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.
Using an operational amplifier, this circuit was found in the March 1996 issue of Electronics Now...
This simulation shows how a transistor can be used as a switch turning on and off in the LED. The...
This circuit was found in the March 1990 issue of Radio Electronics magazine. The circuit can be...