Proximity Switches

A proximity switch is one detecting the proximity (closeness) of some object. By definition, these
switches are non-contact sensors, using magnetic, electric, or optical means to sense the proximity
of objects.The “normal” status of a switch is the condition of minimum stimulus. A proximity
switch will be in its “normal” status when it is distant from any actuating object.

Being non-contact in nature, proximity switches are often used instead of direct-contact limit
switches for the same purpose of detecting the position of a machine part, with the advantage of
never wearing out over time due to repeated physical contact. However, the greater complexity (and
cost) of a proximity switch over a mechanical limit switch relegates their use to applications where
lack of physical contact yields tangible benefits.

Most proximity switches are active in design.They incorporate a powered electronic circuit to sense the proximity of an object. Inductive proximity switches sense the presence of metallic objects through the use of a high-frequency magnetic field. Capacitive proximity switches sense the presence of non metallic objects through the use of a high-frequency electric field. Optical
switches detect the interruption of a light beam by an object.

The schematic diagram symbol for a proximity switch with mechanical contacts is the same as
for a mechanical limit switch, except the switch symbol is enclosed by a diamond shape, indicating
a powered (active) device:

Many proximity switches, though, do not provide “dry contact” outputs. Instead, their output
elements are transistors configured either to source current or sink current. The terms “sourcing”
and “sinking” are best understood by visualizing electric current in the direction of conventional
flow rather than electron flow.


The following schematic diagrams contrast the two modes of switch operation, using red arrows
to show the direction of current (conventional flow notation). In both examples, the load being
driven by each proximity switch is a light-emitting diode (LED):