Transistor Tester

This simple circuit has been of great help to me in many cases. It can test the quality of transistors. In the circuit, with a resistance as low as 40 ohms between BC or BE, it can check the power output transistor in the amplifier circuit.

The circuit principle is as follows:
The 555 timer (IC1) works as a 12 Hz oscillator, and the output at pin 3 drives the 4027 (IC2) bistable circuit. After being divided by 2, the output is output at pins 14 and 15. The output is connected to LED1 and LED2 through the current limiting resistor R3. The two LEDs have been arranged so that only one LED will light up when transistors of different polarities are connected to the circuit. When no transistor is connected, the two LEDs will flash alternately. The outputs of pins 14 and 15 are connected to the B pole of the transistor under test through the voltage divider resistors R4 and R5. When a good transistor is connected, the transistor is turned on and a path is generated for one of the two LEDs. If a good NPN transistor is connected, LED1 will flash. If a PNP transistor is connected, LED2 will flash. If an open-circuited transistor is connected, the two LEDs will flash in turn. If a short-circuited transistor is connected, neither LED will flash.