BC547 is a NPN Bi-polar Junction Transistor (BJT) which is a semiconductor device used to amplify and switch electronic signals and electrical power. The NPN transistor can be used in two different modes: forward biased mode and the reverse biased mode. In forward biased mode, the electric current can easily flow through it. So it acts like a CLOSED SWITCH. However, in reverse biased mode, the current through it is practically zero and thus, it acts like an OPEN SWITCH.