Page 143 - 15 Dangerously Mad Projects for the Evil Genius

Chapter 9 ■ Covert Radio Bug 121

should see the meter start to twitch and then move from a hot glue gun. Drill two small holes for the
quite vigorously. You’ll find that the meter is wire ends of the antenna. This is going to be
sensitive enough to detect a phone call from a few outside of the box, so the holes need to line up
meters away. with the terminal block.

Step 7. Box the Project
Theory
Assuming everything is working, we can put the
Referring back to Figure 9-15, a small signal in the
project in a box if we like. As with all the projects
gigahertz frequency will be induced in the loop
in this book, first find a box that is big enough, and
antenna. This is rectified (see Figure 9-25) by the
then work out where the project parts will fit
germanium diode. A germanium diode is used
before drilling holes for the switch variable resistor
because it has a much lower voltage drop than the
and the meter.
usual silicon diode (0.3V compared to 0.6V). This
Figure 9-24 shows the layout of the components makes it more sensitive.
in the box.
To measure how strong the signal is, think about
Fix the stripboard in place at the end of the box, the average signal. If it is not rectified, then half
with the terminal block pressed against the end of the time it is positive and half the time it is
the box. Use self-adhesive pads or blobs of glue negative, making the average signal always zero.

Figure 9-24 Fitting the cell phone detector into a box