Experiment 6: Very Simple Switching
52 Chapter 2
FundAmentAls
Basic schematic symbols (continued)
The problem is that this is not very intuitive, especially when
you’re just beginning to use schematics. When you see two
wires crossing, you can easily imagine that they are making
a connection, even though there’s no dot at the intersec-
tion. Therefore, in the interests of clarity, I’ve chosen to use
the old “semicircular bump” style of schematic in this book
(see Figure 2-39). It can be summarized like this:- A dot joining two wires indicates an electrical
connection. - A bump in a wire that crosses another wire indicates no
connection.
In this book, you won’t find wires crossing each other with-
out either a dot or a bump.
Figure 2-39. In wiring schematics, a dot always indicates an
electrical connection. However, the cross-shaped intersection
of wires at top-right is considered bad style because if the dot
is accidentally omitted or poorly printed, the intersection can
be mistaken for the type shown at bottom left, in which the
wires do not make a connection. All three of the configurations
in the bottom row indicate no connection, the first example
being the most common style, the center example being least
common, and the third being the most old-fashioned—although
for reasons of clarity, it is used in this book.In a battery-powered circuit, you may find a battery symbol,
but more often you will find a little note indicating where posi-
tive voltage enters the system, while negative is indicated by a
“ground” symbol. In fact there may be ground symbols all over
the place. You have to remember that when you build a circuit,
all the wires leading to grounds must actually be connected
together, to the negative side of the voltage supply.
The idea of the ground symbol dates back to the time when
electronic gadgets were mounted on a metal chassis, which
was connected to the negative side of the power supply. The
ground symbol really meant “connect to the chassis.” Some
variants in the ground symbol are shown in Figure 2-40.Figure 2-40. All of these symbols are used to mean the same
thing: connect the wire to “ground” or “chassis” or the negative
side of the power source. The far-right symbol is used in this
book.In this book, we have color throughout, so I’ll show a red
positive and blue negative to clarify where the power is
connected, and I won’t use ground symbols. Once again,
my purpose is to minimize the risk of misunderstandings,
because I know how frustrating it is to build a circuit that
doesn’t work.
A big inconsistency in schematics is the way in which they
show resistors. The traditional zigzag symbol has been
abandoned in Europe. Instead they use a rectangle with a
number inside indicating the number of ohms. See Figure
2-41. The Europeans also changed the way in which decimal
points are represented: they omit them as much as possible,
because in badly printed schematics, the little dots tend to
get lost (or can be confused with dust and dirt). So, a 4.7KΩ
resistor will be listed as 4K7, and a 1.2MΩ resistor will be
1M2. I like this notation, so I’m going to be using it myself,
but I’ll be keeping the zigzag resistor symbol, which remains
widely used in the United States.220
220
Figure 2-41. Two styles for depicting a 220Ω resistor. The upper
version is traditional, and still used in the United States. The
lower version is European.