This circuit was found in a Philips documentation from 1970. The components are of the time, difficult to find today, but the configuration can be adapted to use modern equivalent components.
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...