The circuit shown in the figure below is from the 1993 Linear Technology Applications. The operational ones used are uncommon and the use of equivalents can be analyzed.
November 2016 - The TL3780 ultra-miniature E-switch touch switches can now be obtained from Mouser...
Using a 4046 PLL this circuit was obtained from the October 1994 issue of Electronics Now...
Found in a 1968 US publication, it is a font filter. The transistor is according to the desired...