Hardware Hacking 13Coils
An alternate approach to picking up electromagnetic signals is to use a simple
coil of wire and an amplifier. A telephone pickup consists of yards of thin
copper wire wrapped around an iron slug. Plugged into an amp this coil acts
like a radio antenna for low frequencies. Stuck on a telephone receiver (or held
against any other loudspeaker) it picks up the electromagnetic field generated by
the voice coil of the speaker, allowing you to record your landlord making
unsavory threats.
Plug the tap coil into the portable amp and repeat the same experiments we did
with the radio. Sometimes you will hear different types of sounds from the same
appliance. The coil is small enough that you can move it close in and to precise
locations, like a stethoscope. Open up a computer and pass it slowly over the
motherboard, or over a laptop, and note the change in sound as you move from
the CPU area to the RAM to the disk drive to the CDROM. Listen to small
motors in fan, vibrators and toys; notice the change in pitch as you change the
motor speed. Take a ride on the subway and listen to the motors and doors as
you come in and out of stations. Cozy up to a neon sign.
Jérôme Noetinger, Andy Keep, Nathan Davis and others have made beautiful
use of this secret magnetic music.
The stethoscope-like accuracy of the coil moving over a circuit board makes it a
useful, non-destructive device for pinpointing the location of interesting sounds
that we can later tap off directly with a wires soldered to the board.
If you move the coil near the speaker of your amplifier it will begin to feed back
with the coil that moves the speaker cone (see next chapter.) As with feedback
between a microphone and speaker, the pitch is affected by the distance
separating the two parts, but here the pitch changes smoothly and linearly,
without the odd jumps caused by the vagaries of acoustics, giving you a
Theremin-like instrument. Try this with a full-size guitar amplifier for greater
range.
Speaking of guitars, you can use a guitar pickup in place of a telephone tap – a
guitar pickup is just a coil of wire, wrapped around a magnet, inside an
expensive package. You can repeat the above experiments with a whole guitar in
your hands, but a loose pickup is handier. At repair shops you can sometimes
buy cheap the low-end pickups removed when better ones are installed. Jump
ahead to Chapter 6 if you need advice on wiring the pickup to a cord and plug.
As most guitarists know, “single coil” pickups are better at picking up hum and
weird electromagnetic noise, while “humbuckers” are so called because they
tend to be less sensitive to exactly the kind of garbage we want to hear here.