This circuit is from a Radio Electronics from February 1959. The circuit has a speaker output and good sensitivity. Power is supplied from a 9 V battery.
The diagram, obtained in an old European documentation, shows the ways to trigger and to shut down...
In the figure we have a three-phase AC motor control for voltages up to 600 V or 1200 V depending on...
This shield circuit can serve as a basis for a digital mixer or simply for a switch without noise...