Late into the night, the project was finished. He took a damaged transformer from a local school’s power supply—a heavy, silent block of copper and iron. He clipped the tester’s leads to the coil. He took a breath and pressed the switch.
| Component | Value/Type | Notes | |-----------|------------|-------| | R1 | 10kΩ | 555 timing | | R2 | 1kΩ | Base drive for Q1 | | R3 | 1kΩ | Q1 collector to Q2 base | | R4 | 1kΩ | Output protection | | R5 | 100Ω | LED current limit for LM3914 (adjustable) | | R6 | 10kΩ | Pull-up for trigger | | R7 | 10kΩ trimmer | Calibration pot (LM3914 RHI) | | C1 | 100µF / 16V | 555 timing cap | | C2 | 470pF ceramic | Coupling cap from ring signal | | C3 | 0.01µF ceramic | 555 pin 5 bypass | | C4 | 10µF | LM3914 reference bypass | | Q1 | BC547 (NPN) | General purpose | | Q2 | BC557 (PNP) | General purpose | | D1, D2 | 1N4148 | Clamping/protection | | U1 | NE555 | Timer IC | | U2 | LM3914 | Dot/Bar driver | | LEDs | 10x standard | 2V drop, 20mA | | Power | 9V battery | Ideal for portability |
Before diving into the schematic, understand the two crucial phases of operation: Blue Ring Tester Schematic Diagram
When the 555 output goes HIGH:
The ringing signal appears at the collector of Q2. This signal is AC-coupled via a small capacitor (e.g., 100pF to 470pF – C2) to remove the DC bias. The signal then passes through: Late into the night, the project was finished
While various manufacturers produce "Blue Ring Testers," the underlying schematic diagram almost universally derives from classic pulse-testing circuits popularized in electronics magazines and open-source forums. The diagram can be broken down into three functional blocks:
The "Blue Ring Tester" gets its name from the characteristic blue color of the original PCB and the "ringing" waveform it produces. Unlike a conventional LC meter or multimeter, it injects a short, sharp voltage pulse into the coil under test. If the coil has no shorted turns, it will "ring"—producing a decaying sinusoidal oscillation. If there are shorted turns, the energy is dissipated instantly, and the oscillation dampens almost immediately. He took a breath and pressed the switch
Despite these, for flyback transformers, switching transformers, deflection yokes, and common-mode chokes, the Blue Ring Tester is unbeatable for its price.
: Most designs operate on a standard 9V battery . Applications and Limitations