This tone recognizer circuit that relies on R5 and C1 was found in the December 1994 issue of Electronics Now. The circuit operates on voltages from 6 to 12 V typically.
The figure shows how to use a CMOS amplifier as a crystal-controlled oscillator. The circuit is...
This circuit is from the 1990 Philips Databook. The circuit can be supplied with voltages up to...
This very low consumption circuit can be powered by a solar cell. The time indication is given by a...