This filter was found in Texas Instruments documentation from the 1980s. The circuit uses a common operational amplifier and has its response curve in the figure.
This circuit with a double-gate MOS transistor was found in a Popular Electronics from the 90's....
This photoelectric cell light relay was found in Radio Electronics magazine from April 1958. The...
This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...