The proposed simple crystal tester circuit can be used for accurately testing the operating frequency of any standard crystal, quickly.
The figure below shows a crystal checker circuit schematic. In this circuit, transistor Q1 is attached to a Pierce-oscillator setup with the crystal delivering the feedback from Q1’s collector to its base to generate and retain oscillation. This is the operation when a crystal under test working correctly.
A tiny variable capacitor, C4, permits the tester to sample a wide frequency range. Diodes D1 and D2 transform the RF to DC while LED1 takes care of indicating whether or not the crystal is oscillating.
The crystal checker crystal can be constructed on a breadboard or a perfboard and enclose it in a compact plastic case. Since there is a lot of crystal sizes used in electronics, two tiny alligator clips will be perfect acting as an adaptable crystal socket.
How to Use
Applying the tester is easy. Connect the crystal to the test and turn the capacitor C4, beginning at its minimum capacitance value until the LED lights up. The circuit can also be utilized to inspect a number of the ceramic and piezo-filter components.