Null detector

  
Null detector uses simple LED readout to indicate if test resistor R* is below, equal to, or greater than test resistance Rref. IfR* = Rref, the 741 output sits at midpoint value of 4 volts and LED A lights
Null detector - schematic

Otherwise, the output of the 741 turns off one transistor and diverts current from the other transistor through B or C, depending on the polarity of the input voltage difference. Null-detector response is illustrated.




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