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 purpose of this circuit is to turn off the power supplied by a battery when its voltage drops...
Found in National Semiconductor documentation from the 1990s, this circuit has a 10 kHz pass band...
Found in a 1980s documentation, this amplifier uses an integrated circuit of the era. The RLC...