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Switches
Switch Contacts - pole, throw etc.
Standard Switches - SPST, SPDT, DPST, DPDT.
Special Switches – multi-way, key, tilt, reed etc.
Also see: Relays | Series and Parallel Connections - Switches
Selecting a Switch
There are three important features to consider when selecting
a switch:
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Circuit symbol for a
simple on-off switch
Contacts (e.g. single pole, double throw)
Ratings (maximum voltage and current)
Method of Operation (toggle, slide, key etc.)
Switch Contacts
Several terms are used to describe switch contacts:
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Pole - number of switch contact sets.
Throw - number of conducting positions, single or double.
Way - number of conducting positions, three or more.
Momentary - switch returns to its normal position when released.
Open - off position, contacts not conducting.
Closed - on position, contacts conducting, there may be several on
positions.
For example: the simplest on-off switch has one set of contacts (single pole) and
one switching position which conducts (single throw). The switch mechanism has
two positions: open (off) and closed (on), but it is called 'single throw' because
only one position conducts.
Switch Contact Ratings
Switch contacts are rated with a maximum voltage and current, and there may be
different ratings for AC and DC. The AC values are higher because the current
falls to zero many times each second and an arc is less likely to form across the
switch contacts.
For low voltage electronics projects the voltage rating will not matter, but you
may need to check the current rating. The maximum current is less for inductive
loads (coils and motors) because they cause more sparking at the contacts when
switched off.
Standard Switches
Type of Switch
Circuit Symbol
Example
ON-OFF
Single Pole, Single Throw =
SPST
A simple on-off switch. This type
can be used to switch the power
supply to a circuit.
When used with mains electricity
this type of switch must be in the
live wire, but it is better to use a
DPST switch to isolate both live
and neutral.
SPST toggle switch
Photograph © Rapid Electronics
(ON)-OFF
Push-to-make = SPST
Momentary
A push-to-make switch returns to
its normally open (off) position
when you release the button, this
is shown by the brackets around
ON. This is the standard doorbell
switch.
Push-to-make switch
Photograph © Rapid Electronics
ON-(OFF)
Push-to-break = SPST
Momentary
A push-to-break switch returns to
its normally closed (on) position
when you release the button.
Photograph © Rapid Electronics
Push-to-break switch
ON-ON
Single Pole, Double Throw =
SPDT
This switch can be on in both
positions, switching on a separate
device in each case. It is often
called a changeover switch. For
example, a SPDT switch can be
used to switch on a red lamp in
one position and a green lamp in
the other position.
SPDT toggle switch
A SPDT toggle switch may be used as a
simple on-off switch by connecting to
COM and one of the A or B terminals
shown in the diagram. A and B are
interchangeable so switches are usually
not labelled.
ON-OFF-ON
SPDT Centre Off
A special version of the standard
SPDT switch. It has a third
switching position in the centre
which is off. Momentary (ON)OFF-(ON) versions are also
available where the switch returns
to the central off position when
released.
SPDT slide switch
(PCB mounting)
SPDT rocker switch
Photographs © Rapid Electronics
Dual ON-OFF
Double Pole, Single Throw =
DPST
A pair of on-off switches which
operate together (shown by the
dotted line in the circuit symbol).
A DPST switch is often used to
switch mains electricity because it
can isolate both the live and
neutral connections.
Photograph © Rapid Electronics
DPST rocker switch
Dual ON-ON
Double Pole, Double Throw =
DPDT
A pair of on-on switches which
operate together (shown by the
dotted line in the circuit symbol).
A DPDT switch can be wired up
as a reversing switch for a motor
as shown in the diagram.
ON-OFF-ON
DPDT slide switch
DPDT Centre Off
A special version of the standard
SPDT switch. It has a third
switching position in the centre
which is off. This can be very
useful for motor control because
you have forward, off and reverse
positions. Momentary (ON)-OFF(ON) versions are also available
where the switch returns to the
central off position when released.
Wiring for Reversing Switch
Photograph © Rapid Electronics
Rapid Electronics stock a wide range of switches
and they have kindly allowed me to use their
photographs on this page. The photographs are
from their Image Gallery CD-ROM.
Special Switches
Type of Switch
Push-Push Switch (e.g. SPST = ON-OFF)
This looks like a momentary action push switch but
it is a standard on-off switch: push once to switch
on, push again to switch off. This is called a
latching action.
Photograph © Rapid Electronics
Example
Microswitch (usually SPDT = ON-ON)
Microswitches are designed to switch fully open or
closed in response to small movements. They are
available with levers and rollers attached.
Photograph © Rapid Electronics
Keyswitch
A key operated switch. The example shown is
SPST.
Photograph © Rapid Electronics
Tilt Switch (SPST)
Tilt switches contain a conductive liquid and when
tilted this bridges the contacts inside, closing the
switch. They can be used as a sensor to detect the
position of an object. Some tilt switches contain
mercury which is poisonous.
Photograph © Rapid Electronics
Reed Switch (usually SPST)
The contacts of a reed switch are closed by bringing
a small magnet near the switch. They are used in
security circuits, for example to check that doors are
closed. Standard reed switches are SPST (simple
on-off) but SPDT (changeover) versions are also
available.
Warning: reed switches have a glass body which is
easily broken! For advice on handling please see
the Electronics in Meccano website.
Photograph © Rapid Electronics
DIP Switch (DIP = Dual In-line Parallel)
This is a set of miniature SPST on-off switches, the
example shown has 8 switches. The package is the
same size as a standard DIL (Dual In-Line)
integrated circuit.
This type of switch is used to set up circuits, e.g.
setting the code of a remote control.
Photograph © Rapid Electronics
Multi-pole Switch
The picture shows a 6-pole double throw switch,
also known as a 6-pole changeover switch. It can be
set to have momentary or latching action. Latching
action means it behaves as a push-push switch,
push once for the first position, push again for the
second position etc.
Photograph © Rapid Electronics
Multi-way Switch
Multi-way switches have 3 or more conducting
positions. They may have several poles (contact
sets). A popular type has a rotary action and it is
available with a range of contact arrangements from
1-pole 12-way to 4-pole 3 way.
The number of ways (switch positions) may be reduced by
adjusting a stop under the fixing nut. For example if you need
a 2-pole 5-way switch you can buy the 2-pole 6-way version
and adjust the stop.
Multi-way rotary switch
Contrast this multi-way switch (many switch positions) with the
multi-pole switch (many contact sets) described above.
Photograph © Rapid Electronics
Switch types
An electrical switch is any device used to interrupt the flow of electrons in
a circuit. Switches are essentially binary devices: they are either completely on
("closed") or completely off ("open"). There are many different types of switches,
and we will explore some of these types in this chapter.
Though it may seem strange to cover this elementary electrical topic at such a late stage
in this book series, I do so because the chapters that follow explore an older realm of
digital technology based on mechanical switch contacts rather than solid-state gate
circuits, and a thorough understanding of switch types is necessary for the undertaking.
Learning the function of switch-based circuits at the same time that you learn about
solid-state logic gates makes both topics easier to grasp, and sets the stage for an
enhanced learning experience in Boolean algebra, the mathematics behind digital logic
circuits.
The simplest type of switch is one where two electrical conductors are brought in contact
with each other by the motion of an actuating mechanism. Other switches are more
complex, containing electronic circuits able to turn on or off depending on some physical
stimulus (such as light or magnetic field) sensed. In any case, the final output of any
switch will be (at least) a pair of wire-connection terminals that will either be connected
together by the switch's internal contact mechanism ("closed"), or not connected
together ("open").
Any switch designed to be operated by a person is generally called a hand switch, and
they are manufactured in several varieties:
Toggle switches are actuated by a lever angled in one of two or more positions. The
common light switch used in household wiring is an example of a toggle switch. Most
toggle switches will come to rest in any of their lever positions, while others have an
internal spring mechanism returning the lever to a certain normal position, allowing for
what is called "momentary" operation.
Pushbutton switches are two-position devices actuated with a button that is pressed and
released. Most pushbutton switches have an internal spring mechanism returning the
button to its "out," or "unpressed," position, for momentary operation. Some pushbutton
switches will latch alternately on or off with every push of the button. Other pushbutton
switches will stay in their "in," or "pressed," position until the button is pulled back out.
This last type of pushbutton switches usually have a mushroom-shaped button for easy
push-pull action.
The schematic symbology for switches vary according to the switch's purpose and
actuation. A normally-open switch contact is drawn in such a way as to signify an open
connection, ready to close when actuated. Conversely, a normally-closed switch is drawn
as a closed connection which will be opened when actuated. Note the following symbols:
There is also a generic symbology for any switch contact, using a pair of vertical lines to
represent the contact points in a switch. Normally-open contacts are designated by the
lines not touching, while normally-closed contacts are designated with a diagonal line
bridging between the two lines. Compare the two:
The switch on the left will close when actuated, and will be open while in the "normal"
(unactuated) position. The switch on the right will open when actuated, and is closed in
the "normal" (unactuated) position. If switches are designated with these generic
symbols, the type of switch usually will be noted in text immediately beside the symbol.
Please note that the symbol on the left is not to be confused with that of a capacitor. If a
capacitor needs to be represented in a control logic schematic, it will be shown like this:
In standard electronic symbology, the figure shown above is reserved for polaritysensitive capacitors. In control logic symbology, this capacitor symbol is used for any
type of capacitor, even when the capacitor is not polarity sensitive, so as to clearly
distinguish it from a normally-open switch contact.
With multiple-position selector switches, another design factor must be considered: that
is, the sequence of breaking old connections and making new connections as the switch is
moved from position to position, the moving contact touching several stationary contacts
in sequence.
The selector switch shown above switches a common contact lever to one of five different
positions, to contact wires numbered 1 through 5. The most common configuration of a
multi-position switch like this is one where the contact with one position is broken before
the contact with the next position is made. This configuration is called break-beforemake. To give an example, if the switch were set at position number 3 and slowly turned
clockwise, the contact lever would move off of the number 3 position, opening that
circuit, move to a position between number 3 and number 4 (both circuit paths open),
and then touch position number 4, closing that circuit.
There are applications where it is unacceptable to completely open the circuit attached to
the "common" wire at any point in time. For such an application, a make-before-break
switch design can be built, in which the movable contact lever actually bridges between
two positions of contact (between number 3 and number 4, in the above scenario) as it
travels between positions. The compromise here is that the circuit must be able to
tolerate switch closures between adjacent position contacts (1 and 2, 2 and 3, 3 and 4, 4
and 5) as the selector knob is turned from position to position. Such a switch is shown
here:
When movable contact(s) can be brought into one of several positions with stationary
contacts, those positions are sometimes called throws. The number of movable contacts
is sometimes called poles. Both selector switches shown above with one moving contact
and five stationary contacts would be designated as "single-pole, five-throw" switches.
If two identical single-pole, five-throw switches were mechanically ganged together so
that they were actuated by the same mechanism, the whole assembly would be called a
"double-pole, five-throw" switch:
Here are a few common switch configurations and their abbreviated designations:
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REVIEW:
The normal state of a switch is that where it is unactuated. For process
switches, this is the condition its in when sitting on a shelf, uninstalled.
A switch that is open when unactuated is called normally-open. A switch that is
closed when unactuated is called normally-closed. Sometimes the terms
"normally-open" and "normally-closed" are abbreviated N.O. and N.C.,
respectively.
The generic symbology for N.O. and N.C. switch contacts is as follows:
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Multiposition switches can be either break-before-make (most common) or
make-before-break.
The "poles" of a switch refers to the number of moving contacts, while the
"throws" of a switch refers to the number of stationary contacts per moving
contact.