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 the October 1992 Radio Electronics magazine, this oscillator is controlled by switch S1....
This circuit was obtained in a 1976 amateur radio manual. The circuit can be made with a 2N2222...
The component used in this circuit is not very common today as the circuit is from the 80's. It...