Many years ago (sometime in the last century), I built a continuity tester from a kit (which may have been from Audio Amateur), but can no longer find any info for it. The tester still works fine, and I frequently use it. Its PCB is labeled PE301A, uses an LM324 with a buzzer, is powered by a 9V battery, fits nicely in a 2.75"×4" black plastic box, has Voc = ~ 200mV, and Isc = ~ 270µA. Before I start tracing PCB tracks to reverse engineer this tester, and satisfy my curiosity about its operation, does anyone recognize it and perhaps have a schematic? Lots of web searching has not turned up anything.