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 optically coupled circuit was found in the February 1968 Radio Electronics magazine. The...
We found this circuit in the April 1978 issue of Radio Electronics magazine. The power supply must...
This circuit supports several sets of coils and can oscillate from 1 to 44 MHz. The transistor is an old...