This 555 timer circuit Bipolar transistor tester electronic project is a very a simple and inexpensive device that can be used for testing bipolar transistors types npn and pnp . If the transistor functioning properly the corresponding led indicator will flash at 2Hz.
Separate led indicators are used for npn and pnp devices. Assuming an npn test device is connected,the transistor will be biased off when IC1 output (pin 3) goes low, and will conduct when pin 3 goes high.
The led D1, with current limiting resistor R5, will flash when a serviceable transistor is connected in the right configuration.
If the test transistor have a short circuit between the base and collector the led D1 will flash dimly or not at all and if the test transistor is a short circuit between collector and emitter then D1 will simply glow continuously, and it will fail to light at all .
The circuit works in the same way in pnp mode except that the pnp transistor is pulsed on when IC1 output is low.
|3.3K ¼ Resistor
330k ¼ Resistor
15k ¼ Resistor
4.7k ¼ Resistor
120 ¼ Resistor