Thursday, October 30, 2014
Safe Oscillator For Watch Crystals circuit with explanation
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
