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.
The circuit shown in figure 5 generates square signals based on a transconductance operational amplifier...
By adding a transistor to the output and some polarization features we can obtain a higher output...
Found in the 1990 special issue of Radio Electronics Experimenter magazine, this circuit uses an...