Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 56)

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Back in Experiments 24 (Anti-Capacitor) and 25
(Magnetic Bridge) we talked about how an electric
current through a coil creates a magnetic field and how
this magnetic field can be used to “bridge” air gaps in a
transformer. What if the transformer air gap was larger,
perhaps a few inches? The distant coil would still pick up
some of the energy, but not much. If the original
(“transmitting”) coil and current through it were much
larger, then the electromagnetic field from it could still be
picked up by a “receiving” coil and produce a small
current even if the distance was many miles. This is the
concept of radio, which uses electromagnetic waves to
send information through the air. The coils used for
transmitting and receiving these signals are called
antennas. Today the air around us is full of radio
transmissions for things such as music, television,
cellular phones, aircraft navigation, communication with
probes in outer space, radio-controlled toys, and
thousands of other uses. The Federal Government
makes sure that all of these uses operate on different
frequencies so that they don’t interfere with each other. A
wide range of schemes are used for encoding the radio
signals with the information being sent. These are called
modulation. You’ve probably heard of AM and FM
radios. These stand for Amplitude Modulation and
Frequency Modulation.
There are many different radio signals floating around,
but we only want to listen to one. Think of this as being
in a large, crowded room and trying to talk to someone on
the other side. We solve this by connecting our antenna
to a capacitor in sort of an inductor-capacitor oscillator.
Remember from Experiment 24 that an inductor passes
DC (low frequencies) and blocks AC (high frequencies)
while a capacitor does the opposite. By combining these
two components we can “filter” out a small range of
frequency that we will listen to. By varying the
capacitance (using a tuning capacitor like the one
included in your Electronic Playground) we can adjust or
“tune” the range of frequency that we are listening to. The
tuning dial on all AM and FM radios is a variable
capacitor just like yours. After filtering out the undesired
radio frequencies we amplify the remaining signal using
a transistor amplifier like the one used in Experiment 16,
decode the modulation into the original audio signal
(electrical energy representing voice or music), and
produce sound with a speaker.
The antenna you will use is a 50
μH and a 800μH coil
connected together and wrapped around an iron bar. The
variable capacitor varies from 50 to 200pF (which is
.00005 to .0002
μF). Notice that these values are much
smaller than the transformer and the other capacitors you
have; that is why they have not been used in any previous
experiments. Their symbols are:
You will now build an AM radio receiver. Connect the
wires according to the Wiring Checklist, connecting the
battery wire last since this will turn on the circuit. You will
get better circuit performance if you keep your wires
short, so don’t use longer wire lengths than you need to.
This is because at high frequencies long wires start to act
like small inductors. AM radio uses a frequency range of
500 to 1600 KHz, where these effects are just starting to
become noticeable. After connecting the last wire, turn
your variable resistor all the way to the left. This part acts
as the speaker loudness control, turn it down (to the right)
if your sound is too loud. Now adjust your tuning
capacitor, turning it slowly. You should be able to hear a
few local radio stations. Although this radio uses the
same types of circuits as AM radios sold in stores, this is
a very simple radio receiver. Take a look inside an old AM
radio in your house, you’ll see a lot more components.
So don’t expect to get as good of performance as with
radios sold in stores.
If you turn the tuning capacitor slowly you should receive
several stations. If not, try walking around your house or
outside with your Playground. This may give you better
radio reception. Or you can connect a wire from battery
spring 26 to a water pipe or other electrical “ground”.
Also check your wiring, as this circuit is more complicated
than most that you have built.
Take a look at the schematic. The antenna and tuning
capacitor for tuning are on the left. Transistor NPN1
amplifies the received radio signal using its current gain.
The diode and .0047
μF capacitor are a simple Amplitude
Modulation detector, converting the radio signal back to
the original audio signal. The audio is amplified using
NPN2’s current gain and converted into sound by the
speaker.
EXPERIMENT #38: Transistor Radio
Antenna
Tuning Capacitor
Symbol for Antenna
Symbol for
Tuning Capacitor