Found in Motorola's 1967 documentation this circuit can use any general-purpose transistor. The signal should be applied to a good amplifier. The noise diode can be replaced with a common one.
This circuit was found in Electronics Now. With it the dialing signals of a telephone are...
This circuit switches the brightness of the LED. We found it in an English documentation....
This circuit already explored in several diagrams in this section was found in a 90’s Popular...