In many industrial processes, it is necessary to monitor various physical quantities with switches. Optical switches are also useful in safety applications, where beams of light can be used to detect personnel entry into a dangerous area. Machine position is detected by either the interruption or reflection of a light beam. Another form of proximity switch is the optical switch, comprised of a light source and photocell.
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A non-electronic proximity switch would use the same symbol as the lever-actuated limit switch. The symbol shown here for the proximity switch is of the electronic variety, as indicated by the diamond-shaped box surrounding the switch. If a metallic part (not necessarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring circuit. 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. Simple proximity switches use a permanent magnet to actuate a sealed switch mechanism whenever the machine part gets close (typically 1 inch or less).
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Proximity switches sense the approach of a metallic machine part either by a magnetic or high-frequency electromagnetic field. Any switch designed to be operated by a person is generally called a hand switch, and they are manufactured in several varieties: Toggle Switches In any case, the final output of any switch will be (at least) a pair of wire-connection terminals that will either be connected together by the switch’s internal contact mechanism (“closed”), or not connected together (“open”).
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Other switches are more complex, containing electronic circuits able to turn on or off depending on some physical stimulus (such as light or magnetic field) sensed. The simplest type of switch is one where two electrical conductors are brought in contact with each other by the motion of an actuating mechanism. There are many different types of switches, and we will explore some of these types in this chapter. Switches are essentially binary devices: they are either completely on (“closed”) or completely off (“open”). An electrical switch is any device used to interrupt the flow of electrons in a circuit.