A blue ring tester is a handheld electrical tester used to detect the presence of live AC voltage without direct contact with conductors. The device typically indicates voltage via a glowing “blue ring” neon/LED indicator or via capacitive-sensing electronics that drive a visual indicator. This write-up focuses exclusively on the schematic/design principles, key circuit blocks, component selection, and a compact reference schematic suitable for a hobby or repair-level build (mains-awareness required).
It works by sending a fast voltage pulse into an inductor and counting the "rings" (decaying oscillations) it produces. A high number of rings (indicated by more lit LEDs) signifies a "good" component with high quality (Q) factor, while few or no rings suggest a shorted winding. ⚡ Technical Features & Circuit Design blue ring tester schematic diagram exclusive
: It is optimized for high-frequency wound components. It may fail to accurately test inductors with very low or very high inductance, such as simple wall-adapter transformers. A blue ring tester is a handheld electrical
The ringing signal is AC-coupled via C4 and clamped by D1, D2 to protect the comparator. The LM393 compares the ringing waveform to ground. For a healthy coil, the ringing crosses zero many times. The comparator outputs a series of pulses for each zero-crossing. It works by sending a fast voltage pulse