This differential amplifier, found in a 1964 Electronic Design, has a gain of 30. The setting can be applied to modern equivalent transistors.
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...