Page 136 - 15 Dangerously Mad Projects for the Evil Genius

Chapter 9 ■ Covert Radio Bug 115

A Bug Detector

One of the interesting things about this bug is that
because of its short range and low power, it
broadcasts at a strength that is no stronger than the
radio signals received from commercial radio
stations in the FM band. This makes it almost
impossible to build a detector to find out if you are
being bugged using this kind of device.
A lot of modern bugs are based on cell phone
technology. They are basically a phone that can
be concealed somewhere and turned on remotely
either by a phone call or SMS message. It is
much easier to detect such bugs—at least when
they are on.
So, we are going to build a detector for these
kinds of bugs. The Evil Genius also finds it a good
way to detect minions using their cell phones when
they should be doing their minion homework. This
particularly annoys the Evil Genius, especially
when their phones are better than his. So having
detected such errant behavior, the Evil Genius can
enjoy a good cell phone stomping session.
The detector looks like a detector should look
(Figure 9-14), with a proper analog meter, whose
needle swings over to indicate the strength of the
microwave frequencies that cell phones use.
The antenna is attached using screw terminals to
allow easy switching between different antenna
designs. This will allow you to experiment with
different designs to work with the different
frequencies at which mobile phones operate.
Figure 9-15 shows the schematic diagram for
the bug detector.

The design uses a germanium diode as a
detector and amplifies the weak signal from the
phone using an operational amplifier. For a more
detailed description, see the “Theory” section that
Figure 9-14 Cell phone bug detector
follows.