This filter for receiving audio in telegraphy was found in a magazine 73 for radio amateurs, in an edition of the 80s. The circuit is tuned to 700 Hz and has a 200 Hz bandpass.
This circuit is from 1988 documentation. It uses a range of 30 LEDs driven by 4017 integrated...
This circuit was found in Electronics Now, it can be used with universal phones on a common...
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