: Community-driven guides for building your own version on a Vero board are available at Electronics Repair Made Easy .
: One of its primary advantages is the ability to test components while they are still soldered to the PCB, provided there are no other major shorts in the power rail.
This article is for educational purposes. Always observe safety procedures when working with high-voltage circuits. Flyback transformers and SMPS primaries can store lethal charges even when unplugged. blue ring tester schematic diagram exclusive
Indicator drive
| 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). | : Community-driven guides for building your own version
A momentary negative-going excitation pulse (typically generated by an onboard 555 timer circuit or a micro-controller tuned to a low frequency like 10Hz to 50Hz) shocks the tank circuit into oscillation through resistor R5cap R sub 5 2. The Comparator Stage (LM393)
Note: This tester checks for shorted turns, not total inductance value. Always complement this tool with a standard LCR meter for a complete breakdown of component health. Should be 9V
Because inductors can produce high-voltage spikes (back EMF) when pulsed, the schematic includes clamping diodes. These protect the sensitive ICs from being fried by the very component they are trying to test. Component List for the Schematic
At the heart of the schematic is a momentary switch and a transistor or IC gate that sends a brief DC pulse into the component under test (L). This pulse "kicks" the inductor into resonance with a parallel capacitor (C) located inside the tester, creating a tuned tank circuit. 2. The Comparator Chain