Transistor Tester

A simple but useful Transistor tester

Current Status:    Complete
Date:                   21st December 2003
License:               TBC

Ever needed to quickly determine if a transistor is operational? Well this circuit could help you.

It works by providing a quick DC test of a transistor and it will tell you if it is an NPN or PNP transistor. It will identify shorted and open circuit junctions and it can be used to test diodes.

I did not design this circuit but I built a prototype many years ago from some one else's completed circuit. I created a schematic (a poor copy of which was here previously) and my very first PCB design was of this circuit.

How it works

It uses the ubiquitous 555 timer to work as a low speed (about 1 Hz) astable multivibrator. When the Q output of the 555 is high (9 volts) Q1 is switched on and provides 0V to the BASE connection of a transistor. If a PNP transistor is in the test socket, and functional, it should turn on LED D1. When the Q output of the 555 is 0V, Transistor Q2 should be on, this switches 9V to the base of a test connector. LED D2 should light if the transistor works.

Circuit details

Circuit diagram updated 3rd December 2003. Now includes Eagle Schematic and PCB design for download. Click on the image for a larger picture. The original circuit had an error with the connections to the Q output of the 555 timer. I copied my working PCB to create this schematic.

To download the Schematic and PCB, click here


Updated 14 April 2021