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Component of the Month: Switches

Switches are a fundamental electronic component, they come in a variety of styles and configurations, and they have many different uses. From turning on a light to controlling a robotic arm, switches are the ideal control mechanism. At the physical layer, switches control the flow of information on the Internet and launch the rockets we send off into space. If the universal symbol for a bright idea is the light bulb, then it’s safe to assume there’s a switch inside every one of us, waiting to be flipped.

To get an idea just how creative folks get with their use of switches in projects, in the coming month, we’ll be taking a look at some of the more interesting applications we’ve come across. We’ll also be on the lookout for interesting new uses. If you or somebody you know has a project with an interesting application of switches, let us know by leaving a link in the comments below.

We’ll start things off with an introduction to switches via an excerpt from Charles Platt’s Encyclopedia of Electronic Components Volume 1:


What It Does

A switch contains at least two contacts, which close or open when an external lever or knob is flipped or moved. Schematic symbols for the most basic type of on-off switch are shown in Figure 6-1.

The most fundamental type of switch is a knife switch, illustrated in Figure 6-2. Although it was common in the earliest days of electrical discov­ ery, today it is restricted to educational purposes in schools, and (in a more robust format) to AC electrical supply panels, where the large contact area makes it appropriate for conducting high amperages, and it can be used for “hot switching” a substantial load.

How It Works

The pole of a switch is generally connected with a movable contact that makes or breaks a con­ nection with a secondary contact. If there is only one pole, this is a single-pole switch. If there is an additional pole, electrically isolated from the first, with its own contact or set of contacts, this is a two-pole switch, also known as a double- pole switch. Switches with more than 4 poles are uncommon.

fig-6-1

Figure 6-1. The two most common schematic symbols for a SPST switch, also known as an on-off switch. The symbols are functionally identical.

If there is only one secondary contact per pole, this is a single-throw or ST switch, which may also be described as an on-off or off-on switch. If there is an additional secondary contact per pole, and the pole of the switch connects with the second contact while disconnecting from the first, this is a double-throw or DT switch, also known as a two-way switch.

fig-6-2

Figure 6-2. A DPST knife switch intended for the educational environment.

A double-throw switch may have an additional center position. This position may have no con­ nection (it is an “off” position) or in some cases it connects with a third contact.

Where a switch is spring-loaded to return to one of its positions when manual pressure is released, it functions like a pushbutton even though its physical appearance may be indistinguishable from a switch.

Variants

Float switch, mercury switch, reed switch, pressure switch, and Hall-effect switch are considered as sensing devices, and will be found in Volume 3.

Terminology

Many different types of switches contain parts that serve the same common functions. The ac­tuator is the lever, knob, or toggle that the user turns or pushes. A bushing surrounds the actua­ tor on a toggle-type switch. The common con­ tact inside a switch is connected with the pole of the switch. Usually a movable contact is attached to it internally, to touch the secondary contact, also known as a stationary contact when the movable contact is flipped to and fro.

Poles and Throws

Abbreviations identify the number of poles and contacts inside a switch. A few examples will make this clear:

SPST also known as 1P1T

Single pole, single throw

DPST also known as 2P1T

Double pole, single throw

SPDT also known as 1P2T

Single pole, double throw

3PST also known as 3P1T

Three pole, single throw

Other combinations are possible.

In Figure 6-3, schematic symbols are shown for double-throw switches with 1, 2, and 3 poles. The dashed lines indicate a mechanical connection, so that all sections of the switch move together when the switch is turned. No electrical connec­ tion exists between the poles.

fig-6-3

Figure 6-3. Schematic symbols to represent three types of double-throw switch. Top left: Single-pole. Bottom left: Double-pole. Right: Triple-pole, more commonly known as 3-pole.

On-Off Behavior

The words ON and OFF are used to indicate the possible states of a switch. The additional word NONE is used by some manufacturers to indicate that a switch does not have a center position. Some manufacturers don’t bother with the word NONE, assuming that if the word is omitted, a center position does not exist.

ON-OFF or ON-NONE-OFF

A basic on-off SPST switch with no center position.

ON-ON or ON-NONE-ON

A basic SPDT switch with no center position.

ON-OFF-ON

A double-throw switch with center-off posi­tion (no connection when the switch is centered).

ON-ON-ON

A triple-throw switch where the center po­sition connects with its own set of terminals.

Parentheses are used in descriptions of spring- loaded switches to indicate a momentary state that lasts only as long as pressure is applied to the actuator.

(ON)-OFF or OFF-(ON)

A spring-loaded switch that is normally off and returns to that position when pressure is released. Also known as NO (normally open), and sometimes described as FORM A. Its performance is similar to that of a push­ button and is sometimes described as a make-to-make connection.

ON-(OFF) or (OFF)-ON

A spring-loaded switch that is normally on and returns to that position when pressure is released. This is sometimes described as a make-to-break connection. Also known as NC (normally closed), and sometimes de­ scribed as FORM B.

(ON)-OFF-(ON)

A spring-loaded double-throw switch with a no-connection center position to which it returns when pressure on its actuator is re­ leased.

Other combinations of these terms are possible.

Most double-throw switches break the connec­tion with one contact (or set of contacts) before making the connection with the second contact (or set of contacts). This is known as a break before make switch. Much less common is a make before break switch, also known as a shorting switch, which establishes the second connection a mo­ ment before the first connection is broken. Use of a shorting switch may cause unforeseen con­ sequences in electronic components attached to it, as both sides of the switch will be briefly con­nected when the switch is turned.


Here’s a slideshow of some of the interesting switch variants from Charles Platt’s Encyclopedia of Electronic Components, Volume 1:

Start the slideshow


For more on microswitches, rockers, sliders, toggles, DIPs, SIPs, paddle switches, and more, check out the Encyclopedia of Electronic Components Volume 1 by Charles Platt. It’s the informative, concise, and well-organized resource that's perfect for teachers, hobbyists, engineers, and students wanting a go-to electronics quick reference.

Buy now!

Adam Flaherty

I make cool stuff and write about other people making cool stuff on makezine.com. If you have something you think I should see, send me a tip.


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