Crystal Radio Demonstrations
are one of the best devices for demonstrating a resonating electrical
circuit. They are low cost and easy to understand.
When radio was new, crystal sets were the first
receivers available. They are so simple they don't even
need a battery or other power source. They literally pull power out of
the air. However, before you start believing in perpetual motion
machines, we must state that the first and second laws of
thermodynamics are rigorously obeyed even by crystal radios. The
power they pull out of the air is put into it by the radio station's
transmitter. In fact, the transmitter puts a lot more power in than
thousands of crystal radios could remove.
Tesla was definitely not as eccentric (a polite word
for crazy) as he may have seemed. Electrical power actually can be transmitted
without wires. We do it on a regular basis with radio and television.
However, there are some serious problems involved with transmitting
enough power, to say, run a city.
|Crystal radios contain an inductor
and a capacitor in an electrical circuit tuned to resonate at the
carrier frequency of a radio broadcast. The signal is then passed
through a diode to transform it from AC to DC (see Figure 1) form
so that it can drive a very sensitive ear phone.
The diode at one time was made of a naturally
occurring crystalline material, hence, the name crystal radio. An
antenna is also usually attached to the circuit and the circuit is
RadioShack produces a crystal radio kit which
sells for about $10 and can easily be put together in less than 30
minutes. It works fine for most purposes.
|Figure 1. Effect of
Rectifying a Wave
More adventuresome individuals will want to make
their own from scratch. It can even be done as a class project in a
reasonable amount of time. The
Xtal Set Society has plans for a very
inexpensive unit made from a Quaker Oats box. Owen
Radio Resources page has lots of construction information and about a bizillion links. Plans for our personal favorite can be found on
Ian Purdies's Amateur Radio Tutorial Pages. Once you have a crystal radio
in hand it's time to proceed to the demos.
- Demo 1- Using a Crystal Radio to See Radio
- It's possible to amplify a crystal radio's output by
connecting it to a set of computer speakers rather than using the ear
phone. The first step is to make a jumper wire with the correct
fitting on each end so that it can be connected to both the radio and
the speakers. Generally, the parts can be obtained at RadioShack. If
the radio is simultaneously connected to an oscilloscope the signal
from the radio station can be heard while looking at the
distinctive AM radio wave form.
Playing a crystal radio over an amplified speaker
system takes some of the wonder out of it. However, unless everyone in
the room has their own radio its a problem to demonstrate that the
unit is indeed picking up a station.
A small transistor radio can also be connected to an
oscilloscope and used for viewing either AM or FM radio signals.
Simply cut the ear plug off and strip off some of the insulation, then
connect the ends to the oscilloscope. Be careful not to short the
wires together. This is the best system to use if you have trouble
tuning into a radio station with the crystal set.
- Demo 2- Evaluating the Resonant Frequency of a
- When it's excited by a voltage pulse, a crystal
radio will oscillate at its resonant frequency (see
circuits). This is similar to striking a
bell with a hammer. The hammer provides a mechanical impulse which
makes the bell ring.
To make a crystal radio's circuit oscillate, select
a long connecting wire. Attach one end of the lead wire to the ground
on a power supply set for a voltage of about 12 volts. Wrap about two
turns of wire around the coil. Attach the other end of the wire to
either a 12 volt light bulb fixture or a 5 ohm resistor. This is used
to limit the current flow in the wire and keep the fuses in the power
supply from blowing. Connect an oscilloscope to the radio instead of the earphone. Attach a second connecting wire to the light fixture.
Using the free end of the second connecting wire,
briefly touch the positive lead on the power supply. You should
observe a decaying sin wave on the oscilloscope. The wave represents
the oscillation of the crystal radio's circuit. It decays since the
circuit has resistance.
Note: the crystal radio does become a low powered
transmitter when it is pulsed. Although the signal will be too weak to
transmit any appreciable distance, it could cause interference to
nearby radio receivers. If the crystal set is tuned to a station, it should be
possible to hear the interference by tuning a separate transistor
radio to the same station. Listen for the static sound when the
crystal set is pulsed.
||Set of Amplified Computer Speakers
||These generally come with any
computer. No special type is needed.
||12 volt Power Supply
||12 volts is suggested, but other DC
voltages are useable as long as they are reasonably low powered
||12 volt Light Fixture (any kind)
||Another device such as a 10 ohm
resistor can be used for limiting current.
||Specialized Connector Wire to
Connect Between the Radio and Speakers
||This wire will probably have to be
||Misc. Connecting wires
For more information about wireless
communication and the electromagnetic spectrum visit
The Hidden World
of the Electromagnetic Spectrum.
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