74HC08
33K
1K
22K
5VDC
regulated
74HC14
100µF
100µF
probably because they are too easy to ignore. My
theory is that rodents will find a noise unpleasant
if I do — so it must be extremely loud and must
stop and start unpredictably. Just when a pack-
rat thinks he can get some sleep, it starts again.
BUILDING AN UNPREDICTABLE
SOUND SCREAMER
To make this happen, I needed a high-speed
oscillator to generate an audio frequency, and
slow-speed oscillators to start it and stop it. The
diagram in Figure
A
shows what I had in mind. It
may not make much sense yet, but you can refer
back to it as we go along.
Because there will be multiple oscillators, I
decided to use a 74HC14 chip. It contains six NOT
logic gates, each of which can oscillate if you add
just a resistor and a capacitor. This is much easier
than using six 555 timers, or a microcontroller
programmed to do six things at once.
A test schematic is shown in Figure
B
, using an
LED to show what’s going on. To breadboard it, the
easiest way is by standing the resistors vertically
so they fit between pins of the chip. You can see
this in Figures
C
and
D
on the following page
.
You’ll need a regulated 5VDC supply, which
TIME REQUIRED:
2–4 Hours
DIFFICULTY:
Easy
COST:
$45–$50
MATERIALS
» Breadboard, full size
» Bulk jumper wire, 22 gauge (4 feet)
» Jumper wires with alligator clips each end (4)
» Stereo amplifier, 12VDC, 20 to 40 watts/
channel TPA3116-D2 or similar, from eBay
» AC-DC adapter, 12V, rated at least 2A
» Piezo tweeters (2 or more) for car stereo, eBay
» LM7805 or L7805 voltage regulator
» 74HC14 hex inverter chip
» 74HC08 quad AND chip
» Transistor, 2N3904
» Standard LED, any color
» Basic speaker, 8
Ω
, 2" or larger for testing
» Capacitors, electrolytic, 100µF (4)
» Capacitors, ceramic: 33nF (2), 0.1µF (1),
0.47µF (1), and 1µF (2)
» Resistors: 1k
Ω
(1), 4.7k
Ω
(1), and 22k
Ω
(4)
» Resistors: 10k
Ω
, 15k
Ω
, 33k
Ω
, 47k
Ω
, 68k
Ω
,
100k
Ω
, 220k
Ω
, and 470k
Ω
(2 of each) for
experimenting
TOOLS
» Wire cutters, wire strippers, miniature flat-
blade screwdriver, needlenose pliers
» Multimeter
» Magnifying glass
» Capacitance tester (optional)
» Oscilloscope (optional)
Adobe Stock-chrisharvey and SergeyBitos
Slow
speed
oscillator
Slow
speed
oscillator
Audio
oscillator
Audio
amplifier
AND
AND
CHARLES PLATT is the author of
the bestselling Make: Electronics, its
sequel Make: More Electronics, the
Encyclopedia of Electronic Components
Volumes 1–3, Make: Tools, and Make:
Easy Electronics. makershed.com/platt
A
B
93
make.co
M85_092-95_EFF_F1.indd 93M85_092-95_EFF_F1.indd 93 4/10/23 1:48 PM4/10/23 1:48 PM
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