3 Transistor tester using 555

You have transistors and you decided to design something or make a simple circuit. But the problem is you cant recognized which of these transistors are NPN or PNP type. With this simple tester,  you  can determine what type of transistor you got and you can tell if its defective or not.
Tester schematic


Circuit operation:
When you place the unknown transistor in the circuit and when red LED lights up, its a PNP type. On the other hand, when yellow LED is on, its NPN. When both LEDs are on, transistor is shorted (defective) and when all  the LEDs are off, transistor is open (defective). Complete scenario of tested transistor is tabulated below.
Transistor conditions
Part List:
555 IC
9V battery
R1 - 1 kilo-ohms 1/4W
R2 - 5 kilo-ohms 1/4W
R3 - 27 ohms 1/4W
R4 - 560 ohms 1/4W
R5 - 1 kilo-ohms 1/4W
C1 - 10uF electrolytic capacitor
C2 - 100nF
LED1 - yellow or orange LED
LED2 - red LED
D1 - 3V zener diode

Note: This tester is for bipolar junction transistor (bjt) only.


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  2. plz write the value of capacitor

  3. Thank u easily understandable