This circuit recognizes a tone determined by the tuned input circuit (R3 / R4 / C2). The circuit was found in Radio Electronics from the 90s. The 4046 is not a common component today.
This circuit from Motorola is from 1968, appearing elsewhere on this site. The transistors admit...
This circuit was obtained from the 2005 Linear Technology component manual itself. The output...
Found in na 1970 EDN this circuit has a parallel regulation providing a symmetrical output of 15 V....