switching using NPN and PNP Transistor

 

This diagram shows how NPN and PNP transistors are used as switches.
In the NPN (low-side switch), the emitter is connected to ground and the load is placed between Vcc and the collector. When the microcontroller outputs a HIGH signal to the base, the transistor turns ON and allows current to flow from Vcc → load → transistor → ground. This is called current sinking, and it is the most commonly used switching method.
In the PNP (high-side switch), the emitter is connected to Vcc and the load is connected to ground through the collector. When the base is pulled LOW, the transistor turns ON and supplies current from Vcc to the load. This is called current sourcing.
In practice, a base resistor is always required to protect the transistor and microcontroller.