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.
We found this measurement circuit with a logarithmic scale in an Electronics Now from November 1995....
This circuit assumes that a pulse produced by S1 has a constant duration determined by R1 and C1....
This circuit is used for remote current monitoring. In the original version of the 80s a lamp is...