PROJECTS AM Radio Transmitter
everything can be connected through a terminal
block, but if you don’t have one, you can use
alligator clips to link the upper wires with the
variable capacitor at the bottom. The variable
capacitor (Figures
L
and
M
) actually contains a
pair of capacitors, which is why it has three tabs
linked as shown, connecting them in parallel.
Terminals A and B can be connected with
ground and an external antenna if you want
to pick up real radio stations. But for your test
circuit, this won't be necessary.
Place your radio circuit near the loop antenna
on your breadboard circuit, adjust the variable
capacitor, and you should easily hear the tone
from the 7555 oscillator.
But what is your radio actually doing, here? Flip
back to Figure G, which shows the output from
IC2. This is the signal that goes into the loop of
wire, which is your transmission antenna. The
oscilloscope trace shows that the output of IC2
varied between 0V and approximately 7.5VDC.
Now take a look at Figure
N
. This trace
was measured across the two wires from the
receiving coil in your simple receiver. Somehow
the coil picks up the transmission and turns it into
a voltage that varies between –0.35V and +0.35V.
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0 1 2 3 4 5 6 7 8 9 10
milliseconds
volts
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0 1 2 3 4 5 6 7 8 9 10
milliseconds
volts
N O
Voltage across the receiving coil in the AM radio
receiver.
Voltage across the earphone in the receiver, after
the signal passes through the diode to block
negative fluctuations. The audio signal “rides the
peaks” of the carrier frequency.
Charles Platt, Adobe Stock-Archivist
Earphone
Diode
10K
Coil
Tuning
capacitor
Terminal
block
A
B
K
All the components of your minimal AM radio
receiver. Terminals A and B are available for
ground wire and antenna.
L
Tuning capacitor, also known as a variable
capacitor. Graph squares are tenths of an inch.
M
Tuning capacitors are often sold with an
additional wheel.
68 makezine.com
M83_062-69_BookEx-Radio_F1.indd 68M83_062-69_BookEx-Radio_F1.indd 68 10/10/22 3:33 PM10/10/22 3:33 PM
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