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  • Simple Transistor Tester

  • Created: Sep 25, 2014

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Summary

This circuit is a simple transistor tester that can easily identify the transistor as NPN or PNP with the support of LEDs acting as an indicator in the system. This eliminates the difficulty in recognizing the polarity of transistors as well as identifying if the transistor is busted or improperly connected. Transistor tester uses a high-speed Si-gate CMOS device (74AHCT00D) as its main component that has quad 2-input NAND function compatible with Low-power Schottky.


The two LEDs are used for displaying the condition of a transistor. Both PNP as well as NPN transistors can be tested using this circuit. Quad 2-input CMOS NAND gate is the brain of the circuit. Out of the four NAND gates inside the IC, only three are used. The gates connected to resistor R1 and capacitor C1 forms a square wave oscillator. In addition, the frequency of this oscillator can be adjusted by using R1. The inverted oscillator output is connected to the base of the transistor under test through the resistor R2 and the non-inverted oscillator output is connected to the emitter of the transistor under test using the resistor R3. Moreover, the status of the LEDs D1 and D2 reveals the condition of the transistor under test. If red LED is ON, it indicates that the transistor under test is a good NPN. If green LED is ON, it indicates that the transistor under test is a good PNP. If both LEDs are ON, it indicates that the transistor under test is short. If both LEDs are OFF, it indicates that either the transistor is busted or improperly connected.


This simple transistor tester project is designed for beginner electronic enthusiasts. The device can be easily constructed which will be helpful in making electronic projects that involves the usage of transistors. This will give them ease in determining good transistors as well as convenience in recognizing their polarities.

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