Using electronic scheme shown below can be made a tester for bipolar transistors. Using this electronic circuit we can sort transistors, depending on the class of amplification (gain in current between 140-270), B (270-500) or C (less than 500).
For proper testing NPN transistor is necessary that the transistor to be inserted into the socket TUT (Transistor Under Test) and turn switch S2 in position C. If the LED does not light move switch S2 in position B, and if does not light even in this position, move the switch S2 on the last position.
If the LED does not light up even in the last position then tested transistor has a current amplification in less than 140 or is defective. With the S1 button base current can be interrupted, at which point the LED should turn (if is not off, between collector and emitter of the transistor there is a short circuit). The tester requires a supply voltage of 9 volts DC.