This circuit uses an optical coupler called Raystor created by Raytheon in the 1960s. The circuit uses components from the era that can be changed. The current is low.
This sonic switch, sound relay or simply vox uses a medium impedance speaker as a microphone. The...
This circuit is an impedance adapter. The circuit is from a Radio Electronics from April 1966. The...
This circuit using components that are still common in our market was found in a 1987...