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.
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.
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.
Float switch, mercury switch, reed switch, pressure switch, and Hall-effect switch are considered as sensing devices, and will be found in Volume 3.
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.
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.
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.
A double-throw switch with center-off position (no connection when the switch is centered).
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.
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:
[make_slideshow slug=”interesting-switches” title=”Interesting Switches” link=”Start the slideshow”]