An ideal school project for small workbench as a School Project.


It is one of the most commonly used circuits available online. It works immaculately. The circuit is very common; however, I needed this little project on my work bench to immediately test Diodes and Transistors. Testing a transistor using a multimeter takes little more time however, with this circuit its just too quick.

Transistor Diode Tester Thumbnail

Transistor Diode Tester Thumbnail

The circuit is wired around one of the most commonly used timer IC 555. It is a very cheap, popular and very useful precision timing monostable multivibrator IC. In the project presented here the IC 555 is operating at a frequency of 2Hz. The output at pin number 3 drives the circuit with a positive then zero voltage. The other end of the circuit is connected to a voltage divider with the mid-point at approx 4.5v. This allows the Red LED and the Green LED to flash alternately when no transistor or diode is connected to the tester probes (alligator clips).

Transistor Diode Tester Circuit

Transistor Diode Tester Circuit

This little project is fit for testing both PNP / NPN transistors and nearly all type of diodes. In this circuit multivibrator IC 555 works in three modes: astable, monostable and bistable


If a good transistor is connected (at C,E,B), it will produce a short across the LED pair when the voltage is in one direction and only one Yellow LED will flash. If the transistor is open, both (Red and Yellow LEDs will flash (alternatively) and if the transistor is shorted, neither (Yellow or Red) LED will flash.

Similarly, the Diode needs to be plugged into C and E. For a forward bias, Yellow LED will flash, for reverse bias Red LED will flash, an open diode will keep both LEDs flashing and a shorted diode will not flash any LED.

Transistor Diode Tester - Indications

Transistor Diode Tester - Indications


  • Red-Yellow LED Flashing : Open Transistor (Faulty)
  • Red LED Flashing : Closed Transistor (Faulty)
  • NO LED Flashing : Shorted Transistor (Faulty)
  • Yellow LED Flashing : Good Transistor (OK)



  • Red-Yellow LED Flashing : Open Diode (Faulty)
  • Red LED Flashing : Reverse Bias (OK)
  • NO LED Flashing : Shorted Diode (Faulty)
  • Yellow LED Flashing : Forward Bias (OK)



  • 1 x IC 555
  • 1 x 8Pin Base
  • 4 x 220 Ohm Resistors
  • 1 x 33K Resistor
  • 1 x Red LED
  • 1 x Yellow LED
  • 1 x General Purpose PCB
  • 2 x 1N4148
  • 1 x 10uf 25V
  • 1 x 9V Battery
  • 1 x 9V Battery Connector
  • 3 x Alligator Clips


PRICE: Rs. 25/-

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