This circuit for analog telephones from the 90s was found in documentation from the time. It lights up a remote LED when the extension is in use.
This circuit for frequencies of 20 to 2 kHz was obtained in English documentation from 1966. The...
Indicated for adjustment in transmitters for the 11-meter band, this circuit with analog meter was...
The circuit shown uses the LM340 but applies to other regulators. We found this setting in...