"Clever Continuity Tester Wins Op Amp Contest with Precision Problem Detection"
John Guy has designed a continuity tester that modulates the beeper's tone according to the resistance measured in the circuit, making it easier to track down a short circuit. The circuit is based on a single AD8534 quad op amp chip and features a voltage-controlled oscillator, a comparator with hysteresis, and a low-pass filter. The PCB design is easy to assemble and can be placed in a laser-cut acrylic case, turning it into a neat handheld instrument.