This relay triggered by voice or by another sound was found in an Elektor from the 90's. The circuit uses a 12 V sensitive relay and has no sensitivity control.
This configuration with the LM383 appears elsewhere on this site. The circuit shown was obtained...
This signal attenuator was suggested by Electronics Now magazine in April 1999. The circuit is...
The voltage gain of this DC amplifier is 100 times, given by the relationship between R2 and R1....