This protective circuit for power lines was found in a Radio Electronics from November 1957. The transistor used can be any general-purpose transistor such as the CK722.
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...