As simple as switches are, how they are categorized can be difficult to understand. All switches have a certain number of poles and throws. This is expressed using this format:
(Number of poles)P(Number of throws)T
For example, SPDT is used to represent a Single Pole Double Throw. A pole can be thought of as how many common pins there are and a throw are the number positions that common pin can connect to.
Each pole is switched together but is electrically isolated from the others. This makes it easy to have different voltages and circuits running through the same device.
Any switch with more than one throw will have a Normally Closed (NC) and at least one Normally Open (NO) contact. If it is a single throw switch then it needs to be determined if it is NC or NO. This is stating if the switch is closed (making electrical connection) or open (no electrical connection) in its default state, when the switch is not activated.
The function of the switch will describes how the switch will act when activated. The first condition states its default connection, every condition after describes how it will act when activated. It will say On if the common pin is making connection with another pin, Off means it is disconnected from all other pins. For example, the basic Off-On function will always be a NO circuit which will make connection when activated.
For each position that the switch has there will be a condition. A 3 position selector switch might have a function of On-On-On. This would be an example of a SP3T circuit that is always making connection with a different pin. Be sure to always look at the datasheet to make sure it is making the connections that you want. Below is an example of an abnormal 3 positions selector switch that is DPDT.
Momentary (Mom) functions can get a little more confusing. Their actions will only stay for as long as the switch is being activated. A SPST – NO pushbutton with a spring is a very common example. Its function is Off-Mom which means there is no electrical contact until the button is pressed. Once the button is released the spring will push it back into its default condition, breaking the connection.
Most of the time a Mom function will do the opposite of the default condition, Off to On and On to Off, but you have to look at the datasheet to make sure. Some switches can have multiple Mom conditions which can do a variety of effects. Below is a pushbutton switch with an Off-Mom-Mom function.
Since there is only one common and two pins it connects to, that makes this a SPDT circuit. There are no connections being made in the switches default position, but as you press it down it will go to position one. Press a little more and it will go to position 2 making another connection.
These circuits are used for switches, relays, sensors, and other devices that have any changes in electrical paths.