This circuit was developed to allow watch crystals to be used in an existing CMOS
oscillator circuit that was to run from a 12V supply. The problem is
that these crystals only work up to a supply voltage of about 6V. Any
more than that and the crystal will be over-driven, causing it to
shatter. This circuit solves the problem by using LEDs
1 & 2 and a 470nF capacitor (C3) to limit the drive to the crystal
to about 4V peak-to-peak. Note that it may be necessary to adjust C1
& C2 to ensure reliable start-up and stable oscillation with some
crystals. However, the C1:C2 ratio should be maintained. As a bonus, the
two LEDs both glow, giving a visual indication that the oscillator is working.
The relatively high values used here for capacitors C1 & C2 will
load the crystal, which means that the oscillator will run at less than
the nominal crystal frequency (32.768kHz).
Author: Duncan Graham – Copyright: Silicon Chip Electronics