This circuit was found in a 1980s documentation for amateur radio. The circuit makes use of components of the era, but modern equivalents can be used.
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...