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 A switch is a device used to change course of current. Switches are essentially binary devices: they are either completely on ("closed") or completely off ("open").
In applications where multiple switching options are required, (i.e. a telephone service) mechanical switches have long been replaced by electronic variants which can be intelligently controlled and automated.
A switch, which can be operated by a person is called a hand switch, and they are available in several varieties:
Toggle switch
Toggle switches are actuated by a lever angled in one of two or more positions. These kind of switches are used in household wiring and they come to rest in any of their lever positions
Pushbutton switch
Pushbutton switches are two-position devices actuated with a button that is pressed and released. Usually pushbutton switches uses internal spring mechanism to return the button to its "out," or "unpressed," position, for momentary operation. Besides these, there are some pushbutton switches that latches alternately on or off with every push of the button while 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.
Selector Switch
Selector switches are actuated by rotary knob or lever to select one of two or more positions. Just like the toggle switch, selector switches can either rest in any of their positions or contain spring-return mechanisms for momentary operation.
Joystick switch
A joystick switch consists of a lever capable of moving in more than one axis of motion. Depending upon the way or the position of the lever, one or more of several switch contact mechanisms are actuated. The circle-and-dot notation on the switch symbol represents the direction of joystick lever motion required to actuate the contact. Joystick hand switches are commonly used for crane and robot control.
Limit switches
Limit switches are used to restrict the motion of a machine by turning off the triggering power to a component if it moves too far. These limit switches are fitted with a lever pushed by the machine part. The levers of the limit switches are tipped with a small roller bearing, preventing the lever from being worn off by repeated contact with the machine part.
Proximity switches
Simple proximity switches use a permanent magnet to trigger a sealed switch mechanism whenever the machine part gets close (typically 1 inch or less) while other types of proximity switches use magnetic or high-frequency electromagnetic field switches to detect the approach of a metallic machine part. Besides these, more complex proximity switches work like a metal detector, energizing a coil of wire with a high-frequency current, and electronically monitoring the magnitude of that current. If a metallic part (not necessarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring circuit.
In addition to all these, another type of proximity switch called optical switch, which comprises of a light source and photocell detects machine position either by the interruption or reflection of a light beam. Optical switches are also useful in safety applications, where beams of light can be used to detect personnel entry into a dangerous area.
Industrial switches
In many industrial processes, it becomes necessary to monitor various physical quantities with switches, as there is always a risk invovled if a process variable has exceeded normal parameters. Such switches can be used to shut down processes or equipment if those variables have reached dangerous or destructive levels.
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