This 50 Hz rejector or notch filter was found in a 1975 documentation. Other frequencies can be obtained by changing components. The filter uses a Wien network and must use a symmetrical power supply.
This circuit can be used in a toy organ. The frequencies are determined by the capacitors selected by...
This circuit, which appears in this section elsewhere, is from an American 1980s documentation....
This very simple circuit uses a phototransistor and a very sensitive relay (reed type) being used...