We found this touch control on a Popular Electronics from 1970. The circuit makes use of an integrated of the time. It can be adapted to use modern equivalent components.
This circuit was found in an Analog Devices manual and can supply a 3 mA current or drain a 10 mA...
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
The integrated circuit used in this project is not common today. It was found in a 2000...