Transistor Type Sensors

Sorry, I guess I don't read the same things you do. Give me a reference (i.e., URL) to what you are reading, and I'm sure someone here can help you out.

I really don't intend to be disrespectful but you just casually asked about one of the most fundamental components used in electronics today. If you don't understand the fundamental idea that these transistors work as current amplifiers then I cannot help. My only suggestion is that you need to take some classes or explain a little better what you already understand. I can help fill in details but I cannot paint the whole picture.

NPN and PNP sensors are pretty simple. Really just as simple as registering here on this forum.
The different sensors are used to provide an output of either positive or negative to match whatever your load requirement is. For example you may have a PLC input card that is source and wants to see a positive voltage input or you can have a sinking input card that wants negative.

Not having an electronics background, it took me a while to figure this out for my self. I look at it this way: Consider a DC circuit consisting of a +DC voltage line, a load (perhaps a solenoid, or a PLC input) and a 0 VDC return. We can place a switch either between the + line and the load or between the load and the return (-) to turn the load on or off. A switch (your sensor) with a PNP transistor is placed between the + line and the load. In this case the switch is said to be "sourcing" current to the load , A switch with an NPN transistor is placed between the load and the 0 volt return. In this case the switch is said to be "sinking" current from the load.

You are "malar"....right??..