This diode checker circuit can check the diode the good or bad and also can indicate its polarity.
How it works
In Figure 1, There are IC1, R1, R2, R3, C1 include as the oscillator circuit that generate the square wave out to pin 6. This signal will be an AC that is symmetrical waveform.
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 a transistors, because structure of transistors have state like two diode 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-emittor lead. If normal transistor, 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 assambled in the universal board, constain 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 use 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 text.
PCB,SW1,and more etc.