This circuit lets through the 300 Hz to 3kHz range that corresponds to the human voice. We found it in the Radio Electronics magazine of February 1990. The circuit uses both parts of the LM381.
Suggested by Raytheon in 1967 documentation this circuit uses a lightbulb and LDR optical coupler of the...
Found in American documentation from the 90's, this circuit consists of a solid state relay with...
Indicated to be used at the sensor input of a PC, this circuit was found in an English...