Switching Basics and More 51Experiment 6: Very Simple SwitchingFundAmentAls
Basic schematic symbols
Schematic symbols are like words in a language: they have
mutated over the years into a confusing range of variations.
A simple on/off (single-pole, single-throw, or SPST) switch,
for instance, can be represented by any of the symbols
shown in Figure 2-37. They all mean exactly the same thing.
Contact Pole
Figure 2-37. Variations on a theme: Just some of the differ-
ent styles used to depict a single-pole, single-throw switch in
schematic diagrams. The bottom version is the style used in
this book.
Figure 2-38 shows double-pole, double-throw switches. A
dotted line indicates a mechanical connection inside the
switch, so that when you flip it, you affect both the poles si-
multaneously. Remember, the poles are electrically isolated
from each other.
S1A
S1B
Figure 2-38. More variations: some different styles for depicting
a double-pole, double-throw switch. The style at bottom-right
is used in this book.Once in a while, you may find a schematic in which switches
seem to be scattered around, but the way they are identi-
fied (such as S1A, S1B, S1C, and so on) tells you that this is
really all one switch with multiple poles.
In the schematics in this book, I’ll place a gray rectangle
behind each switch. This gray rectangle is not a standard
symbol; you won’t find it in other books. I’m just including it
to remind you that the parts inside are all contained in one
package.
A very important stylistic variation in schematics is the way
they show whether wires make a connection with each
other. Old schematics used to show a little semicircular
bump in a wire if it crossed another wire without making
a connection. Because modern circuit-drawing software
doesn’t create this style of schematic, it is no longer often
used. The modern style, which you are likely to find if you
browse through schematics online, can be summarized like
this:- A dot joining two wires indicates an electrical
connection. - No dot indicates no connection.