This is really Simple level voltage detector circuit. I like it so much because cause me know principle about comparator circuit form.
As show in Figure below, We use LM741 OP-AMP(IC-741) highly popular is base of circuit.
As video below. When we set all components to breadboard successfully. Suppose, we need to detects voltage 2-volts. I test this circuit with firstly to apply a 9-volts power supply to the circuit. Then,with rotate or adjust a VR1-potentiometer in breadboard to set a reference voltage to pin 3 (read digital multimeter left is 2-volts), After that apply an input voltage to the INPUT at pin 2 of IC1.(read voltage as Zero) Now LED go out. I used to measure output voltage equals the power supply voltage 8.4V approximately.
Then I increate voltage at pin 2 continuously until to 2-volts as volts multimeter. It is making LED glow on immediately. Although we still add voltage up too many voltage. I used to measure voltage at output as 2-volts it lower than the positive (power supply) voltage so LED glow.
Therefore, When voltage at pin 2 larger than pin 3 cause output pin 6 as low voltage when compare with ground so LED light up.
The resistor-1K is used to control current to LED1.
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