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Nicolas CollinsIllustrated by Simon Lonergan

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Published in 2006 byRoutledge Taylor & Francis Group 270 Madison AvenueNew York, NY 10016

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Library of Congress Cataloging-in-Publication Data

Collins, Nicolas.Handmade electronic music : the art of hardware hacking / by Nicolas Collins ; illustrated by Simon

Lonergan.p. cm.

Includes bibliographical references and index.ISBN 0-415-97591-3 (hardcover) -- ISBN 0-415-97592-1 (pbk.)1. Electronic apparatus and appliances-Design and construction-Amateurs' manuals. 2. Electronic

musical instruments-Construction. I. Title.

TK9965.C59 2006786.7'1923--dc22 2005030693

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129

Chapter 20

MORE OSCILLATORS: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 amplifi er.• Hand tools.

Th e 74C14 off ers 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 synthesis.

Gated Oscillator

Th e 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 fi gure 20.1)

Th is chip contains four identical NAND gates. Th ere 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 fi ne 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 aft er the AND logic to fl ip 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 specifi cally because both kids have been whining to see it for one hundred miles.

Th e 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 fi gure 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 fi gure 20.3).

Try both confi gurations. 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

pinout.


131More Oscillators

the gate input (pin 1 in fi gure 20.2 and fi gure 20.3) and alternate connecting the other end to the + and ground buses. Th e 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.

Total Control

Th e 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 fi gure 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).

Th e control input on Oscillator 1 is tied directly to +9 volts, so it runs all the time, as we demonstrated earlier in fi gure 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.


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 fl angey 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 fi gure 20.5) to create tone clusters or rhythmic patterns, depending on capacitor sizes. As you can see, you’ll have to confi gure parallel or satellite ground buses for the top-side gates as we did for the multi-voice oscil-lators in chapter 18 (see fi gure 18.14 and 18.15).

Experiment with diff erent value capacitors and pots for the diff erent 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. Th e connections to the supply voltages have two distinct functions in our circuit: through pins 14 and 7 they provide power 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. Th ey are also rather diffi cult to blow up.

Figure 20.4 A gated oscillator.


133More Oscillators

Figure 20.5 Four cascaded gated oscillators.


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