This tone recognizer circuit that relies on R5 and C1 was found in the December 1994 issue of Electronics Now. The circuit operates on voltages from 6 to 12 V typically.
This circuit was found in the October 1989 issue of Radio Electronics magazine. It drives an...
The transistors in this circuit from an English publication in 1980 may be the BC548. Operational...
This is one of many circuits of this type that can be found in this section. This one is taken from...