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.
Found in a magazine 73 for radio amateurs, this circuit has two settings, one for the interval and the...
This good quality amplifier found in a 1982 documentation has a traditional complementary output...
A low frequency divider can be made using common unijunction transistors as shown by the figure...