Different types of discrete outputs available in Balluff sensors
Most sensors use a solid-state device for the switching function.
Different options available are:
PNP, 3-wire 
NPN, 3-wire 
AC/DC, 2-wire 
NPN and PNP are technical terms for the type of transistor used to switch the output.
The type of transistor determines the direction of current flow.
PNP sensors are sometimes called “sourcing sensors” because they source positive power to the output. NPN sensors are sometime called “sinking sensors” because they sink ground to the output.
PNP, 3-wire
NPN, 3-wire
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