This circuit was found in an American documentation of the 90s. Optical resources with the LDR as a lens can help in obtaining directivity and sensitivity.
This voice-actuated bistable relay or sonic switch was found in English documentation. The circuit...
This circuit was found in a documentation of the 80's having characteristics of high overvoltage given by...
This circuit is from a 1969 Electronics World. The transistors may be modern general-purpose...