the greatest location for crystal
radio reception due to all the metal in the frame of the building, but
it can be made to work. To improve your chances of getting the crystal
radio set to actually work at the display, try to get a spot next to the
window and a radiator. You can throw the antenna out the window! Grounding
to the base of the cold water pipe coming into a water fountain or nearby
restroom sink should work fine (even if the wire leading to the pipe is
long).
The other
option is to build a loop crystal set. Loops will work without a ground
or antenna. Although the loop may not work well in the gym, (except perhaps
near the window) it can be easily taken outside. When the judges meet
with your child for the science fair, he can take them outside to hear
the radio. Our book Crystal Set Loopers &
More has a number of plans to build crystal set loops of different
sizes. Loops are directional; you can point them in the direction of the
station to want for optimum performance. The reasons for this are well-understood,
but experimentation with this could be the basis for another good science
fair project!
©
Copyright 1997-2008. The Xtal Set Society. All rights reserved.
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