The idea for this crystal tester circuit sprung out of the need of testing a large number of oscillator crystals lying unused in a big hobby box. testing the crystals one by one without the appropriate device would have been very slow and a time consuming task. Commercial crystal testers are however very expensive, that is why this simple electronic crystal tester was born.
The transistor T1 and the crystal to be tested together makes a complete crystal oscillator. The capacitors C1 and C2 works as a capacitive voltage divider that is connected to the transistor T1. If the crystal being tested is intact, the circuit oscillates. The sine wave oscillation voltage is fed to the rectifier circuit (d1, D2) and filtered by capacitor C4. With an intact crystal, the DC voltage at the base of the transistor T2 is high enough to cause the transistor to conduct. The LED lights up signalling that the crystal is good.
The electronic circuit cand be used to test crystals with frequencies from 100kHz up to 30MHz. The current consumption is low: around 25mA.