This circuit is from an old documentation, but perfectly current. In place of the battery, 2 or 4 small cells can be used.
Two inverters, of the 6 available in a Low-power Schottky 74LS04, are used in this anti-ring...
Found in Radio Electronics from the 90's, this circuit can be implemented with other operational...
Taken from a 1956 Raytheon transistor manual, this circuit uses varistors to obtain modulation of a signal...