(SPST) switch is used. If both sides of the
line were to be switched, then a double-
pole, single-throw (DPST) switch would be
used. Reference to Figure 4-10 will show
the circuit diagrams for various switches. If
a single line is to switch first to one point
and then to another, the SPDT switch can
be used. If a double line is to be switched to
two other points, then the DPDT switch
would be used.
Another type of switch used is the push-
button switch. These switches can be of two
main types: normally open or normally
closed. The normally opened push button
(PBNO) is found in Figure 4-11. This switch
only completes a circuit when pushed.
Figure 4-12 is the opposite of PBNO, the
normally closed push button (PBNC). This
switch opens the circuit only when pushed
or depressed. Otherwise, the circuit is
closed. The term “normally” is used to
indicate that the push-button switch is at
rest (untouched by someone).
Frequently, it is desirable to switch a cir-
cuit from two different locations. In this case,
a three-way switch is used. Perhaps you
have such switches in your home that permit
you to turn a light on or off from two places
in a room or hallway. The schematic diagram
of this circuit is shown in Figure 4-13.
The light is on, but it can be turned off
by moving either switch 1 or 2. In Figure 4-14
the light is off, but it can be turned on by
either switch 1 or 2. Follow the circuit
through the switches in each position.
In Project 1—Experimenter, Problem 6,
you will gain first-hand experience in con-
necting three-way switches, so that lights
can be controlled from different locations.
40
Electricity
DPST
DPDT
SPDT
SPST
Figure 4-10.
Diagrams of switch types.
Figure 4-11.
Push button normally open (PBNO).
Figure 4-12.
Push button normally closed (PBNC).
Power
source
3-way
switch
number 1
3-way
switch
number 2
Light in
center of a
long hallway
From main
fuse or circuit
breaker panel
At opposite end
of hallway
At one end
of hallway
Figure 4-13.
A three-way switch circuit. The light is on.