Low side and High side switches
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) NPN open collector switch. (b) A failed attempt to make a PNP version.

The low side switch is very simple as shown in Figure 1a. The low-voltage logic just has to feed sufficient current into the NPN transistor to turn it fully on.

There is a temptation to think that we could do the same trick with an PNP transistor as shown in Figure 1b. The problem is that the emitter-base junction is always forward biased. This will apply the 12 V to the chip output and destroy it or, if there are protection diodes on the output, the current will flow through the protection diodes into the micro-controller supply (shown as 5 V in this case). The effect of this current flow is to turn on Q3 and the load can not be switched off.

There are ways around the problem of Figure 1b but they are all more complex.