The transistor is often used as a switch in electronic circuits.
A signal of 0.6V, or greater, to the BASE of the transistor "switches on" the component and allows a current to flow in the COLLECTOR / EMITTER circuit.Transistors also act as CURRENT AMPLIFIERS since a small base current (Ib) controls a much larger collector current (Ic). The amplification (GAIN - hfe) of a transistor, is given by the equation hfe = Ic/Ib and is typically several hundreds.
N.B. When choosing a transistor it is important to make sure that the gain is high enough for the required use and that the maximum current which it can handle is sufficient to drive the output device. e.g. BFY 51 gain > 40 , max Ic = 1 amp.
If the max Ic value is exceeded, the transistor will probably "burn out".
The transistor acts as a NOT gate (inverter) since a "high" at the base gives a "low" at the collector.
(When the transistor switches on, the collector voltage decreases.)