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 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. 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