Using a 4046 PLL this circuit recognizes a 1.6 to 1.8 kHz tone by driving an output LED. The circuit is from a Radio Electronics from December 1989.
Texas Instruments has downloadable documentation on designing digital isolators. The Digital...
This circuit is from a 1973 documentation. In it we have the way of using a FET as an input of...
This polarization step for electret microphones was found in an Electronics Now from the 90s. The...