Selector switch
Selector switches are actuated with a rotary knob or lever of some sort to select one of two or
more positions. Like the toggle switch, selector switches can either rest in any of their positions or
contain spring-return mechanisms for momentary operation.
Lever actuator limit switch
These limit switches closely resemble rugged toggle or selector hand switches fitted with a lever
pushed by the machine part. Often, the levers are tipped with a small roller bearing, preventing the
lever from being worn off by repeated contact with the machine part.
Proximity switches
Proximity switches sense the approach of a metallic machine part either by a magnetic or high-
frequency electromagnetic field. Simple proximity switches use a permanent magnet to actuate a
sealed switch mechanism whenever the machine part gets close (typically 1 inch or less). More com-
plex 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 nec-
essarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring
circuit. The symbol shown here for the proximity switch is of the electronic variety, as indicated by
the diamond-shaped box surrounding the switch. A non-electronic proximity switch would use the
same symbol as the lever-actuated limit switch.
Another form of proximity switch is the optical switch, comprised of a light source and photocell.
Machine position is detected by either 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.
In many industrial processes, it is necessary to monitor various physical quantities with switches.
Such switches can be used to sound alarms, indicating that a process variable has exceeded normal
parameters, or they can be used to shut down processes or equipment if those variables have reached
dangerous or destructive levels.
Selector switches are actuated with a rotary knob or lever of some sort to select one of two or
more positions. Like the toggle switch, selector switches can either rest in any of their positions or
contain spring-return mechanisms for momentary operation.
These limit switches closely resemble rugged toggle or selector hand switches fitted with a lever
pushed by the machine part. Often, the levers are tipped with a small roller bearing, preventing the
lever from being worn off by repeated contact with the machine part.
Proximity switches
Proximity switches sense the approach of a metallic machine part either by a magnetic or high-
frequency electromagnetic field. Simple proximity switches use a permanent magnet to actuate a
sealed switch mechanism whenever the machine part gets close (typically 1 inch or less). More com-
plex 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 nec-
essarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring
circuit. The symbol shown here for the proximity switch is of the electronic variety, as indicated by
the diamond-shaped box surrounding the switch. A non-electronic proximity switch would use the
same symbol as the lever-actuated limit switch.
Machine position is detected by either 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.
In many industrial processes, it is necessary to monitor various physical quantities with switches.
Such switches can be used to sound alarms, indicating that a process variable has exceeded normal
parameters, or they can be used to shut down processes or equipment if those variables have reached
dangerous or destructive levels.
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