| 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 |
