Transistor switching, PNP vs NPN HD

This is a short demonstration using a transistor,a potentiometer, and an LED. With the PNP transistor as current flow to the base increases the current between the collector and emitter increases. With the NPN transistor as current flow to the base decreases the current between the collector and emitter increases. So, if you want something to be triggered by the addition of current choose the PNP and if you want something to be triggered by the reduction of current choose the NPN. An example circuit might use a solar cell and an NPN transistor to detect darkness. As darkness falls and the current from the solar cell drops the current from the collector to the emitter will increase.