Using a 4046 PLL this circuit recognizes a 1.6 to 1.8 kHz tone by driving an output LED. The circuit is from a Radio Electronics from December 1989.
Found in Burr Brown documentation from the 90s, this circuit applies to several operational ones....
This circuit has appeared on our site several times as a battery status monitor. In this...
This circuit was obtained from an English magazine from 1981. The magazine no longer exists, but...