Using circuit diagram shown below can be made a very simple transistor tester circuit that can differentiate independently NPN or PNP transistor types.
Operation is very simple transistor tester: transistor to be tested is mounted in the socket and S1button pressed. If the transistor pins correspond to the socket pins ECB, this transistor is npn type, which will be shown optical LED D1. If the LED D2 lights up, it will indicate a correlation between transistor pins and terminals BCE, so this type is a pnp transistor.
If a NPN type transistor is tested, it will conduct when T2 is blocked. Meanwhile, T3 conducts, which will cause lighting of diode D1.If the transistor is PNP type, it will conduct simultaneously with T2 and T1 latter will block T3. Because T1's collector potential is high, T4 conducts and D2 will light.