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.
The configuration shown in the figure is suggested by Texas Instruments and supports equivalent...
We found this circuit in the Linear Applications Handbook of the National Semiconductor of 1994....
This circuit uses a Hall sensor in the feedback of signals that produce oscillations. The circuit is...