Low Cost Transistor Tester

Using this circuit, find out whether a given transistor is good or bad before soldering it. You can also identify npn and pnp types easily. The tester gives LED indication of the pin-outs as well as the working conditions of the transistors. The circuit employs the properties of the gates of low-power IC 4093. It is a NAND IC with four gates that can be designed in various ways. Here the first gate (N1) is designed as a simple oscillator with resistor R1 and capacitor C2. The remaining gates are used as inverters and buffers. The output of gate N1 (pin 3) is fed to gate N2 (pin 4), which, in turn, is used to drive the green half of bicolour LED1. The cathode of green LED1 gives the collector current for the transistor under test (TUT). The base of the TUT gets biasing voltage from the output of gate N2 to invert the signal. The emitter of the TUT is connected to the outputs of gate N3 and N4.