Circuit : Andy Collinson
Two simple test circuits to check operation of quartz crystals.
In the first circuit, above the BC548 is wired as a colpitts oscillator, the frequency tuned
by insertion of a crystal. A good crystal will create high frequency oscillations, the
output at the collector is rectified by the germanium OA91 diode and a deflection will appear
on the meter. Thw more active the crystal, the higher the output deflection which may be adjusted
with the preset.
The next circuit uses a working crystal again used to control the frequency of a colpitts
oscillator. This time the output from the oscillator is taken from the emitter and is full
wave rectified, the small dc bias will then directly cause the second BC548 to light the LED.