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.
We found this circuit on a Radio Electronics. Similar configurations can be found in this section. The...
This circuit was found in the January 1989 issue of Radio Electronics magazine. The circuit sends...
This circuit is from General Electric from the 80s, using a relay with high current contacts (20...