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A switch is a component which controls whether a circuit is open or closed.

They control current flow in a circuit. Switches are critical components in any circuit which requires user interaction or control.

A switch can only exist in one of two states: open or closed. In the off state, a switch looks like an open gap in the circuit. This, in effect, looks like an open circuit, preventing current from flowing.

In the on state, a switch acts just like a piece of perfectly-conducting wire. This closes the circuit, turning the system "on" and allowing current to flow through the rest of the system.

A switch must have at least two terminals, one for the current to go in, another to come out. That only describes the simplest version of a switch though.

The number of poles* on a switch defines how many separate circuits the switch can control. So a switch with one pole, can only control one single circuit. A four-pole switch can separately control four different circuits, while still only having one physical control

The number of throws defines how many positions each of the switch’s poles can be connected to. For example, if a switch has two throws, each circuit (pole) in the switch can be connected to one of two terminals.

Knowing how many poles and throws a switch has, it can be more specifically classified. Commonly you’ll see switches defined as “single-pole, single-throw”, “single-pole, double-throw”, “double-pole, double-throw”, which are more often abbreviated down to SPST, SPDT, and DPDT, respectively.

Latching switches -- like the light switches on your wall, or the switch controlling a fan -- stay in one position until actuated into a new position, and then remain in that state until acted upon once again.

Momentary switches — like the switch that controls a power drill or door buzzer — only remain active as long as they’re actuated or pressed. Once released, they return to their original position. Momentary switches have terminals named COMMON, NORMALLY CLOSED, NORMALLY OPEN