1 2 3 4
8 7 6 5
1 2 3 4
8 7 6 5
IC2
R4
Antenna
C5
R1B
R2B
C1B
Timing, 2.2K
Timing, 2.2K
Current limiting, 330 ohms
Timing, 100pF
Bypass, 10nF
7555 timer
R1B
R2B
R4
C1B
C5
IC1
Additional Components
9VDC
1 2 3 4
8 7 6 5
1 2 3 4
8 7 6 5
IC2
R4
Antenna
C5
R1B
R2B
C1B
Timing, 2.2K
Timing, 2.2K
Current limiting, 330 ohms
Timing, 100pF
Bypass, 10nF
7555 timer
R1B
R2B
R4
C1B
C5
IC1
Additional Components
9VDC
F
The schematic version of the breadboard circuit in Figure E.
PROJECTS AM Radio Transmitter
and capacitor C4 if you wish. Then add the extra
7555 timer, as shown in Figures
E
and
F
. The
components that you placed on the breadboard
previously are still there, but they are grayed out.
The new component values in this circuit will
generate a frequency around 800kHz. The circle
of yellow wire in Figure E, connected through a
330-ohm resistor with the output from IC2, is a
loop about 2" in diameter, which will function as
your transmitting antenna. The 330-ohm resistor
prevents IC2 from being overloaded.
Here’s how the circuit works. Pin 4 of a 555 or
7555 timer is the Reset pin, which puts the timer
on hold when the pin voltage is near negative
ground, but allows the timer to run when you
apply a voltage near the power supply. Notice the
horizontal yellow wire in Figure E which connects
the Output pin of IC1 with the Reset pin of IC2.
9VDC
7555
7555
100pF
2.2K
2.2K
330
10nF
Antenna
E
Adding a second timer that
runs at a radio frequency.
This means that the output of IC1 switches IC2
on and off. IC2 doesn’t mind being switched on
and off quickly, even at audio frequencies as
high as several kilohertz.
Notice that C1B, which you can see in Figure
F, is 100pF. That’s picofarads, not nanofarads.
Be careful not to get your p’s and n’s mixed up.
When you apply power to the circuit, IC2
generates a carrier wave of around 800kHz,
which is switched on and off by IC1. In Figure
G
, an oscilloscope was attached to the output
from IC2 so that you can actually see how this
happens. (In reality, there would be as many
as 1,000 oscillations inside each burst, rather
than the dozen or so shown in Figure G. The
oscilloscope downsamples them so that you can
get an idea of what is going on.)
But what if you don’t have an oscilloscope?
Charles Platt
66 makezine.com
M83_062-69_BookEx-Radio_F1.indd 66M83_062-69_BookEx-Radio_F1.indd 66 10/10/22 3:32 PM10/10/22 3:32 PM
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