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.
We found this oscillator in a publication for radio amateurs from the 70s. The circuit generates...
We found this circuit in a 1982 publication, but all the components used are still common in our...
We found this circuit in a 1976 technical documentation, but it can be assembled with modern...