The purpose of developing this circuit is to allow watch crystals to be used in an existing CMOS oscillator circuit that was to run from a 12 V supply. But there is a problem, these crystal only work uo to a supply voltage of about 6V. If the supply voltage is more than 6 V then the crystal will be over driven, causing it to shatter.
By using LEDs 1 & 2 and a 470nF capacitor (C3) to limit the drive to the crystal to about 4V peak to peak, the problem can be solved using that circuit. Note that to ensure reliable start up and stable oscillation with some crystal, may be necessary to adjust C1 & C2. 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. [Source: siliconchip.com.au]