We found this circuit in an English publication from the 1980s. The circuit uses current components and can be tuned for frequencies between 300 and 1500 Hz.
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...