Many electronics circuits and projects for learning in simple ways.

Diode tester circuit using IC-741 and LED

This Diode tester circuit can check the diode the good or bad and also can indicate its polarity. They use 741 op-amp and a few parts so easy. With two LED display indicates a tested diode.

How diode tester works

In Figure 1, There are IC1, R1, R2, R3, C1 include as the oscillator circuit that generates the square wave out to pin 6. This signal will be an AC that is symmetrical waveform.

Diode tester circuit
Diode tester circuit diagram using 741

Thus, if we connect the test point or both A and B terminal together, makes LED1 and LED2 will alternatively flash ON and OFF.

Then, if we take the diode to the A-B terminal, as cathode to A and anode to B. Now, the LED1 is in a forward bias. So LED1 will glow but LED2 will go out because it’s a reverse bias.

If put the diode in backward, that anode to A and cathode to B, so LED2 will glow but LED1 will go out. If so this indicates that this diode is normal. But, in case of the diode lack, LED1-LED2 will go out. Then, if the diode short circuit, both LEDs will light up.

The transistor tester.
Also, this circuit can be used to check the transistors, because the structure of transistors has a state like two diodes connected together, base-collector is one diode and base-emitter is other one diode.

When testing same the general diode, but when test between the collector-emitter lead. If normal transistor, the LED1-LED2 will must go out. But LEDs light up show that short circuit between collector-emitter, we cannot use this transistor.

How to assemble circuits.
This circuit can be assembled in the universal board, contain into a small box. Trying to install LED1, LED2 to be close to the most testing. And LEDs of different colors to make observation easier. The 9-volts battery should be used in normal type because this circuit uses low current and so saving your money.


The parts you will need.

IC1: LM741 op-amp IC
C1: 0.1uF 50V, Ceramic capacitors
R1: 68K , 1/4W Resistors tolerance: 5%
R2: 10K, 1/4W Resistors tolerance: 5%
LED1,2:  Read in a text.
PCB,SW1,and more etc.

This Post Has 5 Comments

  1. Thanks a lot.

  2. Hi, Salim Khan,

    Thanks for your feedback.

  3. Hey I just wanted to know why the op-amp 741 is required in this case? The circuit would work even if you would remove the op-amp and resistors.
    Kindly illustrate the necessity of the op-amp 🙂

  4. it is nice project using ic 741. –

  5. must to be serial resistor of led s

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