EVASIVE BEEPING THING SCHEMATIC: The 555 is set up so that its output will turn on the 2 transistor audio
oscillators formed by the pair of NPN transistors.
high-pitched sound will travel, which is why a 2" or
3"-diameter tweeter is optimal for this project.
The small speakers shown in Figure B are perfect
for this project. The rating of the small speaker is
not important, since the audio oscillator will drive
speakers from 4Ω to 16Ω with very little power
output. The speaker on the top left was the one
we decided to use in the final design because it fit
nicely into the cabinet we chose to help disguise
the evil device. Now, let’s get on to the design of
the electronics that make this unit work.
Above, you’ll see the schematic of the Beeping
Thing. The 555 is set up so that its output will turn
on the 2 transistor audio oscillators formed by the
pair of NPN transistors. Just like most 555 timer
circuits, the timing cycle is controlled by the 2
resistors on pins 6, 7, and 8, and by the capacitor
connected to pins 1 and 2. If you play around with
the values of the 2 resistors, you can control the
duty cycle of the timing pulses in order to alter both
the off time and on time, to create more or less
beeping each time the cycle repeats.
The capacitor controls the actual frequency of
the timing pulses: the larger the value, the longer
the duration between each timing cycle. In a really
large room, you might want a longer beep and cycle,
132 Make: Volume 14
so a 220μF capacitor could be used, and the 100kΩ
resistor could be swapped for a 220kΩ resistor. For
a smaller room, where it may be easier to locate the
device (e.g. a friend’s office), the capacitor could be
changed to 47μF and the 100kΩ resistor to a 10kΩ
for a very short beep. The best plan is to simply
build the unit as is, and then fine-tune the components until you’re happy with its operation. And
yes, a variable resistor would be easy to adjust.
Now, where do you hide the beast? Well, since
this unit emits hard-to-locate high frequencies, your
options are endless. The high-pitched sound will exit
through the smallest hole in whatever box you place
the parts into. We decided to cram the works into an
old wall adapter (Figure A) that has all of the guts
removed, including any connection to the AC lines.
The little speaker fits nicely into the top of the box,
and there’s just enough room for the 9V battery and
small circuit board. Figure C shows the completed
circuit going into the wall wart box.
There was just enough room to get all the parts
inside, so we couldn’t install an on/off switch. But
that was OK since the top of the box just snapped
together and we could simply unclip the battery.
The unit will run for many days on a full battery, and
if you strategically place the beeper, it may take that