Make Electronics

Switching Basics and More 47

Experiment 6: Very Simple Switching

FundAmentAls

All about switches (continued)

Figure 2-27. These are all toggle switches. Generally, the larger
the switch, the more current it can handle.

To make things more interesting, you can also buy switches
that have three or four poles. (Some rotary switches have
even more, but we won't be using them.) Also, some double-
throw switches have an additional “center off” position.

Putting all this together, I made a table of possible types of
switches (Figure 2-28). When you’re reading a parts catalog,
you can check this table to remind yourself what the ab-
breviations mean.

4PST

ON-OFF

Single

Pole

Double

Pole 3-Pole 4-Pole

Single

Throw

SPST

ON-OFF

DPST

ON-OFF

3PST

ON-OFF

4PDT

ON-ON

SPDT

ON-ON

DPDT

ON-ON

3PDT

ON-ON

4PDT

ON-OFF-ON

SPDT

ON-OFF-ON

DPDT

ON-OFF-ON

3PDT

ON-OFF-ON

Double

Throw

Double

Throw with

Center Off

Figure 2-28. This table summarizes all the various options for
toggle switches and pushbuttons.

Now, what about pushbuttons? When you press a door
bell, you’re making an electrical contact, so this is a type of
switch—and indeed the correct term for it is a momentary
switch, because it makes only a momentary contact. Any
spring-loaded switch or button that wants to jump back to

its original position is known as a momentary switch. We

indicate this by putting its momentary state in parentheses.

Here are some examples:

• OFF-(ON): Because the ON state is in parentheses, it’s
  the momentary state. Therefore, this is a single-pole
  switch that makes contact only when you push it, and
  flips back to make no contact when you let it go. It is
  also known as a “normally open” momentary switch,
  abbreviated “NO.”


• ON-(OFF): The opposite kind of momentary single-pole
  switch. It’s normally ON, but when you push it, you
  break the connection. So, the OFF state is momentary.
  It is known as a “normally closed” momentary switch,
  abbreviated “NC.”


• (ON)-OFF-(ON): This switch has a center-off position.
  When you push it either way, it makes a momentary
  contact, and returns to the center when you let it go.
  Other variations are possible, such as ON-OFF-(ON) or ON-
  (ON). As long as you remember that parentheses indicate
  the momentary state, you should be able to figure out what
  these switches are.

Figure 2-29. This evil mad scientist is ready to apply power to

his experiment. For this purpose, he is using a single-pole,

double-throw knife switch, conveniently mounted on the wall of

his basement laboratory.