To build this circuit based on the standard "Anatek" or "Bob Parker" designs, you will need: 1x 74HCT14 (Hex Inverting Schmitt Trigger) or LM339. LEDs: 2 Red, 2 Orange/Yellow, 4-6 Green.
When Q1 turns off abruptly, the magnetic field in the coil collapses, generating a flyback voltage spike. The coil and its parasitic capacitance form an LC tank circuit , causing the coil to ring (oscillate) at its resonant frequency. blue ring tester schematic diagram exclusive