# Transistor Polarity Checker

Presented here is a simple circuit that will help you figure out whether a transistor is npn or pnp type. -- Raj K. Gorkhali

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Presented here is a simple circuit that will help you figure out whether a transistor is npn or pnp type. If the transistor under test is npn, red LED will blink. If it is pnp, green LED will blink. This circuit is highly useful for small labs where components lie on the desk unidentified after a project is done and it is very tedious to read the part number printed on the component and identify it from its datasheet. The circuit provides a straightforward method for identification of bipolar transistors.

Circuit and working
As shown in Fig. 1, transistor polarity checker circuit is built around quad 2-input NAND gate CD4011 (IC1), four diodes (D1 through D4) and a few discrete components. It works off 9V supply, and a PP3 battery is recommended for compactness of the unit.

A rectangular oscillator is made using gates N3 and N4, which is further connected to N1. The output of N1 is connected to the input of N2. Like this, you get square waves at the outputs of N1 and N2 that are 180 degrees out of phase.

Two LEDs (LED1 and LED2) are used to indicate the polarity of the transistor under test. These LEDs are connected to the outputs of N1 and N2 as shown in Fig. 1. If there is no transistor connected at TUT, both LEDs will blink alternately. If the transistor under test is pnp, it provides a short across the LEDs when the output of N1 is low, which does not allow the red LED to glow. When the output of N1 becomes high, the transistor under test (if pnp) stops conducting and the green LED glows. Thus the green LED blinks if the transistor under test is pnp.

Similarly, if the transistor under test is npn, it will provide a short across the LEDs when the output of N1 is high, which does not allow the green LED to glow. When the output of N1 becomes low, the transistor under test (if npn) stops conducting and the red LED glows. Thus the red LED blinks if the transistor under test is npn.