In-Circuit Transistor Tester:
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.
PARTS LIST:
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 |
