Transistor and diode tester

While making a circuit, the first thing to be considered is parts especially active component or semiconductor. Here is a simple circuit to test either the transistor or diode is good or faulty.

Circuit description of transistor and diode tester

IC1 LM555 is timer IC work as heart of the circuit transistor and diode tester and is used as multivibrator. 70 Hz frequency is obtained from output of IC1 with the help of R1, R2 and C1 together. For transistor testing switch SW2 must be in off mode. In the absence of testing transistor both LED used in circuit flash but when a pnp-npn transistor connect to the test terminal B, C, and E from the output of IC1. After connecting transistor voltage drop across one of the LEDs become lower than threshold voltage which cause stop glowing of one LED, this mean transistor is good.

It seen that faulty transistor have either a short-circuited or a broken collector-emitter junction. If the transistor is short-circuited, none of these LEDs glow, but if the transistor is open circuit both LEDs glow.


Diode Testing

Push SW2 on and the diode under test connected between C and E terminals. One glowing led indicate a good diode, while all other indicates point to fault diode.

circuit diagram of transistor and diode tester


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 1 KΩ

R2 = 100 KΩ

R3 = 680 Ω

R4 = 68 Ω


C1 = 0.1 µF


IC1 = NE555 timer IC

D1 – D4 = 1N4148

D5, D6 = Zener diode 5.1 Volt

D7, D8 = LEDs of different color


SW1, SW2 = switch

9V battery for power supply

4 thoughts on “Transistor and diode tester”

    1. Hi,

      The IC on which the capacitor is connected is 555 timer and the capacitor is typically an electrolytic type with a value of 10uf. Now always remember that capacitors should be rated in terms of voltage to the voltage they are exposed to. Now here the circuit is powered by a 9v power supply, thus you will get a 16v rated capacitor in the market to purchase.

      Hope this helped.

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