Op Amp Quiz: Intelligent Continuity Tester tells you where the problem is

The continuity tester, as found on most multimeters today, is a great tool for finding faulty connections and short circuits. But once you find a short, it’s up to you to figure out which part of the circuit it’s in – hard work on a large PCB with hundreds of components. [John Guy] It aims to facilitate this task by using Continuity test modulates whistle tone According to the measured resistance in the circuit. Tracing a short circuit is just a matter of examining multiple points along the path and observing if the pitch goes up or down.

The circuit is based on the quad-op AD8534 chip. The first stage measures the voltage across the circuit under test in response to a small current and amplifies it. The resulting signal is fed into a voltage controlled oscillator (VCO) made with an amp device connected as an integrator and another acting as a comparator with hysteresis. The op amp number four amplifies the resulting square wave and drives the amplifier. The low-pass filter makes the sound more pleasing to the ears by removing the higher tones.

[John] I pay special attention to the PCB design to make assembly easier despite having a large number of SMD components on a small board. He even silk-screened the parts list on the back, so anyone could put it together even without accompanying documents. The resulting board can be housed in a laser-cut acrylic case, turning it into a nifty handheld tool that is sure to find a place in any engineer’s toolbox. Measuring resistance by sound isn’t as accurate as using a full four-wire setup with an ohmmeter, but it’s much faster and easier if you just want to find that pesky solder bridge hiding somewhere on your breadboard.