Found in Precision Monolithics documentation from the 90's, this circuit equalizes octave signals determined by the capacitor values according to the table next to the diagram.
In the figure we have the way to activate a red LED that needs 1.7 V or even others with higher...
This circuit was obtained in a 1979 publication, but it can be assembled with the BD135 instead of the...
Found in Radio Electronics from the 90's, this circuit employs a 3140 operational amplifier, still...