Replace the red LED with a piezoelectric buzzer in parallel with a red LED. A shorted coil then produces a loud beep—useful when your eyes are on the probe.
For power users, here are three exclusive modifications that transform the basic Blue Ring Tester into a professional-grade tool. blue ring tester schematic diagram exclusive
A series of voltage comparators (or software thresholds) that count how many oscillations exceed a certain voltage level. Replace the red LED with a piezoelectric buzzer
The Blue Ring Tester is a simple, low-cost device that applies a short pulse to a coil and then counts the number of decaying oscillations (ringing). A good coil will produce many oscillations; a coil with a shorted turn will produce very few because the short absorbs the energy. blue ring tester schematic diagram exclusive