Found in American documentation from 1968, this circuit has an acute cut-off of the frequency to which it is tuned and which depends on RC. Transistors may be modern equivalents of general use.
This Radio Electronics circuit, in an 80's edition, decodes the 67 kHz signal used at the time to...
This circuit was found in a 1966 General Electric documentation. The circuit makes use of PUTs...
This circuit was found in an old Electronic Design. It makes use of sidacs and operational not very...