Connection Tester

  
A low resistance ( 0.25 - 4 ohm) continuity tester for checking soldered joints and connections. This simple circuit uses a 741 op-amp in differential mode as a continuity tester. The voltage difference between the non-inverting and inverting inputs is amplified by the full open loop gain of the op-amp. Ignore the 470k and the 10k control for the moment, and look at the input of the op-amp. If the resistors were perfectly matched, then the voltage difference would be zero and output zero.
Connection Tester - schematic

However the use of the 470k and 10k control allows a small potential difference to be applied across the op-amp inputs and upset the balance of the circuit. This is amplified causing the op-amp output to swing to full supply voltage and light the LED'



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