the next, merrily up and down, relating to one
another in a mysteriously harmonious way. Your
circuit is playing not just any pitches but musical
notes!
How mysterious indeed! What could be
animating such a device? Well, we’ll tell you...
Right after these hot tips!
VARIATION 1: THE ATARI PUNK CANDLE
If you’re the experimental type, now would be a
good time to swap out pot 3 for an LDR, aka light-
dependent resistor. This way, you can control
the transitions between notes with changing
light intensity! Grab a candle, enter a dark room,
and see if you can find the sweet spot where
the candle’s flickering makes your Undertoner
arpeggiate like a late 70s disco hit.
NOTE: If you’re a budding synthesist, you might
recognize this scale (and project namesake) from
another classic beginner project — the so-called
“stepped tone generator” or Atari Punk Console —
see page 82 for a new version!
VARIATION 2: LIKE A VACTROLLING STONE
Instead of using a NAND gate oscillator to gate
our synth, what if we could use it to arpeggiate
our scale? This variation is quite similar to the
Atari Punk Candle, but with a twist! Rather than
using the oscillator on pins 11, 12, and 13 to
gate the synth, you can make it flash an LED on
and off. Then position the LED right next to the
photoresistor, so the maximal amount of light is
hitting the photoresistor during the “on” portion
of the cycle. Now, as your light switches off and
on, you’ll hear the circuit quickly cycle through
the notes in the undertone series.
This goofy strobe light-controlled modulation
source is called a vactrol and, believe it or not,
these are everywhere. If you’ve ever plugged a
MIDI cable between two musical instruments, all
you’ve really done is hooked up several pins to a
bunch of tiny light sources that flicker on and off.
Because the information is communicated here
optically — in photons and not electrons — the
communicating systems are entirely electrically
distinct. This way, you can have your MIDI cable
connect any two devices without having to worry
about blowing them up with the wrong voltage.
HOW THE HECK IT WORKS
The Undertoner is, in a word, baffling. A few logic
gates somehow give us a collection of pitches
that, no matter how hard they try, simply cannot
play “out of tune.” How in the world do a bunch
of subatomic particles understand a musical
phenomenon with such deep cultural roots?
Let’s break this circuit down to make sense of
it. First, let’s talk about how to make a NAND gate
oscillate. In talking about NAND gates, it helps to
make what computer scientists call a truth table.
It shows us what the output of our NAND gate will
be when our inputs are “on” (represented by a 1)
or “off” (by a 0):
NAND Input 1 NAND Input 2 Output
0 0 1
0 1 1
1 0 1
1 1 0
As you can see from this table, if we take one
of the inputs to our NAND gate and “tie it high”
(i.e. connect it to the positive battery terminal), we
effectively have an inverter at the other input (a
logic 0 gets a 1, and a 1 a 0).
Now imagine — if we take that inverting input
of the NAND gate and connect it to the output, we
create a feedback loop. By connecting a capacitor
to the so-called “feedback input” of the NAND gate
we add a receptacle for electrons. Specifically, this
cap serves as a receptacle that delays the amount
of time it takes for voltage at the output to be
registered at the input. This is significant because
the input waits for the voltage at its pin to reach
a certain threshold before defining its state as 1
or 0. As it fills and empties, the capacitor sets the
duration of this process. A bigger capacitor means
more time between cycles, i.e. a slower frequency,
and thus a lower pitch.
Finally, by connecting a potentiometer
between the output and input, we can control
this frequency with resistance, slowing down or
speeding up the amount of time it takes for the
capacitor to fill up. (Think of the capacitor as a
bucket, and the resistor as a hose. Bigger hose/
smaller bucket, faster fill; and vice versa).In
other words, the potentiometer gives us variable
pitch control. And you said logic was boring!
DIY USIC: The Undertoner Synth
56 makezine.com
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