Continuity Checker

  
Ul is an op amp used as a comparator. When the test probes are shorted together, resistors R1 and R2 bias the noninverti
Continuity Checker - schematic

ng input to half the supply voltage. The inverting input is biased by a voltage divider that consists of R3, R7, and R4. Resistor R7 is adjusted so that the voltage to the inverting input is lower than that to the noninverting input when the probes are shorted together. With continuity across the probes, Ul"s output goes high and supplies power to Ql, which is configured as a relaxation oscillator. The output of Ql is fed to a high-impedance loudspeaker for an audio tone. When the probe is open, the noninverting input goes to the negative supply rail via R2. This action forces Ul"s output low, which results in no output from the oscillator.




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