Transistor Tester

We can use this basically a high gain amplifier with feedback circuit to cause the LED flash at rate determined by the 10u and 330k resistor. Before insert the unknown transistor, remove one of the transistor. The LED flash when it’s NPN with pins as shown in the photo. Remove one of the transistors to turn the unit off.
TRANSISTOR TESTER-2

[Source: Talking Electronics]