Found in 1980s Electronics Design, this circuit covers 4 decades of frequencies with a unique control. A multi-turn trimpot is used for fine adjustment. The frequency range depends on C1.
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...