What are proximity switches?
A proximity switch is one that detects the proximity (closeness) of an object. By definition, these switches are non-contact sensors, which use magnetic, electrical or optical means to detect the proximity of objects.
Having no contact with nature, proximity switches are often used in place of direct contact limit switches for the same purpose of detecting the position of a part of the machine, with the advantage of not wearing out over time due to repeated physical contact.
Types of Proximity switches:
Most proximity switches are active in the design. That is, they incorporate a motorized electronic circuit to detect the proximity of an object.
Inductive proximity switches:
Inductive proximity switches detect the presence of metal objects by using a high frequency magnetic field.
Capacitive proximity switches:
Capacitive proximity switches detect the presence of non-metallic objects by the use of a high-frequency electric field.
Ultrasonic proximity switches:
Ultrasonic proximity switches detect the presence of dense matter by the reflection of sound waves. High frequency (200 kHz) sound waves reflect from the object.
NPN or PNP type switch:
The following schematic diagrams contrast the two modes of operation of the switch, using red arrows to show the direction of the current (conventional flow notation). In both examples, the load that is operated by each proximity switch is a light-emitting diode ( LED):
A common coloring convention for electronic proximity switches is brown for the + V power supply, blue for the ground connection (- pole of the power supply) and black for the switched output signal. This convention is common for sinking and obtaining proximity switches alike. An electronic switch designed to absorb current through its signal cable is alternatively known as an NPN switch due to the type of transistor used in its output.
Conversely, an electronic switch designed to generate current through its signal cable may be termed a PNP switch.
For a sinking switch, this means that the emitter must be connected to the negative rail, which requires an NPN transistor to make the switch.
For a source switch, this means that the emitter must be connected to the positive rail, in which case only one PNP transistor will be sufficient.