Audio Engineering

Components 131

usually the case), you have to adjust the rest of the circuitry. Alternately the 741 could
be designated as an oscillator, and again it is changed accordingly. Digital ICs operate
on the principle of responding to just two voltage levels, a low level (also called a ‘ 0 ’ )
and a high level (also called a ‘ 1 ’ ), and hence, are also called logic devices. Digital ICs
can be thought of as a series of logic gates that are confi gured to perform a certain logic
function. Special-function ICs are complete in themselves. The LM 386 audio power
amplifi er that we’ll be focusing on heavily later in the book is just that; it is a self-
contained unit that is designed to perform just one task (and it does so admirably at that).
Another much-used special function IC is the LM 555, a timer IC, so commonly used to
provide a train of square wave pulses. Figure 4.8 shows the basic IC outline.

4.1.5.2 Switches

Switches occur in so many places despite their somewhat mundane nature. After all,
a switch is just an on/off device. There are actually many different confi gurations
for switches, and it’s a good idea to get to know the variations. First of all, there’s a
terminology specifi c to switches: poles and throws. The simplest type of switch, like the
type you’d fi nd in a lamp switch, is called a single pole, single throw, or SPST switch. The
simple SPST switch has two terminals, one of these goes to the source (this being typically
the positive voltage supply from a battery) and the other goes to the output (typically this
would be the circuit that is to receive the power from the battery); hence, the output can
only be connected to one terminal. It also has a toggle that fl ips back and forth. The light
fl ips on one way and off the other. Switches always have to be described with respect to an
input signal and an output signal. The pole refers to the number of terminals the input can
be connected to. With the SPST switch there is just one. The throw refers to the number

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Figure 4.8 : Integrated circuit package outline.