Found in a 1967 documentation this circuit can be applied to any operational amplifier. Components without values determine the frequency to which the circuit is tuned.
This circuit is based on a 5 V LED flasher. We found it in October 1980 Radio Electronics...
With this system you can disconnect phone lines in case of need for protection. The circuit is...
This circuit was found in a Popular Electronics from the 90s. The circuit has 4 frequency ranges...