Audible Transistor Tester
All types of transistors including Darlington and power will be tested in this project. Connect the 9v battery around the other way at points A and B to test PNP types.
The transformer in the photo is a 10mH choke with 150 turns of 0.01mm wire wound over the 10mH winding. Then, the two original pins (with red and black leads) go to the primary winding and the fine wires are called the Sec. Connect the transformer either way in the circuit and reverse either the primary or secondary (but not both) if it does not work. Almost any audio transformer will work any speaker will be suitable. [Source: talkingelectronics.com]