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.
This circuit was found in a Popular Electronics of the 90s. The operational amplifier admits...
This circuit is from an ancient American documentation. The coil is frequency dependent and the...
This transceiver circuit was found in a 1997 documentation. The circuit operates in the 80 meters...