This is a very simple transistor tester circuit the circuit can be used to test NPN and PNP transistors. The voltage source is a 6V power supply which is 230V AC to 6V step down transformer. It is essential to put the transistor leads in right direction like transistor emitter to circuit emitter where (E) is marked
transistor base to circuit base marked (B) and transistor collector to circuit collector marked (C). The switch S1 is a rotary switch to choose a correct base resistor for under test transistor. The green LED will glow when an under test NPN transistor is working correctly and red LED will glow when an under test PNP transistor is working correctly.