An electrical or electronic device used to turn on or off the flow of the current in a circuit. The selection of the appropriate switch control for an intended application is very important. The performance and acceptance of the completed end product can depend on the ability of the switch to handle the electrical load characteristics of the application without failure over a reasonable amount of time. Switches are essential components in almost all the electronic devices used today. They are made for both through hole and surface mount applications. They are used in applications as simple as turning something on or off or they are designed for tasks as complex as routing signals. Some basic considerations in the selection of a switch are: Load requirement of the application (Voltage, current, AC/DC, AC only, DC only) Type of load (Resistive, inductive, low energy, dry circuit) Type of duty Environmental conditions. 

Some commonly used terms when dealing with switches are: 

• Single Pole (SP): A switch device that opens, closes or changes connections in a single conductor of an electrical circuit. 

• Double Pole (DP): A switch device that opens, closes or changes connections in both conductors of the same circuit. 

• Single Throw (ST): A switch that opens, closes or completes a circuit at only one of the extreme positions of the actuator. 

• Double Throw (DT): A switch that opens, closes or completes a circuit at both extreme positions of its actuator. 

• Normally Open (NO): A momentary type switch in which one or more circuits are open when the switch actuator is at its normal or rest position. 

• Normally Closed (NC): A momentary type switch in which one or more circuits are closed when the switch actuator is at its normal or rest position. 

• Actuator: A lever, pushbutton, slide, trigger or other means to which an external force is applied to operate a switch mechanism. 

• Operating Force: The force that must be applied to the actuator to move it from the free position to the operating position. 

• Operate Position: The position of the actuator at which the circuit transfers. 

• Release Force: In a momentary type switch, the value to which the force on the actuator must be reduced to allow the contacts to return to the normal or rest position. 

• Life Expectancy: The useful life cycle performance of a switch mechanism to open or close, make or break an electrical circuit, based on statistical probability.