Transistor Tester Circuit

A simple transistor tester that identifies if the transistor is a NPN or PNP and indicates if it is working properly. (October 3, 2009)


This is a simple transistor tester, useful to identify if the transistor is an NPN or PNP, also the LEDs indicates if the transistor is working correctly or not.


transistor tester circuit


Bill of materials:


  • 1 CMOS 4069B (4011B can be used also)
  • 2 1k Resistors 1/4 Watts
  • 1 10k Variable Resistor
  • 1 1mF capacitor (Non-electrolytic works ok)
  • 1 Red LED
  • 1 Green LED

    Here is how it works:


    Basically, this circuit is a square-wave oscillator, the frequency is defined by the 10k resistor. The output frequency is applied to the emmiter, then it is inverted and connected to the base and collector of the transistor.


    If the RED LED is on, it means the transistor is a NPN type and works correctly.


    If the GREEN LED is on, it means the transistor is a PNP type and it is working correctly.


    If BOTH LEDs are on, it means the transistor have a short circuit condition.


    If NO LEDs are on, it means the transistor is not working or it wasn't connected correctly.


    The only IC needed for this project is a CMOS Inverter Buffer 4069B. It can be replaced by a cheaper CMOS 4011B and connected as inverter [See schematic]. All non-used input pins on the cmos IC must be connected to ground.


    This transistor tester is very useful for the electronic hobbyist and it can test only bipolar junction transistor (BJT).


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