We found this circuit in an English publication from the 1980s. The circuit uses current components and can be tuned for frequencies between 300 and 1500 Hz.
We found this circuit on Radio Electronics from July 1990. It triggers a relay with a touch of the...
These three circuits have different treble responses. We found them in the February 1970 issue of...
This photoelectric cell light relay was found in Radio Electronics magazine from April 1958. The...