This circuit was found in a 1980s US publication. It indicates when the phone is off-hook by triggering an indicator.
Found in a Popular Electronics from the 80's, this circuit triggers an SCR keeping an LED on when...
This circuit was found in a 1994 documentation. Speed measurement pulses are obtained by a reed...
I found this Vox circuit or acoustic receiver as it was originally called, in a 1973 publication. The...