Page 159 - 15 Dangerously Mad Projects for the Evil Genius

Chapter 10 ■ Laser Voice Transmitter 137

very strange to observe, because it does seem like
the receiver is anticipating the clap.

Duplex

As we mentioned in the introduction, the Evil
Genius is not overly interested in anything that
anyone else has to say; however, if you wish to
have two-way communication, you will need to
Figure 10-26 The fully assembled transmitter make two receiver/transmitter pairs.

When setting them up, you will have to avoid
Using the Project the problem of feedback. Feedback occurs when

the sound entering the first transmitter is received
Alignment
in the first receiver, whose speaker is audible to the
It is surprisingly difficult to get a laser dot to hit microphone of the second transmitter, whose
the diffuser on the receiver from a reasonable output then goes back to the first receiver, and
distance. Ideally, both the receiver and transmitter around and around forever.
should be fixed in place. One way to do this is to
It’s a bit like the audio equivalent of standing
rest the receiver on the window sill of the Evil
between two mirrors that are opposite each other.
Genius’ friend and mount the transmitter on a
While it does produce some interesting sounds, it
tripod. You can do this by drilling an additional
will not help you hear what people are saying.
hole in the transmitter case that is slightly smaller
You can avoid this feedback by keeping the
than the bolt on a tripod. You can then carefully
receiver and transmitter a little way apart.
screw the tripod into the case, which will “self-
tap” a thread into the hole.
Be careful not to tighten the bolt too tight. For a Theory
more solid solution, you could drill a bigger hole
and glue a nut to the inside of the transmitter. This is a good project for learning a bit more about
electronics. It covers amplification and modulation,
Actually, aligning the transmitter with the
two components needed for almost every type of
receiver is just a case of careful aiming from the
electronic communication.
transmitter.

Amplitude Modulation
Time Travel
If you have a radio, it will probably have a switch
One very spooky effect of this project is that if you
somewhere that lets you select AM or FM. These
have the whole apparatus in one room and
acronyms stand for Amplitude Modulation and
someone standing at the transmitter claps their
Frequency Modulation. Although AM and FM
hands, the person at the receiver will hear the clap
operate on different frequencies in public
come through the loudspeaker of the receiver
broadcasting, it is not the choice of frequency that
before they hear the real clap from the person!
differentiates AM from FM, but the way in which
A moment’s thought will tell you that this is just
the sound signal is sent over the airwaves.
because light travels faster than sound, but it is