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.

form a voltage divider that establishes a tiny DC reference voltage on the non-inverting input. This bias ensures that the comparator only triggers when the positive peaks of the ringing waveform exceed a specific threshold.

If you are currently building this circuit or troubleshooting a specific piece of equipment, let me know:

The Blue Ring Tester functions based on the principles of parallel resonance and damping. 1. Pulse Generation

The Blue Ring Tester is an essential, cost-effective tool for any electronics enthusiast or professional who regularly works with flyback transformers and switch-mode power supplies. Its simple design and intuitive LED display make it easy to build yourself. For more insights and community discussions, you can explore the following active electronics forums where enthusiasts and professionals share their experiences:

Connect a known-good SMPS transformer primary winding. Adjust the variable resistor connected to the LED driver pin until all LEDs just light up. This establishes your baseline "good" reading. Troubleshooting Common Layout Issues

Beyond flybacks, it is excellent for testing deflection yokes in TVs and inductor coils in audio gear.

The required components are few, easily available, and low-cost. Here's a typical bill of materials for a DIY project:

The remaining inverters are paralleled (buffers) to provide enough current drive to the test coil/output probe. This ensures a sharp, high-energy pulse is delivered to the transformer under test.