OSCILLATORS THAT MODULATE EACH OTHER
You will need:
• A breadboard.
• One CMOS Quad NAND Gate Schmitt Trigger Integrated Circuit (CD4093).
• Assorted resistors, capacitors, pots, and photoresistors.
• Some solid hookup wire.
• A jack to match your amp.
• A 9-volt battery and connector.
• An amplifier.
• Hand tools.
The 74C14 offers you a fast, cheap, easy route to oscillators whose pitch can be easily
swept over a wide range. With another chip from the same CMOS family we can imple-
ment some more advanced control functions commonly associated with classic analog
The Schmitt Trigger circuit element that turns each Inverter in the 74C14 into a potential
oscillator is also found in other CMOS digital circuits. Most useful is the CD4093 Quad
NAND Gate (see figure 20.1)
This chip contains four identical NAND gates. There are two gates on each side of the
chip, but unlike the spawning salmon of the 74C14, they are arranged in mirror symmetry,
like rutting elks: the outputs of each gate face each other, rather than the same direction.
Note that this chip has the same power connections as the 74C14 Hex Inverter chip we
used in the previous two chapters: + voltage to pin 14, ground to pin 7. All of which brings
us to an important new Rule:
Rule #20: All chips may look alike on the outside without being the same on the in-
side—read the fine print!
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130 HANDMADE ELECTRONIC MUSIC
A NAND gate is a variant of the basic binary function of an AND gate, which for two
inputs generates the following outputs:
Input A Input B Output
0 0 0
1 0 0
0 1 0
1 1 1
You see that the output only goes “true”/1 when both inputs are true/1—democracy in
action: we go to the zoo because it’s the place both kids agree would be fun.
A NAND (NOT+AND) gate adds an inverter stage after the AND logic to flip the
output like this:
Input A Input B Output
0 0 1
1 0 1
0 1 1
1 1 0
Democracy is replaced by contrarian despotism: dad avoids turning off the highway
to visit Mammoth Caves specifically because both kids have been whining to see it for
one hundred miles.
The added inverter stage introduces the principle of knee-jerk denial (discussed in
chapter 18) that transforms this logic circuit into a “gateable oscillator.”
Hook up the circuit shown in figure 20.2. Note that the basic design is similar to our
earlier oscillator: a capacitor between an input and ground; a feedback resistor from the
output back to the input. But where each stage in the Hex Inverter package had just one
input, each NAND gate has two inputs. Because of the combinatorial logic of the NAND
gate, the second input of the gate can be used as a control input to turn the oscillator on
and off: the output of the circuit will only change state (i.e., oscillate) when the control
input is held “high” (+9 volts). If you connect the second input of the gate to ground (0)
the circuit stops oscillating and the output remains in a “1” state (see figure 20.3).
Try both configurations. Remember to hook up power to the chip as you did with the
74C14 (+9 volts to pin 14, ground/– to pin 7). Plug a bit of wire into the breadboard near
Figure 20.1 CD4093 Quad NAND Gate
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the gate input (pin 1 in figure 20.2 and figure 20.3) and alternate connecting the other end
to the + and ground buses. The two inputs to each gate are identical—it doesn’t matter
which you use for the “control input” (the one with the wire moved between + and ground)
and which for the “feedback input” (the one with the capacitor and feedback resistor), as
long as you don’t mix up the two and connect the capacitor and to one and the feedback
resistor to the other, for example. And you can use any of the four NAND gates on the
chip—they function identically—the photos show just one of four possible hookups.
The oscillator oscillates when the control input is connected to +9 volt; it turns off when
the control input is connected to ground. Big deal, you say, we can do this by simply con-
necting and disconnecting the battery. But, because the oscillator’s output consists of a
square wave swinging between “true”/+9v and “false”/ground, we can also use the output
of one oscillator to switch another oscillator on and off. Breadboard the circuit shown in
figure 20.4. Use a large capacitor (4–10uf) and 1 megOhm pot for Oscillator 1 (shown
here using pins 1, 2, and 3), and a 0.1uf capacitor and pot or photoresistor (as here) for
Oscillator 2 (using pins 4, 5, and 6).
The control input on Oscillator 1 is tied directly to +9 volts, so it runs all the time, as
we demonstrated earlier in figure 20.2). But the control input of Oscillator 2 is connected
to the output of Oscillator 1, which gates Oscillator 2 on and off as it swings between
ground and 9 volts. If the Oscillator 1 (the control oscillator) has a large capacitor and runs
Figure 20.2 Basic NAND Gate oscillator, enabled.
Figure 20.3 Basic NAND Gate oscillator, disabled.
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132 HANDMADE ELECTRONIC MUSIC
slowly (like a metronome), you can hear Oscillator 2 (the modulated oscillator) switch on
and off at a regular tempo. As we tune Oscillator 1 higher and higher, the obvious on/off
function transforms into a kind of frequency modulation that is heard as a change in the
tone color rather than tempo.
Very cool flangey noises occur when the control oscillator and the modulated oscillator
are both in the audio range and are very close in frequency—try using .1uf capacitors and
identical photoresistors for both stages. Now substitute 10uf capacitors and 1 megOhm
pots for both stages: careful tuning of the pots results in interesting polyrhythms.
You can cascade three or four oscillators (see figure 20.5) to create tone clusters or
rhythmic patterns, depending on capacitor sizes. As you can see, you’ll have to configure
parallel or satellite ground buses for the top-side gates as we did for the multi-voice oscil-
lators in chapter 18 (see figure 18.14 and 18.15).
Experiment with different value capacitors and pots for the different stages. You can
use photoresistors, electrodes, or any of the other alternative resistors discussed in chapter
15, for the resistors/pots in these circuits.
Note that even though a control input might be connected to +9 volts we still need to
connect +9 volts to pin 14 and ground (– voltage) to pin 7. The connections to the supply
to the chip, needed to run its internal operations—this is the “gas.” But + and – voltage
also have logical value, and are evaluated as part of the (admittedly simple) mathematical
calculations that the circuit performs in order to oscillate. As with our previous circuit
with the 74C14, sometimes this chip will make sound without proper power connections,
but it will be “coasting,” and probably will not perform reliably (or go up hills)—which
brings us to Rule #21:
Rule #21: All chips expect “+” and “–” power connections to their designated power
supply pins, even if these voltages are also connected to other pins for other reasons,
withhold them at your own risk (or entertainment).
By the way, I’ve chosen the CMOS family of integrated circuits (of which the 74C14 and
the CD4093 are members) for our experiments because they consume very little current
and can run on a wide range of voltages, which makes them ideal for battery operation.
They are also rather difficult to blow up.
Figure 20.4 A gated oscillator.
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