This circuit was found in a Hands On Electronics from the 90s. It turns on an LED to indicate that the phone is in use.
We found this oscillator in a publication for radio amateurs from the 70s. The circuit generates...
We found this circuit in a 1982 publication, but all the components used are still common in our...
We found this circuit in a 1976 technical documentation, but it can be assembled with modern...