In this circuit that can indicate the condition of a transistor by using two LEDs. A good NPN transistor conducts during the positive half cycle (pulses are generated by 555 timer) and D2 is off while D1 flashes. With a good PNP transistor D1 is off and D2 Flashes.
1. R1 = 180kΩ
2. R2 = 470Ω
3. R3 = 100Ω
4. C2, C4 = 10nF
5. D3 = 1N4148
6. D4 = 1N4148
7. D5 = 1N4148
8. D5 = 1N4148
9. LED2 = RED LED
10. LED1 = GREEN LED
11. IC1 = NE555 = 2 Nos
Both LEDs flash if the transistor is open and will be off if it is shorted.
The tester checks the transistor even if in-circuit resistance across the transistor is as low as 50 ohms; below this the glow becomes dim. Diodes in the ensure that only low saturation voltage of a good transistor turns the LED off.
Condition of LEDs:-
|S.No.||Transistor||LED D5||LED D6|
|1||Good NPN||Off||NPN Flashing|
|2||Good PNP||PNP Flashing||Off|
|3||Open NPN or PNP||Flashing||Flashing|
|4||Short NPN or PNP||Off||Off|