Experiment 6: Very Simple Switching
46 Chapter 2
FundAmentAls
All about switches
When you flip the type of toggle switch that you used in
Experiment 6, it connects the center terminal with one of
the outer terminals. Flip the switch back, and it connects the
center terminal with the other outer terminal, as shown in
Figure 2-23.
The center terminal is called the pole of the switch. Because
you can flip, or throw, this switch to make two possible con-
nections, it is called a double-throw switch. As mentioned ear-
lier, a single-pole, double-throw switch is abbreviated SPDT.
Some switches are on/off, meaning that if you throw them in
one direction they make a contact, but in the other direction,
they make no contact at all. Most of the light switches in your
house are like this. They are known as single-throw switches. A
single-pole, single-throw switch is abbreviated SPST.
Some switches have two entirely separate poles, so you
can make two separate connections simultaneously when
you flip the switch. These are called double-pole switches.
Check the photographs in Figures 2-24 through 2-26 of
old-fashioned “knife” switches (which are still used to teach
electronics to kids in school) and you’ll see the simplest
representation of single and double poles, and single and
double throws. Various toggle switches that have contacts
sealed inside them are shown in Figure 2-27.Connected Connected
Figure 2-23. The center terminal is the pole of the switch.
When you flip the toggle, the pole changes its connection.Figure 2-24. This primitive-looking single-
pole, double-throw switch does exactly
the same thing as the toggle switches in
Figures 2-23 and 2-27.Figure 2-25. A single-pole, single-throw
switch makes only one connection with
one pole. Its two states are simply open
and closed, on and off.Figure 2-26. A double-pole, single-throw
switch makes two separate on/off
connections.