Transistor Tester

R1	470kΩ
R2	470Ω
R3	470Ω
C1	0.47µF (Electrolytic)
D1	1N4148
D2	1N4148
D3	1N4148
D4	1N4148
D5	RED LED
D6	GREEN LED
IC1	NE556

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	Flashing
2	Good PNP	Flashing	Off
3	Open Circuit NPN of PNP	Flashing	Flashing
4	Short Circuit NPN or PNP	Off	Off