The filter presented is from 1994 but can be implemented with a more modern equivalent. We found this setting in the National Semiconductor Linear Applications Handbook.
This circuit with LDR and neon lamp has its operating frequency given by the light. The circuit is from...
Found in American documentation from the 90s, this circuit can be used as a shield with the...
This tone recognizer circuit that relies on R5 and C1 was found in the December 1994 issue of...