12 Nicolas CollinsChapter 3: Circuit Sniffing
You will need:
- A battery-powered AM radio.
- A battery-powered amp/speaker.
- An inductive telephone pickup coil or a loose electric guitar pickup.
- Optionally: a hundred feet of light-gauge insulated wire, an audio plug
and 2 long pieces of wood.
Radios
Radios make the inaudible audible. Unlike microphones and amplifiers, which
merely make very quiet acoustic sounds much louder, radios pick up
electromagnetic waves that have no acoustic presence whatsoever and translate
them into signals that can be heard through a loudspeaker. Radios are
manufactured for listening to intentionally transmitted electromagnetic waves,
i.e., those sent from radio stations. But they can also be used to sniff out other
types of waves, such as those emitted by lightning, sun spots, Aurora Borealis,
meteorites, camera flash units, computers, etc. Generally speaking AM radios
(the cheaper the better) do a better job of picking up these “spurious” noises than
FM radios.
Put batteries in the radio and turn it on; if it has a band switch set it initially to
AM. Try moving it around various electrical appliances: fluorescent lights,
electric motors, computers, portable CD players, cell phones, MP3 players (iPod)
and remote controllers for RC planes and cars are especially noisy. Fire off a
camera flash next to it. Experiment with tuning the radio to different stations,
between them and to the dead bands at either end of the dial.
As the FCC often warns you, certain electrical appliances can cause “radio
interference.” What this means is that, as a byproduct of whatever useful thing
they are doing, they emit lots spurious electromagnetic radiation in the same
frequency region as radio and TV broadcasts -- they whistle while they work. As
you tune the radio it picks up different frequencies (mostly very high) of
electromagnetic waves, shifts them down into the range of our hearing and
amplifies them. Compared to radio stations these appliances put out very weak
signals -- the noise from a computer drops off rapidly as you move the radio a
few feet away (hence the FCC advice on what to do.)
If your radio has a FM band, try it as well. The technique of FM radio
transmission and reception is designed to minimize interference, but strong
periodic signals (like the clock frequency of a computer) can sometimes be tuned
in.