Experiment 29: Filtering Frequencies
254 Chapter 5
So here’s the specification for this particular coil in an audio crossover network.
Forty feet of 20-gauge copper magnet wire, wrapped in 200 turns around a
spool of 1/16–inch-thick plastic with a hub measuring 7/8 inch in length be-
tween the flanges and 1/2-inch external diameter. Total retail cost of materials
if purchased separately: probably about $1, assuming you can find or make a
spool of the appropriate size.
Conclusion: there’s a lot of mystique attached to audio components. They are
frequently overpriced, and you can make your own coil if you start with these
parameters and adjust them to suit yourself.
Suppose you want to put some thumping bass speakers into your car. Could
you build your own filter so that they only reproduce the low frequencies?
Absolutely—you just need to wind a coil, adding more turns until it cuts as
much of the high frequencies as you choose. Just make sure the wire is heavy
enough so that it won’t overheat when you push 100 or more audio watts
through it.
Here’s another project to think about: a color organ. You can tap into the out-
put from your stereo and use filters to divide audio frequencies into three sec-
tions, each of which drives a separate set of colored LEDs. The red LEDs will
flash in response to bass tones, yellow LEDs in response to the mid-range, and
green LEDs in response to high frequencies (or whatever colors you prefer).
You can put signal diodes in series with the LEDs to rectify the alternating cur-
rent, and series resistors to limit the voltage across the LEDs to, say, 2.5 volts
(when the music volume is turned all the way up). You’ll use your meter to
check the current passing through each resistor, and multiply that number
by the voltage drop across the resistor, to find the wattage that it’s handling,
to make sure the resistor is capable of dissipating that much power without
burning out.
Audio is a field offering all kinds of possibilities if you enjoy designing and
building your own electronics.