Found in an Electronics World of 1966, this circuit makes use of FETs of the time. Modern equivalents can be used. The high voltage supply suggests its use with tube amplifiers.
This circuit of a voice-controlled switch, Vox or sonic switch was obtained in a Popular...
This circuit appeared in the July 1969 issue of Radio Electronics magazine. The sensor is an LDR....
This circuit of two transistors was found in a Japanese magazine in 1967. The transistors are made...