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.
The figure below shows an AC transconductance amplifier with the National Semiconductor LM3900. The...
This common circuit using operational amplifiers was found in the 1993 issue of Radio Electronics Special...
This circuit provides a voltage at the output only when the input voltage exceeds a value that...