Exploring the Circuit 11/7/18

Transistor: A switch controlled by an electrical circuit.

There are different types of transistors; the most common one is the “bipolar junction transistor” or “BJT”
The BJT has three pins: Base (b), collector (c), and emitter (e)
It comes in two versions: NPN, and PNP

Semiconducting material allows for current to flow from the base to the emitter opening a flow of current from the collector to the emitter. Need to apply voltage to the base to get the current flowing from base to emitter; this turns the transistor ON. Once switching on the transistor current is able to flow from the collector to the emitter.  (current will only flow from collector-emitter if transistor is on)
https://www.build-electronic-circuits.com/how-transistors-work/

Transistors can be slightly on and slightly off.

PNP transistor has the same pins as a NPN transistor, however the transistor will turn on when voltage applied to the emitter induces a current flowing from the emitter to the base. Once turned on there is then a large current between the emitter and the collector. 

Voltage applied to the base must be 0.7v lower then the emitter in order for current to flow from emitter to base. Since voltage on base must be lower then emitter, emitter should be connected to the plus of the power source. 
https://www.build-electronic-circuits.com/pnp-transistor/