This buzzer trigger can be used for small alarms. We find it in an 80’s American documentation. The supply can be done with voltages from 3 to 18 V.
Found in a 1965 Radio Shack documentation this oscillator can be used with modern general purpose...
In the figure we have the way to use a single control voltage to act on two varicaps...
Found in a 90s documentation, this switch that is also sensitive enough to operate capacitively uses...