This optical theremin was found in a 90s Hands-On Electronics. The basic frequency is given by C1 and the output is made in a speaker through a small output transformer.
This simple expandable circuit was obtained from a Popular Electronics from the 80's. The supply...
The frequency of the LC resonant circuit must be the same as that of the crystal and the JFET...
This circuit was obtained in a Popular Electronics of 1991. The magazine no longer exists, but the...