Hardware Hacking 77(the one with the capacitor and feedback resistor), as long as you don’t mix up
the two and connect the capacitor and switch 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 oscillator oscillates when the control input is
connected to the +9v bus; it turns off when the input is disconnected from +9
volts (the resistor between the control input and ground insures that the input is
pulled to 0 when not connected to +9 volts.) Big deal, you say, we can do this by
simply connecting and disconnecting the battery. But we can turn each of four
oscillators on & off independently: by adding a momentary switch to each
control input to make a simple keyboard, with each oscillator individually
tuneable and gateable.
More significantly, 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 gate another oscillator on and off. Breadboard the circuit shown
below Use a large capacitor (4-10uf) and pot for Oscillator 1, and a 0.1uf capacitor
and 100k resistor or pot for Oscillator 2.
Oscillator 1 Oscillator 2The control input on Oscillator 1 is tied directly to +9 volts, so it runs all the time
(like the basic 74C14 oscillator we built in Chapter 17.) But the control input of
Oscillator 2 is connected to the output of Oscillator 1, which gates it on and off.
If the Control Oscillator (#1) has a large capacitor and runs slowly (like a
metronome,) you can hear Oscillator #2 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.
You can cascade more than 2 oscillators, a shown below, to create rhythmic
patterns or (if the oscillators are running at audio frequencies) tone clusters.