NPN, PNP Sensors
I agree, Terry gave a very good example.
In a sinking configuration, current passes through the load first, through the output switch of the sensor second and lastly to ground. A sensor with a sinking output switches the ground, or negative, leg of the circuit.
In contrast, sourcing refers to a configuration in which the sensor is the source of the current required to power the load. In a sourcing situation, current supplied by the sensor passes through the load second and then to ground. A sensor with a sourcing output switches the positive leg
According to Gem sensor publication;
Sinking, the more widely used of the two, involves the switching of load current (power) supplied by a power source external to the sensor. In a sinking configuration, current passes through the load first, through the output switch of the sensor.
Even though my most common use is Sourcing.
There's something else you can do with NPN and PNP transistors. If a circuit only contains one type, you can replace it with the other type, providing you reverse the polarity of the supply.
Roger