This variable filter circuit was found in July 1975 Radio Electronics magazine. The circuit uses 3 general purpose op-amps like the 741 and must use a symmetrical source.
Found in a 90s 303 Circuits, this circuit uses the KTY10 sensor, widely used at the time. | Clique na...
We found this circuit in the Linear Applications Handbook of the 1994 National Semiconductor. The...
In the figure we have the commercial circuit of a Japanese electronic voltmeter from the 60's. The...