DI^Y

CIRCUITS

EVASIVE BEEPING THING

Infernal noisemaker turns pals into enemies. By Brad Graham and Kathy McGowan

The Evasive Beeping Thing is appropriately named, since it dutifully does exactly what its name implies: it sends out a 5-second, high-pitched beep every few minutes. The source is extremely difficult to locate because of the way that high frequencies can penetrate objects and trick our ears.

Photograph by Sam Murphy

You’ve probably encountered something similar in the real world, such as a failing appliance or a beeping wristwatch buried deep in a couch. As you know, high-pitched sounds seem like they are coming from all directions, which makes tracking them to the source a real chore. Add the fact that the sound only happens once every several minutes, and it may drive a person loopy as they spend all day looking for the source of the sound. Well, that’s our goal, anyhow!

To generate the high-pitched audio wave, you’ll connect a small speaker like those found in tiny electronic devices (cellphones, transistor radios,

MATERIALS

2N3904 transistors ( 2) or any generic NPN style
Resistors: 1MΩ, 100kΩ, 10kΩ ( 2), 1kΩ, and 100Ω
Capacitors: 100μF, 0.01μF
555 timer
Small speaker from a transistor radio or the like
9V battery and battery clip
Small perforated board to wire the parts onto
Container to hide the unit inside

a tweeter from a small speaker system, etc.) to a simple audio oscillator set to a frequency near the upper limits of our hearing capabilities. The oscillator is triggered to run for approximately 5 seconds every few minutes by a 555 timer circuit with its output connected to the oscillator. The higher the frequency rating of the speaker, the farther the

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