The Ins and Outs of Switches

“The Ins and Outs of Switches”


A switch is a electrical component that allows or stops any electrical current from passing.  In layman’s terms, think “on-off” (or, “off-on”).  A switch simply “switches” the state of something to another state.  It is NOT a funny name for a Swiss person.


Switches are a vital electrical component that are necessary for any diy electronic hobbyist.  One hundred percent of your projects will contain at least one switch.  Unless, of course, you enjoy wasting batteries!!!




The simplest switches are made of two conductive metallic plates that can either be engaged (closed) or disengaged (open).  When a switch does not allow a circuit to pass current through it, we call it OPEN.  When a switch is engaged, we call it CLOSED.  This is counterintuitive to, for instance, a door, but such are the mercurial whims of electrical engineering that a diy hobbyist must contend with.  (We all can’t be electrical engineers, can we?).  Switches get more complicated, so let’s read on.


There are several more variations of open and closed.  A switch that is “normally closed” begins in obviously a closed state, meaning the switch is engaged until manipulated otherwise.  “Normally open” means that the switch does not pass current unless actuated manually or by electrical means (more on that later).  These are labeled N.C. and N.O. respectively.




Switches themselves contain two numerical components that determine how useful they are to your precious project:  Poles and Throws.  A POLE is the number of contacts that can be closed or opened.  This is notated by a P with a number or letter in front of it (for example, SPDT or 3PDT).  A THROW stands for the number of unique positions the switch takes when changed.  This is notated  with a T and also by a number of letter in front of it (SPDT, DTDT).


There are many types of combination of poles and throws (and we’re not talking about javelin throwing), so let’s go over the most common:




A standard single-pole single-throw switch

A standard single-pole single-throw switch

SPST – commonly known as “single-pole, single throw”, this is the aforementioned basic switch with two states: on or off.  Easy examples of this are:  house lights, flash lights, pushbutton vacuum cleaners, et cetera.




SPDT-Switch.svg copy

A single-pole double-throw switch


SPDT: commonly known as “single-pole, double throw”, this is known as a “changeover switch”, as it changes one current path to another.  Think of a train set when you were younger.  If you want the uphill path, you flip it to the first position (which is the 1st throw).  If you wanted the downhill path, you flipped it to the second position (which is the 2nd throw).  And if you wanted to derail the train, you changed it while the train was running over before it could clear the switching point.


A double-pole single-throw switch

A double-pole single-throw switch

DPST:  Double-pole, single-throw switches are basically two SPST switches that work in parallel when changed by a single mechanism.  Off and on, but for two poles instead of just one!  (And no Polish jokes on this one!)





 A double-pole double-throw switch

A double-pole double-throw switch

DPDT:  Double-pole, double-throw switches are two changeover switches.  There are three positions to this particular type of switch.  Useful?  You bet.  Ever used a vacuum cleaner?  The middle position is off, the first is for sucking, the third position is for… blowing!  The opposite.  Not to get too far ahead of our infantile knowledge, but here’s a tidbit:  DPDT switches are useful switching polarity on motors.




Beyond these basic types, there exist many other combinations of poles and throws (3PDT, 4P3T, 8P5T), but they are beyond the scope of a absolute beginner.


Moving on…


Actuators control switches and tell them what to do!  Forgive the jargon, but an actuator is simply “something that causes a mechanical motion”.  there are several types of actuators that control your switches.  They come in several flavors:

BIASED:  These are momentary push-button style, like on a vintage Nintendo controller, arcade games or a simple computer keyboard.  I’m very sure you could come up with more examples.  This type, which affects the circuit when pressed, is “push to make”.  If any diligent students have been taking notes, one can easily see that this would be a type of “normally open” circuit.  When the button is pressed, the circuit is closed.


On the other hand, some biased switch are the opposite and are called “push to break”.  The most succinct example of this is a refrigerator door light.  Though you’ll have to consult the gnomes that live inside your icebox, take my word for it:  when you close the refrigerator, the light goes out.  When you open the door, the switch engages, CLOSES the circuit, and the light turns on.  Therefore, this switch is a N.C. switch, or normally closed.


Confused?  Don’t be.  Consult the gnome.


TOGGLE:  Toggle switches are ones that are actuated by a lever, a handle, or a rocker (and not the members of rockband KISS).  The living room light is an easy example of this type.  They are flipped from one state to another.


ROTARY:  Rotary switches are switches that have positions aligned in a circle or an arc, with a handle providing the ability to switch between different combinations of connections.


And this leads us to electronic switching…



Switches can also be purely electrical.

RELAYS:  Relays are electrically-operated switches.  This is a huge topic in and of itself, but the easiest example you may remember was from science class:  current is passed through a coil around, say, a nail, and it become magnetized.  Thus, it can pick up (in my class, for instance) paper clips.  Now, imagine that the metallic paper clips had wire soldered to them (more on soldering in another article).  This would CLOSE the circuit and voila!  You’ve just understood a basic relay.

In the modern world, there are many applications built off of this simple idea of adding electricity to a copper-winded coil.  Telephones?  Yep.  Televisions?  Yep.  Explosives?  Ummmm, sure, you evil terrorist.  But it doesn’t stop there, does it?  With the invention of integrated circuits (IC’s) and semiconductors, the possibilities are limitless!  (More on that in future articles, but let’s not get ahead of ourselves, you simpleton!)



Now, getting back to the first idea of paper clips and a nail, imagine that switch wasn’t actuated by your hand or your actions, but on external factors.  Imagine that the amount of light in a room would switch on an air-conditioner.  Imagine that movement could trigger lights going on or a camera being engaged when you’re running a red light camera (all those New York parking tickets I’ve been collecting).  Imagine if the barometric pressure would lower enough to engage a device to turn on the air-conditioner for grandma’s arthritis and joint pain.  What makes those possible?  SWITCHES, you silly person!  The possibilities are ENDLESS and if you aren’t inspired yet, you must be brain dead!


You have an assignment:  Go out into the world and try to find as many switches in the world (automatic door at McDonalds).  Now, imagine you could use those in your projects!