It is a general-purpose inductance or LCR meter. The Blue Ring Tester is a Q meter, not an inductance meter. It provides a qualitative assessment of the coil's 'Q' (Quality factor)—a measure of its efficiency—rather than its precise inductance value. A high Q indicates a good, low-loss inductor; a low Q suggests a damaged or "lossy" one.
A 9V battery powers the unit. A notable design improvement involves changing R7 from 2.2 kΩ to 510 Ω
For the most accurate results, consider these two pro-tips during your build:
: An internal capacitor in the tester pairs with the external coil's inductance, forming a temporary RLC tank circuit. This triggers a damped AC voltage oscillation known as "ringing". blue ring tester schematic diagram exclusive
Assorted values to set timer frequency. Test Probes: Heavy-duty clips for connection. How the Circuit Works Pulse Generation: The timer generates a short positive pulse.
const int testVoltage = 5; // Test voltage (V) const int testCurrent = 10; // Test current (mA)
Standard multimeters check a coil's health using simple direct current (DC) resistance. However, a transformer with a single shorted turn will still read normal resistance while completely failing in a high-frequency circuit. It is a general-purpose inductance or LCR meter
The classic Blue Ring Tester circuit, originally popularized by Dick Smith Electronics and designer Bob Parker, relies on inexpensive, widely available components. Below is the functional schematic architecture.
: When the pulse hits a healthy inductor, it creates a decaying AC waveform (ringing). The tester counts how many of these oscillations exceed a specific voltage threshold.
The Blue Ring Tester can be used not only to detect shorted turns but also to test the quality of capacitors (if you resonate them with a known inductor) and even to identify the resonant frequency of an LC circuit. A high Q indicates a good, low-loss inductor;
+9V to +12V DC Power │ ┌─────────────┴─────────────┐ ▼ ▼ [Pulse Generator] [Reference Voltage] (555 Timer / Gates) │ │ ▼ ▼ [Comparator Array] [Pulse Driver] (LM324 / LM339) (NPN Transistor) │ │ ▼ ├──────► [Test Leads] [LED Bar Display] │ (CX & LX) (Blue LEDs 1-8) ▼ ▲ [Decay Detector]──────────────────┘ 1. The Pulse Generator (Oscillator)
Notes:
The capacitor in the tank circuit must be stable and high-quality (e.g., polypropylene) for accurate, consistent ringing.
| Symptom | Likely Cause | Solution | |---------|--------------|----------| | Both LEDs off | No power or dead 555 | Check voltage across pin 1 & 8 of U1. Should be 9V. | | Green LED always on | Comparator stuck high | Check R4, R6. Possibly C4 shorted (replace). | | Red LED always on | No ringing signal | Probe test points with scope. Is the 555 pulsing? | | Inconsistent results | Poor probe connections | Use shorter, thicker leads. Solder alligator clips. | | False positives on large coils | Insufficient pulse energy | Increase C2 to 22nF or reduce R3 to 68Ω (do not go lower). |